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# 21 Lists

### Sections

1. List Categories
2. Basic Operations for Lists
3. List Elements
4. List Assignment
5. IsBound and Unbind for Lists
6. Identical Lists
7. Duplication of Lists
8. Membership Test for Lists
9. Enlarging Internally Represented Lists
10. Comparisons of Lists
11. Arithmetic for Lists
12. Filters Controlling the Arithmetic Behaviour of Lists
13. Additive Arithmetic for Lists
14. Multiplicative Arithmetic for Lists
15. Mutability Status and List Arithmetic
16. Finding Positions in Lists
17. Properties and Attributes for Lists
18. Sorting Lists
19. Sorted Lists and Sets
20. Operations for Lists
21. Advanced List Manipulations
22. Ranges
23. Enumerators

Lists are the most important way to treat objects together. A list arranges objects in a definite order. So each list implies a partial mapping from the integers to the elements of the list. I.e., there is a first element of a list, a second, a third, and so on. Lists can occur in mutable or immutable form, see Mutability and Copyability for the concept of mutability, and Duplication of Lists for the case of lists.

This chapter deals mainly with the aspect of lists in GAP as data structures. Chapter Collections tells more about the collection aspect of certain lists, and more about lists as arithmetic objects can be found in the chapters Row Vectors and Matrices.

Lists are used to implement ranges (see Ranges), sets (see Sorted Lists and Sets),indexSets strings (see Strings and Characters), row vectors (see Row Vectors), and matrices (see Matrices); Boolean lists (see Boolean Lists) are a further special kind of lists.

Several operations for lists, such as `Intersection` and `Random`, will be described in Chapter Collections, in particular see Lists and Collections.

## 21.1 List Categories

A list can be written by writing down the elements in order between square brackets `[`, `]`, and separating them with commas `,`. An empty list, i.e., a list with no elements, is written as `[]`.

    gap> [ 1, 2, 3 ];              # a list with three elements
    [ 1, 2, 3 ]
    gap> [ [], [ 1 ], [ 1, 2 ] ];  # a list may contain other lists
    [ [  ], [ 1 ], [ 1, 2 ] ]

Each list constructed this way is mutable (see Mutability and Copyability).

- `IsList(` obj`) C`

  tests whether obj is a list.

      gap> IsList( [ 1, 3, 5, 7 ] );  IsList( 1 );
      true
      false

- `IsDenseList(` obj`) C`

  A list is dense if it has no holes, i.e., contains an element at every position up to the length. It is absolutely legal to have lists with holes. They are created by leaving the entry between the commas empty. Holes at the end of a list are ignored. Lists with holes are sometimes convenient when the list represents a mapping from a finite, but not consecutive, subset of the positive integers.

      gap> IsDenseList( [ 1, 2, 3 ] );
      true
      gap> l := [ , 4, 9,, 25,, 49,,,, 121 ];;  IsDenseList( l );
      false
      gap> l[3];
      9
      gap> l[4];
      List Element: <list>[4] must have an assigned value
      not in any function
      Entering break read-eval-print loop ...
      you can 'quit;' to quit to outer loop, or
      you can 'return;' after assigning a value to continue
      brk> l[4] := 16;;  # assigning a value
      brk> return;       # to escape the break-loop
      16
      gap>

  Observe that requesting the value of `l[4]`, which was not assigned, caused the entry of a `break`-loop (see Section Break Loops). After assigning a value and typing `return;`, GAP is finally able to comply with our request (by responding with `16`).

- `IsHomogeneousList(` obj`) C`

  returns `true` if obj is a list and it is homogeneous, or `false` otherwise.

  A homogeneous list is a dense list whose elements lie in the same family (see Families). The empty list is homogeneous but not a collection (see Collections), a nonempty homogeneous list is also a collection.

      gap> IsHomogeneousList( [ 1, 2, 3 ] );  IsHomogeneousList( [] );
      true
      true
      gap> IsHomogeneousList( [ 1, false, () ] );
      false

- `IsTable(` obj`) C`

  A table is a nonempty list of homogeneous lists which lie in the same family. Typical examples of tables are matrices (see Matrices).

      gap> IsTable( [ [ 1, 2 ], [ 3, 4 ] ] );        # in fact a matrix
      true
      gap> IsTable( [ [ 1 ], [ 2, 3 ] ] );           # not rectangular but a table
      true
      gap> IsTable( [ [ 1, 2 ], [ () , (1,2) ] ] );  # not homogeneous
      false

- `IsConstantTimeAccessList(` list`) C`

  This category indicates whether the access to each element of the list list will take roughly the same time. This is implied for example by `IsList and IsInternalRep`, so all strings, Boolean lists, ranges, and internally represented plain lists are in this category.

  But also other enumerators (see Enumerators) can lie in this category if they guarantee constant time access to their elements.

  ## 21.2 Basic Operations for Lists

  The basic operations for lists are element access (see List Elements), assignment of elements to a list (see List Assignment), fetching the length of a list (see Length), the test for a hole at a given position, and unbinding an element at a given position (see IsBound and Unbind for Lists).

  The term basic operation means that each other list operation can be formulated in terms of the basic operations. (But note that usually a more efficient method than this one is implemented.)

  Any GAP object list in the category `IsList` (see IsList) is regarded as a list, and if methods for the basic list operations are installed for list then list can be used also for the other list operations.

  For internally represented lists, kernel methods are provided for the basic list operations. For other lists, it is possible to install appropriate methods for these operations. This permits the implementation of lists that do not need to store all list elements (see also Enumerators); for example, the elements might be described by an algorithm, such as the elements list of a group. For this reduction of space requirements, however, a price in access time may have to be paid (see ConstantTimeAccessList).

- `\[\](` list`,` pos`) O`
- `IsBound\[\](` list`,` pos`) O`
- `\[\]\:\=(` list`,` pos`,` val`) O`
- `Unbind\[\](` list`,` pos`) O`

  These operations implement element access, test for element boundedness, list element assignment, and removal of the element at position pos. In all cases, the index pos must be a positive integer.

  Note that the special characters `[`, `]`, `:`, and `=` must be escaped with a backslash `\` (see Symbols); so `\[\]` denotes the operation for element access in a list, whereas `[]` denotes an empty list. (Maybe the variable names involving special characters look strange, but nevertheless they are quite suggestive.)

  `\[\](` list`,` pos`)` is equivalent to ``list`[` pos`]`, which clearly will usually be preferred; the former is useful mainly if one wants to access the operation itself, for example if one wants to install a method for element access in a special kind of lists.

  Similarly, `IsBound\[\]` is used explicitly mainly in method installations. In other situations, one can simply call `IsBound`, which then delegates to `IsBound\[\]` if the first argument is a list, and to `IsBound\.` if the first argument is a record.

  Analogous statements hold for `\[\]\:\=` and `Unbind\[\]`.

  ## 21.3 List Elements

- ``list`[` pos`]`

  The above construct evaluates to the pos-th element of the list list. pos must be a positive integer. List indexing is done with origin 1, i.e., the first element of the list is the element at position 1.

      gap> l := [ 2, 3, 5, 7, 11, 13 ];;  l[1];  l[2];  l[6];
      2
      3
      13

  If list is not a list, or pos does not evaluate to a positive integer, or ``list`[`pos`]` is unbound an error is signalled.

- ``list`{` poss`}`

  The above construct evaluates to a new list new whose first element is ``list`[`poss`[1]]`, whose second element is ``list`[`poss`[2]]`, and so on. poss must be a dense list of positive integers. However, it does not need to be sorted and may contain duplicate elements. If for any i, ``list`[` poss`[`i`] ]` is unbound, an error is signalled.

      gap> l := [ 2, 3, 5, 7, 11, 13, 17, 19 ];;
      gap> l{[4..6]};  l{[1,7,1,8]};
      [ 7, 11, 13 ]
      [ 2, 17, 2, 19 ]

  The result is a new list, that is not identical to any other list. The elements of that list, however, are identical to the corresponding elements of the left operand (see Identical Lists).

  It is possible to nest such sublist extractions, as can be seen in the following example.

      gap> m := [ [1,2,3], [4,5,6], [7,8,9], [10,11,12] ];;  m{[1,2,3]}{[3,2]};
      [ [ 3, 2 ], [ 6, 5 ], [ 9, 8 ] ]
      gap> l := m{[1,2,3]};; l{[3,2]};
      [ [ 7, 8, 9 ], [ 4, 5, 6 ] ]

  Note the difference between the two examples. The latter extracts elements 1, 2, and 3 from m and then extracts the elements 3 and 2 from this list. The former extracts elements 1, 2, and 3 from m and then extracts the elements 3 and 2 from each of those element lists.

  To be precise: With each selector `[`pos`]` or `{`poss`}` we associate a level that is defined as the number of selectors of the form `{`poss`}` to its left in the same expression. For example

          l[pos1]{poss2}{poss3}[pos4]{poss5}[pos6]
      level   0      0      1     1      1     2

  Then a selector ``list`[`pos`]` of level level is computed as `ListElement(`list`,`pos`,`level`)`, where `ListElement` is defined as follows. (Note that `ListElement` is not a GAP function.)

      ListElement := function ( list, pos, level )
       if level = 0 then
        return list[pos];
       else
        return List( list, elm -> ListElement(elm,pos,level-1) );
       fi;
      end;

  and a selector ``list`{`poss`}` of level level is computed as `ListElements(`list`,`poss`,`level`)`, where `ListElements` is defined as follows. (Note that `ListElements` is not a GAP function.)

      ListElements := function ( list, poss, level )
       if level = 0 then
        return list{poss};
        else
         return List( list, elm -> ListElements(elm,poss,level-1) );
        fi;
      end;

- `\{\}(` list`,` poss`) O`

  This operation implements sublist access. For any list, the default method is to loop over the entries in the list poss, and to delegate to the element access operation. (For the somewhat strange variable name, cf. Basic Operations for Lists.)

  ## 21.4 List Assignment

- ``list`[` pos`] :=` object`;`

  The list element assignment assigns the object object, which can be of any type, to the list entry at the position pos, which must be a positive integer, in the mutable (see Mutability and Copyability) list list. That means that accessing the pos-th element of the list list will return object after this assignment.

      gap> l := [ 1, 2, 3 ];;
      gap> l[1] := 3;; l;             # assign a new object
      [ 3, 2, 3 ]
      gap> l[2] := [ 4, 5, 6 ];; l;   # <object> may be of any type
      [ 3, [ 4, 5, 6 ], 3 ]
      gap> l[ l[1] ] := 10;; l;       # <index> may be an expression
      [ 3, [ 4, 5, 6 ], 10 ]

  If the index pos is larger than the length of the list list (see Length), the list is automatically enlarged to make room for the new element. Note that it is possible to generate lists with holes that way.

      gap> l[4] := "another entry";; l;  # <list> is enlarged
      [ 3, [ 4, 5, 6 ], 10, "another entry" ]
      gap> l[ 10 ] := 1;; l;             # now <list> has a hole
      [ 3, [ 4, 5, 6 ], 10, "another entry",,,,,, 1 ]

  The function `Add` (see Add) should be used if you want to add an element to the end of the list.

  Note that assigning to a list changes the list, thus this list must be mutable (see Mutability and Copyability). See Identical Lists for subtleties of changing lists.

  If list does not evaluate to a list, pos does not evaluate to a positive integer or object is a call to a function which does not return a value (for example `Print`) an error is signalled.

- ``list`{` poss`} :=` objects`;`

  The sublist assignment assigns the object ``objects`[1]`, which can be of any type, to the list list at the position ``poss`[1]`, the object ``objects`[2]` to ``list`[`poss`[2]]`, and so on. poss must be a dense list of positive integers, it need, however, not be sorted and may contain duplicate elements. objects must be a dense list and must have the same length as poss.

      gap> l := [ 2, 3, 5, 7, 11, 13, 17, 19 ];;
      gap> l{[1..4]} := [10..13];; l;
      [ 10, 11, 12, 13, 11, 13, 17, 19 ]
      gap> l{[1,7,1,10]} := [ 1, 2, 3, 4 ];; l;
      [ 3, 11, 12, 13, 11, 13, 2, 19,, 4 ]

  It is possible to nest such sublist assignments, as can be seen in the following example.

      gap> m := [ [1,2,3], [4,5,6], [7,8,9], [10,11,12] ];;
      gap> m{[1,2,3]}{[3,2]} := [ [11,12], [13,14], [15,16] ];; m;
      [ [ 1, 12, 11 ], [ 4, 14, 13 ], [ 7, 16, 15 ], [ 10, 11, 12 ] ]

  The exact behaviour is defined in the same way as for list extractions (see List Elements). Namely with each selector `[`pos`]` or `{`poss`}` we associate a level that is defined as the number of selectors of the form `{`poss`}` to its left in the same expression. For example

          l[pos1]{poss2}{poss3}[pos4]{poss5}[pos6]
      level   0      0      1     1      1     2

  Then a list assignment ``list`[`pos`] :=` vals`;` of level level is computed as `ListAssignment(` list`,` pos`,` vals`,` level`)`, where `ListAssignment` is defined as follows. (Note that `ListAssignment` is not a GAP function.)

      ListAssignment := function ( list, pos, vals, level )
       local i;
       if level = 0 then
        list[pos] := vals;
       else
        for i in [1..Length(list)] do
         ListAssignment( list[i], pos, vals[i], level-1 );
        od;
       fi;
      end;

  and a list assignment ``list`{`poss`} :=` vals`` of level level is computed as `ListAssignments(` list`,` poss`,` vals`,` level`)`, where `ListAssignments` is defined as follows. (Note that `ListAssignments` is not a GAP function.)

      ListAssignments := function ( list, poss, vals, level )
       local i;
       if level = 0 then
        list{poss} := vals;
       else
        for i in [1..Length(list)] do
         ListAssignments( list[i], poss, vals[i], level-1 );
        od;
       fi;
      end;

- `\{\}\:\=(` list`,` poss`,` val`) O`

  This operation implements sublist assignment. For any list, the default method is to loop over the entries in the list poss, and to delegate to the element assignment operation. (For the somewhat strange variable name, cf. Basic Operations for Lists.)

- `Add(` list`,` obj`) O`

  adds the element obj to the end of the mutable list list, i.e., it is equivalent to the assignment ``list`[ Length(`list`) + 1 ] :=` obj``, see list element!assignment. Nothing is returned by `Add`, the function is only called for its side effect.

      gap> l := [ 2, 3, 5 ];; Add( l, 7 ); l;
      [ 2, 3, 5, 7 ]

- `Append(` list1`,` list2`) O`

  adds the elements of the list list2 to the end of the mutable list list1, see sublist!assignment. list2 may contain holes, in which case the corresponding entries in list1 will be left unbound. `Append` returns nothing, it is only called for its side effect.

  Note that `Append` changes its first argument, while `Concatenation` (see Concatenation) creates a new list and leaves its arguments unchanged.

      gap> l := [ 2, 3, 5 ];; Append( l, [ 7, 11, 13 ] ); l;
      [ 2, 3, 5, 7, 11, 13 ]
      gap> Append( l, [ 17,, 23 ] ); l;
      [ 2, 3, 5, 7, 11, 13, 17,, 23 ]

  ## 21.5 IsBound and Unbind for Lists

- `IsBound(` list`[`n`] ) M`

  `IsBound` returns `true` if the list list has a element at the position n, and `false` otherwise. list must evaluate to a list, otherwise an error is signalled.

      gap> l := [ , 2, 3, , 5, , 7, , , , 11 ];;
      gap> IsBound( l[7] );
      true
      gap> IsBound( l[4] );
      false
      gap> IsBound( l[101] );
      false

- `Unbind(` list`[`n`] ) M`

  `Unbind` deletes the element at the position n in the mutable list list. That is, after execution of `Unbind`, list no longer has an assigned value at the position n. Thus `Unbind` can be used to produce holes in a list. Note that it is not an error to unbind a nonexisting list element. list must evaluate to a list, otherwise an error is signalled.

      gap> l := [ , 2, 3, 5, , 7, , , , 11 ];;
      gap> Unbind( l[3] ); l;
      [ , 2,, 5,, 7,,,, 11 ]
      gap> Unbind( l[4] ); l;
      [ , 2,,,, 7,,,, 11 ]

  Note that `IsBound` and `Unbind` are special in that they do not evaluate their argument, otherwise `IsBound` would always signal an error when it is supposed to return `false` and there would be no way to tell `Unbind` which component to remove.

  ## 21.6 Identical Lists

  With the list assignment (see List Assignment) it is possible to change a mutable list. This section describes the semantic consequences of this fact. (See also Identical Objects.)

  First we define what it means when we say that ``an object is changed''. You may think that in the following example the second assignment changes the integer.

      i := 3;
      i := i + 1;

  But in this example it is not the integer `3` which is changed, by adding one to it. Instead the variable `i` is changed by assigning the value of `i+1`, which happens to be `4`, to `i`. The same thing happens in the following example

      l := [ 1, 2 ];
      l := [ 1, 2, 3 ];

  The second assignment does not change the first list, instead it assigns a new list to the variable `l`. On the other hand, in the following example the list is changed by the second assignment.

      l := [ 1, 2 ];
      l[3] := 3;

  To understand the difference, think of a variable as a name for an object. The important point is that a list can have several names at the same time. An assignment ``var`:=`list`;` means in this interpretation that var is a name for the object list. At the end of the following example `l2` still has the value `[ 1, 2 ]` as this list has not been changed and nothing else has been assigned to it.

      l1 := [ 1, 2 ];
      l2 := l1;
      l1 := [ 1, 2, 3 ];

  But after the following example the list for which `l2` is a name has been changed and thus the value of `l2` is now `[ 1, 2, 3 ]`.

      l1 := [ 1, 2 ];
      l2 := l1;
      l1[3] := 3;

  We say that two lists are identical if changing one of them by a list assignment also changes the other one. This is slightly incorrect, because if two lists are identical, there are actually only two names for one list. However, the correct usage would be very awkward and would only add to the confusion. Note that two identical lists must be equal, because there is only one list with two different names. Thus identity is an equivalence relation that is a refinement of equality. Identity of objects can be detected using `IsIdenticalObj`, see Identical Objects.

  Let us now consider under which circumstances two lists are identical.

  If you enter a list literal then the list denoted by this literal is a new list that is not identical to any other list. Thus in the following example `l1` and `l2` are not identical, though they are equal of course.

      l1 := [ 1, 2 ];
      l2 := [ 1, 2 ];

  Also in the following example, no lists in the list `l` are identical.

      l := [];
      for i in [1..10] do l[i] := [ 1, 2 ]; od;

  If you assign a list to a variable no new list is created. Thus the list value of the variable on the left hand side and the list on the right hand side of the assignment are identical. So in the following example `l1` and `l2` are identical lists.

      l1 := [ 1, 2 ];
      l2 := l1;

  If you pass a list as an argument, the old list and the argument of the function are identical. Also if you return a list from a function, the old list and the value of the function call are identical. So in the following example `l1` and `l2` are identical lists:

      l1 := [ 1, 2 ];
      f := function ( l ) return l; end;
      l2 := f( l1 );

  If you change a list it keeps its identity. Thus if two lists are identical and you change one of them, you also change the other, and they are still identical afterwards. On the other hand, two lists that are not identical will never become identical if you change one of them. So in the following example both `l1` and `l2` are changed, and are still identical.

      l1 := [ 1, 2 ];
      l2 := l1;
      l1[1] := 2;

  ## 21.7 Duplication of Lists

  Here we describe the meaning of `ShallowCopy` and `StructuralCopy` for lists. For the general definition of these functions, see Duplication of Objects.

  The subobjects (see ShallowCopy) of a list are exactly its elements.

  This means that for any list list, `ShallowCopy` returns a mutable new list new that is not identical to any other list (see Identical Lists), and whose elements are identical to the elements of list.

  Analogously, for a mutable list list, `StructuralCopy` returns a mutable new list scp that is not identical to any other list, and whose elements are structural copies (defined recursively) of the elements of list; an element of scp is mutable (and then a new list) if and only if the corresponding element of list is mutable.

  In both cases, modifying the copy new resp. scp by assignments (see List Assignment) does not modify the original object list.

  `ShallowCopy` basically executes the following code for lists.

      new := [];
      for i in [ 1 .. Length( list ) ] do
        if IsBound( list[i] ) then
          new[i] := list[i];
        fi;
      od;

      gap> list1 := [ [ 1, 2 ], [ 3, 4 ] ];;  list2 := ShallowCopy( list1 );;
      gap> IsIdenticalObj( list1, list2 );
      false
      gap> IsIdenticalObj( list1[1], list2[1] );
      true
      gap> list2[1] := 0;;  list1;  list2;
      [ [ 1, 2 ], [ 3, 4 ] ]
      [ 0, [ 3, 4 ] ]

  `StructuralCopy` basically executes the following code for lists.

      new := [];
      for i in [ 1 .. Length( list ) ] do
        if IsBound( list[i] ) then
          new[i] := StructuralCopy( list[i] );
        fi;
      od;

      gap> list1 := [ [ 1, 2 ], [ 3, 4 ] ];;  list2 := StructuralCopy( list1 );;
      gap> IsIdenticalObj( list1, list2 );
      false
      gap> IsIdenticalObj( list1[1], list2[1] );
      false
      gap> list2[1][1] := 0;;  list1;  list2;
      [ [ 1, 2 ], [ 3, 4 ] ]
      [ [ 0, 2 ], [ 3, 4 ] ]

  The above code is not entirely correct. If the object list contains a mutable object twice this object is not copied twice, as would happen with the above definition, but only once. This means that the copy new and the object list have exactly the same structure when viewed as a general graph.

      gap> sub := [ 1, 2 ];; list1 := [ sub, sub ];;
      gap> list2 := StructuralCopy( list1 );
      [ [ 1, 2 ], [ 1, 2 ] ]
      gap> list2[1][1] := 0;; list2;
      [ [ 0, 2 ], [ 0, 2 ] ]
      gap> list1;
      [ [ 1, 2 ], [ 1, 2 ] ]

  ## 21.8 Membership Test for Lists

- ``obj `in` list``

  tests whether there is a positive integer index such that ``list`[` index`] =` obj``.

  If the list list knows that it is strictly sorted (see IsSSortedList), the membership test is much quicker, because a binary search can be used instead of the linear search used for arbitrary lists.

      gap> 1 in [ 2, 2, 1, 3 ];  1 in [ 4, -1, 0, 3 ];
      true
      false
      gap> s := SSortedList( [2,4,6,8,10,12,14,16,18,20,22,24,26,28,30,32] );;
      gap> 17 in s;  # uses binary search and only 4 comparisons
      false

  For finding the position of an element in a list, see Finding Positions in Lists.

  ## 21.9 Enlarging Internally Represented Lists

  Section List Assignment told you (among other things) that it is possible to assign beyond the logical end of a mutable list, automatically enlarging the list. This section tells you how this is done for internally represented lists.

  It would be extremely wasteful to make all lists large enough so that there is room for all assignments, because some lists may have more than 100000 elements, while most lists have less than 10 elements.

  On the other hand suppose every assignment beyond the end of a list would be done by allocating new space for the list and copying all entries to the new space. Then creating a list of 1000 elements by assigning them in order, would take half a million copy operations and also create a lot of garbage that the garbage collector would have to reclaim.

  So the following strategy is used. If a list is created it is created with exactly the correct size. If a list is enlarged, because of an assignment beyond the end of the list, it is enlarged by at least ``length`/8 + 4` entries. Therefore the next assignments beyond the end of the list do not need to enlarge the list. For example creating a list of 1000 elements by assigning them in order, would now take only 32 enlargements.

  The result of this is of course that the physical length of a list may be larger than the logical length, which is usually called simply the length of the list. Aside from the implications for the performance you need not be aware of the physical length. In fact all you can ever observe, for example by calling `Length` (see Length), is the logical length.

  Suppose that `Length` would have to take the physical length and then test how many entries at the end of a list are unassigned, to compute the logical length of the list. That would take too much time. In order to make `Length`, and other functions that need to know the logical length, more efficient, the length of a list is stored along with the list.

  ## 21.10 Comparisons of Lists

- ``list1`=` list2``
  `` list1`<>` list2``

  Two lists list1 and list2 are equal if and only if for every index i, either both entries ``list1`[`i`]` and ``list2`[`i`]` are unbound, or both are bound and are equal, i.e., ``list1`[`i`] =` list2`[`i`]` is `true`.

      gap> [ 1, 2, 3 ] = [ 1, 2, 3 ];
      true
      gap> [ , 2, 3 ] = [ 1, 2, ];
      false
      gap> [ 1, 2, 3 ] = [ 3, 2, 1 ];
      false

  This definition will cause problems with lists which are their own entries. Comparing two such lists for equality may lead to an infinite recursion in the kernel if the list comparison has to compare the list entries which are in fact the lists themselves, and then GAP crashes.

- ``list1`<` list2``
  `` list1`<=` list2``

  Lists are ordered lexicographically. Unbound entries are smaller than any bound entry. That implies the following behaviour. Let i be the smallest positive integer i such that list1 and list2 at position i differ, i.e., either exactly one of ``list1`[i]`, ``list2`[i]` is bound or both entries are bound and differ. Then list1 is less than list2 if either ``list1`[`i`]` is unbound (and ``list2`[`i`]` is not) or both are bound and ``list1`[`i`] <` list2`[`i`]` is `true`.

      gap> [ 1, 2, 3, 4 ] < [ 1, 2, 4, 8 ]; # <list1>[3] < <list2>[3]
      true
      gap> [ 1, 2, 3 ] < [ 1, 2, 3, 4 ];    # <list1>[4] is unbound and therefore very small
      true
      gap> [ 1, , 3, 4 ] < [ 1, 2, 3 ];     # <list1>[2] is unbound and therefore very small
      true

  Note that for comparing two lists with `<` or `<=`, the (relevant) list elements must be comparable with `<`, which is usually not the case for objects in different families, see Families. Also for the possibility to compare lists with other objects, see Families.

  ## 21.11 Arithmetic for Lists

  It is convenient to have arithmetic operations for lists, in particular because in GAP row vectors and matrices are special kinds of lists. However, it is the wide variety of list objects because of which we prescribe arithmetic operations not for all of them. (Keep in mind that ``list'' means just an object in the category `IsList`, see IsList.)

  (Due to the intended generality and flexibility, the definitions given in the following sections are quite technical. But for not too complicated cases such as matrices (see Operators for Matrices) and row vectors (see Operators for Row Vectors) whose entries aren't lists, the resulting behaviour should be intuitive.)

  For example, we want to deal with matrices which can be added and multiplied in the usual way, via the infix operators `+` and `*`; and we want also Lie matrices, with the same additive behaviour but with the multiplication defined by the Lie bracket. Both kinds of matrices shall be lists, with the usual access to their rows, with `Length` (see Length) returning the number of rows etc.

  For the categories and attributes that control the arithmetic behaviour of lists, see Filters Controlling the Arithmetic Behaviour of Lists.

  For the definition of return values of additive and multiplicative operations whose arguments are lists in these filters, see Additive Arithmetic for Lists and Multiplicative Arithmetic for Lists, respectively. It should be emphasized that these sections describe only what the return values are, and not how they are computed.

  For the mutability status of the return values, see Mutability Status and List Arithmetic. (Note that this is not dealt with in the sections about the result values.)

  Further details about the special cases of row vectors and matrices can be found in Operators for Row Vectors and in Operators for Matrices, the compression status is dealt with in Row Vectors over Finite Fields and Matrices over Finite Fields.

  ## 21.12 Filters Controlling the Arithmetic Behaviour of Lists

  The arithmetic behaviour of lists is controlled by their types. The following categories and attributes are used for that.

  Note that we distinguish additive and multiplicative behaviour. For example, Lie matrices have the usual additive behaviour but not the usual multiplicative behaviour.

- `IsGeneralizedRowVector(` list`) C`

  For a list list, the value `true` for `IsGeneralizedRowVector` indicates that the additive arithmetic behaviour of list is as defined in Additive Arithmetic for Lists, and that the attribute `NestingDepthA` (see NestingDepthA) will return a nonzero value when called with list.

- `IsMultiplicativeGeneralizedRowVector(` list`) C`

  For a list list, the value `true` for `IsMultiplicativeGeneralizedRowVector` indicates that the multiplicative arithmetic behaviour of list is as defined in Multiplicative Arithmetic for Lists, and that the attribute `NestingDepthM` (see NestingDepthM) will return a nonzero value when called with list.

  Note that these filters do not enable default methods for addition or multiplication (cf. IsListDefault).

      gap> IsList( "abc" ); IsGeneralizedRowVector( "abc" );
      true
      false
      gap> liemat:= LieObject( [ [ 1, 2 ], [ 3, 4 ] ] );
      LieObject( [ [ 1, 2 ], [ 3, 4 ] ] )
      gap> IsGeneralizedRowVector( liemat );
      true
      gap> IsMultiplicativeGeneralizedRowVector( liemat );
      false
      gap> bas:= CanonicalBasis( FullRowSpace( Rationals, 3 ) );
      CanonicalBasis( ( Rationals^3 ) )
      gap> IsMultiplicativeGeneralizedRowVector( bas );
      true

- `IsListDefault(` list`) C`

  For a list list, `IsListDefault` indicates that the default methods for arithmetic operations of lists, such as pointwise addition and multiplication as inner product or matrix product, shall be applicable to list.

  `IsListDefault` implies `IsGeneralizedRowVector` and `IsMultiplicativeGeneralizedRowVector`.

  All internally represented lists are in this category, and also all lists in the representations `IsGF2VectorRep`, `Is8BitVectorRep`, `IsGF2MatrixRep`, and `Is8BitMatrixRep` (see Row Vectors over Finite Fields and Matrices over Finite Fields). Note that the result of an arithmetic operation with lists in `IsListDefault` will in general be an internally represented list, so most ``wrapped list objects'' will not lie in `IsListDefault`.

      gap> v:= [ 1, 2 ];;  m:= [ v, 2*v ];;
      gap> IsListDefault( v );  IsListDefault( m );
      true
      true
      gap> IsListDefault( bas );  IsListDefault( liemat );
      true
      false

- `NestingDepthA(` obj`) A`

  For a GAP object obj, `NestingDepthA` returns the additive nesting depth of obj. This is defined recursively as the integer 0 if obj is not in `IsGeneralizedRowVector`, as the integer 1 if obj is an empty list in `IsGeneralizedRowVector`, and as 1 plus the additive nesting depth of the first bound entry in obj otherwise.

- `NestingDepthM(` obj`) A`

  For a GAP object obj, `NestingDepthM` returns the multiplicative nesting depth of obj. This is defined recursively as the integer 0 if obj is not in `IsMultiplicativeGeneralizedRowVector`, as the integer 1 if obj is an empty list in `IsMultiplicativeGeneralizedRowVector`, and as 1 plus the multiplicative nesting depth of the first bound entry in obj otherwise.

      gap> NestingDepthA( v );  NestingDepthM( v );
      1
      1
      gap> NestingDepthA( m );  NestingDepthM( m );
      2
      2
      gap> NestingDepthA( liemat );  NestingDepthM( liemat );
      2
      0
      gap> l1:= [ [ 1, 2 ], 3 ];;  l2:= [ 1, [ 2, 3 ] ];;
      gap> NestingDepthA( l1 );  NestingDepthM( l1 );
      2
      2
      gap> NestingDepthA( l2 );  NestingDepthM( l2 );
      1
      1

  ## 21.13 Additive Arithmetic for Lists

  In this general context, we define the results of additive operations only in the following situations. For unary operations (zero and additive inverse), the unique argument must be in `IsGeneralizedRowVector`; for binary operations (addition and subtraction), at least one argument must be in `IsGeneralizedRowVector`, and the other either is not a list or also in `IsGeneralizedRowVector`.

  (For non-list GAP objects, defining the results of unary operations is not an issue here, and if at least one argument is a list not in `IsGeneralizedRowVector`, it shall be left to this argument whether the result in question is defined and what it is.)

  Zero

  The zero (see Zero) of a list x in `IsGeneralizedRowVector` is defined as the list whose entry at position i is the zero of x[i] if this entry is bound, and is unbound otherwise.

      gap> Zero( [ 1, 2, 3 ] );  Zero( [ [ 1, 2 ], 3 ] );  Zero( liemat );
      [ 0, 0, 0 ]
      [ [ 0, 0 ], 0 ]
      LieObject( [ [ 0, 0 ], [ 0, 0 ] ] )

  AdditiveInverse

  The additive inverse (see AdditiveInverse) of a list x in `IsGeneralizedRowVector` is defined as the list whose entry at position i is the additive inverse of x[i] if this entry is bound, and is unbound otherwise.

      gap> AdditiveInverse( [ 1, 2, 3 ] );  AdditiveInverse( [ [ 1, 2 ], 3 ] );
      [ -1, -2, -3 ]
      [ [ -1, -2 ], -3 ]

  Addition

  If x and y are in `IsGeneralizedRowVector` and have the same additive nesting depth (see NestingDepthA), the sum x + y is defined pointwise, in the sense that the result is a list whose entry at position i is x[i] + y[i] if these entries are bound, is a shallow copy (see ShallowCopy) of x[i] or y[i] if the other argument is not bound at position i, and is unbound if both x and y are unbound at position i.

  If x is in `IsGeneralizedRowVector` and y is in `IsGeneralizedRowVector` and has lower additive nesting depth, or is neither a list nor a domain, the sum x + y is defined as a list whose entry at position i is x[i] + y if x is bound at position i, and is unbound if not. The equivalent holds in the reversed case, where the order of the summands is kept, as addition is not always commutative.

      gap> 1 + [ 1, 2, 3 ];  [ 1, 2, 3 ] + [ 0, 2, 4 ];  [ 1, 2 ] + [ Z(2) ];
      [ 2, 3, 4 ]
      [ 1, 4, 7 ]
      [ 0*Z(2), 2 ]
      gap> l1:= [ 1, , 3, 4 ];;             l2:= [ , 2, 3, 4, 5 ];;
      gap> l3:= [ [ 1, 2 ], , [ 5, 6 ] ];;  l4:= [ , [ 3, 4 ], [ 5, 6 ] ];;
      gap> NestingDepthA( l1 );  NestingDepthA( l2 );
      1
      1
      gap> NestingDepthA( l3 );  NestingDepthA( l4 );
      2
      2
      gap> l1 + l2;
      [ 1, 2, 6, 8, 5 ]
      gap> l1 + l3;
      [ [ 2, 2, 3, 4 ],, [ 6, 6, 3, 4 ] ]
      gap> l2 + l4;
      [ , [ 3, 6, 3, 4, 5 ], [ 5, 8, 3, 4, 5 ] ]
      gap> l3 + l4;
      [ [ 1, 2 ], [ 3, 4 ], [ 10, 12 ] ]
      gap> l1 + [];
      [ 1,, 3, 4 ]

  Subtraction

  For two GAP objects x and y of which one is in `IsGeneralizedRowVector` and the other is also in `IsGeneralizedRowVector` or is neither a list nor a domain, x − y is defined as x + (−y).

      gap> l1 - l2;
      [ 1, -2, 0, 0, -5 ]
      gap> l1 - l3;
      [ [ 0, -2, 3, 4 ],, [ -4, -6, 3, 4 ] ]
      gap> l2 - l4;
      [ , [ -3, -2, 3, 4, 5 ], [ -5, -4, 3, 4, 5 ] ]
      gap> l3 - l4;
      [ [ 1, 2 ], [ -3, -4 ], [ 0, 0 ] ]
      gap> l1 - [];
      [ 1,, 3, 4 ]

  ## 21.14 Multiplicative Arithmetic for Lists

  In this general context, we define the results of multiplicative operations only in the following situations. For unary operations (one and inverse), the unique argument must be in `IsMultiplicativeGeneralizedRowVector`; for binary operations (multiplication and division), at least one argument must be in `IsMultiplicativeGeneralizedRowVector`, and the other either not a list or also in `IsMultiplicativeGeneralizedRowVector`.

  (For non-list GAP objects, defining the results of unary operations is not an issue here, and if at least one argument is a list not in `IsMultiplicativeGeneralizedRowVector`, it shall be left to this argument whether the result in question is defined and what it is.)

  One

  The one (see One) of a dense list x in `IsMultiplicativeGeneralizedRowVector` such that x has even multiplicative nesting depth and has the same length as each of its rows is defined as the usual identity matrix on the outer two levels, that is, an identity matrix of the same dimensions, with diagonal entries `One`( x[1][1] ) and off-diagonal entries `Zero`( x[1][1] ).

      gap> One( [ [ 1, 2 ], [ 3, 4 ] ] );
      [ [ 1, 0 ], [ 0, 1 ] ]
      gap> One( [ [ [ [ 1 ] ], [ [ 2 ] ] ], [ [ [ 3 ] ], [ [ 4 ] ] ] ] );
      [ [ [ [ 1 ] ], [ [ 0 ] ] ], [ [ [ 0 ] ], [ [ 1 ] ] ] ]

  Inverse

  The inverse (see Inverse) of an invertible square table x in `IsMultiplicativeGeneralizedRowVector` whose entries lie in a common field is defined as the usual inverse y, i.e., a square matrix over the same field such that x y and y x is equal to `One`( x ).

      gap> Inverse( [ [ 1, 2 ], [ 3, 4 ] ] );
      [ [ -2, 1 ], [ 3/2, -1/2 ] ]

  Multiplication

  There are three possible computations that might be triggered by a multiplication involving a list in `IsMultiplicativeGeneralizedRowVector`. Namely, x * y might be

  (I)
    the inner product x[1] * y[1] + x[2] * y[2] + …+ x[n] * y[n], where summands are omitted for which the entry in x or y is unbound (if this leaves no summand then the multiplication is an error), or
  (L)
    the left scalar multiple, i.e., a list whose entry at position i is x * y[i] if y is bound at position i, and is unbound if not, or
  (R)
    the right scalar multiple, i.e., a list whose entry at position i is x[i] * y if x is bound at position i, and is unbound if not.

  Our aim is to generalize the basic arithmetic of simple row vectors and matrices, so we first summarize the situations that shall be covered.

          | scl   vec   mat
      ---------------------
      scl |       (L)   (L)
      vec | (R)   (I)   (I)
      mat | (R)   (R)   (R)

  This means for example that the product of a scalar (scl) with a vector (vec) or a matrix (mat) is computed according to (L). Note that this is asymmetric.

  Now we can state the general multiplication rules.

  If exactly one argument is in `IsMultiplicativeGeneralizedRowVector` then we regard the other argument (which is then neither a list nor a domain) as a scalar, and specify result (L) or (R), depending on ordering.

  In the remaining cases, both x and y are in `IsMultiplicativeGeneralizedRowVector`, and we distinguish the possibilities by their multiplicative nesting depths. An argument with odd multiplicative nesting depth is regarded as a vector, and an argument with even multiplicative nesting depth is regarded as a scalar or a matrix.

  So if both arguments have odd multiplicative nesting depth, we specify result (I).

  If exactly one argument has odd nesting depth, the other is treated as a scalar if it has lower multiplicative nesting depth, and as a matrix otherwise. In the former case, we specify result (L) or (R), depending on ordering; in the latter case, we specify result (L) or (I), depending on ordering.

  We are left with the case that each argument has even multiplicative nesting depth. If the two depths are equal, we treat the computation as a matrix product, and specify result (R). Otherwise, we treat the less deeply nested argument as a scalar and the other as a matrix, and specify result (L) or (R), depending on ordering.

      gap> [ (), (2,3), (1,2), (1,2,3), (1,3,2), (1,3) ] * (1,4);
      [ (1,4), (1,4)(2,3), (1,2,4), (1,2,3,4), (1,3,2,4), (1,3,4) ]
      gap> [ 1, 2, , 4 ] * 2;
      [ 2, 4,, 8 ]
      gap> [ 1, 2, 3 ] * [ 1, 3, 5, 7 ];
      22
      gap> m:= [ [ 1, 2 ], 3 ];;  m * m;
      [ [ 7, 8 ], [ [ 3, 6 ], 9 ] ]
      gap> m * m = [ m[1] * m, m[2] * m ];
      true
      gap> n:= [ 1, [ 2, 3 ] ];;  n * n;
      14
      gap> n * n = n[1] * n[1] + n[2] * n[2];
      true

  Division

  For two GAP objects x and y of which one is in `IsMultiplicativeGeneralizedRowVector` and the other is also in `IsMultiplicativeGeneralizedRowVector` or is neither a list nor a domain, x / y is defined as x * y−1.

      gap> [ 1, 2, 3 ] / 2;  [ 1, 2 ] / [ [ 1, 2 ], [ 3, 4 ] ];
      [ 1/2, 1, 3/2 ]
      [ 1, 0 ]

  mod

  If x and y are in `IsMultiplicativeGeneralizedRowVector` and have the same multiplicative nesting depth (see NestingDepthM), x `mod` y is defined pointwise, in the sense that the result is a list whose entry at position i is x[i] `mod` y[i] if these entries are bound, is a shallow copy (see ShallowCopy) of x[i] or y[i] if the other argument is not bound at position i, and is unbound if both x and y are unbound at position i.

  If x is in `IsMultiplicativeGeneralizedRowVector` and y is in `IsMultiplicativeGeneralizedRowVector` and has lower multiplicative nesting depth or is neither a list nor a domain, x `mod` y is defined as a list whose entry at position i is x[i] `mod` y if x is bound at position i, and is unbound if not. The equivalent holds in the reversed case, where the order of the arguments is kept.

      gap> 4711 mod [ 2, 3,, 5, 7 ];
      [ 1, 1,, 1, 0 ]
      gap> [ 2, 3, 4, 5, 6 ] mod 3;
      [ 2, 0, 1, 2, 0 ]
      gap> [ 10, 12, 14, 16 ] mod [ 3, 5, 7 ];
      [ 1, 2, 0, 16 ]

  Left Quotient

  For two GAP objects x and y of which one is in `IsMultiplicativeGeneralizedRowVector` and the other is also in `IsMultiplicativeGeneralizedRowVector` or is neither a list nor a domain, `LeftQuotient`( x, y ) is defined as x−1 * y.

      gap> LeftQuotient( [ [ 1, 2 ], [ 3, 4 ] ], [ 1, 2 ] );
      [ 0, 1/2 ]

  ## 21.15 Mutability Status and List Arithmetic

  Many results of arithmetic operations, when applied to lists, are again lists, and it is of interest whether their entries are mutable or not (if applicable). Note that the mutability status of the result itself is already defined by the general rule for any result of an arithmetic operation, not only for lists (see Mutability and Copyability).

  However, we do not define exactly the mutability status for each element on each level of a nested list returned by an arithmetic operation. (Of course it would be possible to define this recursively, but since the methods used are in general not recursive, in particular for efficient multiplication of compressed matrices, such a general definition would be a burden in these cases.) Instead we consider, for a list x in `IsGeneralizedRowVector`, the sequence x = x1, x2, …xn where xi+1 is the first bound entry in xi if exists (that is, if xi is a nonempty list), and n is the largest i such that xi lies in `IsGeneralizedRowVector`. The immutability level of x is defined as infinity if x is immutable, and otherwise the number of xi which are immutable. (So the immutability level of a mutable empty list is 0.)

  Thus a fully mutable matrix has immutability level 0, and a mutable matrix with immutable first row has immutability level 1 (independent of the mutability of other rows).

  The immutability level of the result of any of the binary operations discussed here is the minimum of the immutability levels of the arguments, provided that objects of the required mutability status exist in GAP.

  Moreover, the results have a ``homogeneous'' mutability status, that is, if the first bound entry at nesting depth i is immutable (mutable) then all entries at nesting depth i are immutable (mutable, provided that a mutable version of this entry exists in GAP).

  Thus the sum of two mutable matrices whose first rows are mutable is a matrix all of whose rows are mutable, and the product of two matrices whose first rows are immutable is a matrix all of whose rows are immutable, independent of the mutability status of the other rows of the arguments.

  For example, the sum of a matrix (mutable or immutable, i.e., of immutability level one of 0, 1, or 2) and a mutable row vector (i.e., immutability level 0) is a fully mutable matrix. The product of two mutable row vectors of integers is an integer, and since GAP does not support mutable integers, the result is immutable.

  For unary arithmetic operations, there are three operations available, an attribute that returns an immutable result (`Zero`, `AdditiveInverse`, `One`, `Inverse`), an operation that returns a result that is mutable (`ZeroOp`, `AdditiveInverseOp`, `OneOp`, `InverseOp`), and an operation whose result has the same immutability level as the argument (`ZeroSM`, `AdditiveInverseSM`, `OneSM`, `InverseSM`). The last kind of operations is equivalent to the corresponding infix operations `0 *` list``, `-` list``, ``list`^0`, and ``list`^-1`. (This holds not only for lists, see Mutability and Copyability.)

      gap> IsMutable( l1 );  IsMutable( 2 * Immutable( [ 1, 2, 3 ] ) );
      true
      false
      gap> IsMutable( l2 );  IsMutable( l3 );
      true
      true

  An example motivating the mutability rule is the use of syntactic constructs such as ``obj`*` list`` and `-` list`` as an elegant and efficient way to create mutable lists needed for further manipulations from mutable lists. In particular one can construct a mutable zero vector of length n by `0 * [ 1 ..` n`]`. The latter can be done also using `ListWithIdenticalEntries`.

- `ListWithIdenticalEntries(` n`,` obj`) F`

  is a list list of length n that has the object obj stored at each of the positions from 1 to n. Note that all elements of lists are identical, see Identical Lists.

      gap> ListWithIdenticalEntries( 10, 0 );
      [ 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 ]

  ## 21.16 Finding Positions in Lists

- `Position(` list`,` obj`[,` from`] ) O`

  returns the position of the first occurrence obj in list, or fail if obj is not contained in list. If a starting index from is given, it returns the position of the first occurrence starting the search after position from.

  Each call to the two argument version is translated into a call of the three argument version, with third argument the integer zero `0`. (Methods for the two argument version must be installed as methods for the version with three arguments, the third being described by `IsZeroCyc`.)

      gap> Position( [ 2, 2, 1, 3 ], 1 );
      3
      gap> Position( [ 2, 1, 1, 3 ], 1 );
      2
      gap> Position( [ 2, 1, 1, 3 ], 1, 2 );
      3
      gap> Position( [ 2, 1, 1, 3 ], 1, 3 );
      fail

- `PositionCanonical(` list`,` obj`) O`

  returns the position of the canonical associate of obj in list. The definition of this associate depends on list. For internally represented lists it is defined as the element itself (and `PositionCanonical` thus defaults to `Position`, see Position), but for example for certain enumerators (see Enumerators) other canonical associates can be defined.

  For example `RightTransversal` defines the canonical associate to be the element in the transversal defining the same coset of a subgroup in a group.

      gap> g:=Group((1,2,3,4),(1,2));;u:=Subgroup(g,[(1,2)(3,4),(1,3)(2,4)]);;
      gap> rt:=RightTransversal(g,u);;AsList(rt);
      [ (), (3,4), (2,3), (2,3,4), (2,4,3), (2,4) ]
      gap> Position(rt,(1,2));
      fail
      gap> PositionCanonical(rt,(1,2));
      2

- `PositionNthOccurrence(` list`,` obj`,` n`) O`

  returns the position of the n-th occurrence of obj in list and returns `fail` if obj does not occur n times.

      gap> PositionNthOccurrence([1,2,3,2,4,2,1],1,1);
      1
      gap> PositionNthOccurrence([1,2,3,2,4,2,1],1,2);
      7
      gap> PositionNthOccurrence([1,2,3,2,4,2,1],2,3);
      6
      gap> PositionNthOccurrence([1,2,3,2,4,2,1],2,4);
      fail

- `PositionSorted(` list`,` elm`) O`
- `PositionSorted(` list`,` elm`,` func`) O`

  In the first form `PositionSorted` returns the position of the element elm in the sorted list list.

  In the second form `PositionSorted` returns the position of the element elm in the list list, which must be sorted with respect to func. func must be a function of two arguments that returns `true` if the first argument is less than the second argument and `false` otherwise.

  `PositionSorted` returns pos such that list [pos −1] < elm and elm ≤ list [pos ]. That means, if elm appears once in list, its position is returned. If elm appears several times in list, the position of the first occurrence is returned. If elm is not an element of list, the index where elm must be inserted to keep the list sorted is returned.

  `PositionSorted` uses binary search, whereas `Position` can in general use only linear search, see the remark at the beginning of Sorted Lists and Sets. For sorting lists, see Sorting Lists, for testing whether a list is sorted, see IsSortedList and IsSSortedList.

      gap> PositionSorted( [1,4,5,5,6,7], 0 );
      1
      gap> PositionSorted( [1,4,5,5,6,7], 2 );
      2
      gap> PositionSorted( [1,4,5,5,6,7], 4 );
      2
      gap> PositionSorted( [1,4,5,5,6,7], 5 );
      3
      gap> PositionSorted( [1,4,5,5,6,7], 8 );
      7

- `PositionSet(` list`,` obj`) F`
- `PositionSet(` list`,` obj`,` func`) F`

  `PositionSet` is a slight variation of `PositionSorted`. The only difference to `PositionSorted` is that `PositionSet` returns `fail` if obj is not in list.

      gap> PositionSet( [1,4,5,5,6,7], 0 );
      fail
      gap> PositionSet( [1,4,5,5,6,7], 2 );
      fail
      gap> PositionSet( [1,4,5,5,6,7], 4 );
      2
      gap> PositionSet( [1,4,5,5,6,7], 5 );
      3
      gap> PositionSet( [1,4,5,5,6,7], 8 );
      fail

- `PositionProperty(` list`,` func`) O`

  returns the first position of an element in the list list for which the property tester function func returns `true`.

      gap> PositionProperty( [10^7..10^8], IsPrime );
      20
      gap> PositionProperty( [10^5..10^6],
      >        n -> not IsPrime(n) and IsPrimePowerInt(n) );
      490

  `First` (see First) allows you to extract the first element of a list that satisfies a certain property.

- `PositionBound(` list`) O`

  returns the first index for which an element is bound in the list list. For the empty list it returns `fail`.

      gap> PositionBound([1,2,3]);
      1
      gap> PositionBound([,1,2,3]);
      2

- `PositionNot(` list`,` val`[,` from-minus-one`] ) O`

  For a list list and an object val, `PositionNot` returns the smallest nonnegative integer n such that list [n] is either unbound or not equal to val. If a nonnegative integer is given as optional argument from-minus-one then the first position larger than from-minus-one with this property is returned.

- `PositionNonZero(` vec`) O`

  For a row vector vec, `PositionNonZero` returns the position of the first non-zero element of vec, or `Length(`vec`)+1` if all entries of vec are zero.

  `PositionNonZero` implements a special case of `PositionNot` (see PositionNot). Namely, the element to be avoided is the zero element, and the list must be (at least) homogeneous because otherwise the zero element cannot be specified implicitly.

      gap> l:= [ 1, 1, 2, 3, 2 ];;  PositionNot( l, 1 );
      3
      gap> PositionNot( l, 1, 4 );  PositionNot( l, 2, 5 );
      5
      6
      gap> PositionNonZero( l );  PositionNonZero( [ 2, 3, 4, 5 ] * Z(2) );
      1
      2

- `PositionSublist(` list`,` sub`) O`
- `PositionSublist(` list`,` sub`,` from`) O`

  returns the smallest index in the list list at which a sublist equal to sub starts. If sub does not occur the operation returns `fail`. The second version starts searching after position from.

  To determine whether sub matches list at a particular position, use `IsMatchingSublist` instead (see IsMatchingSublist).

  ## 21.17 Properties and Attributes for Lists

- `IsMatchingSublist(` list`,` sub`) O`
- `IsMatchingSublist(` list`,` sub`,` at`) O`

  returns `true` if sub matches a sublist of list from position 1 (or position at, in the case of the second version), or `false`, otherwise. If sub is empty `true` is returned. If list is empty but sub is non-empty `false` is returned.

  If you actually want to know whether there is an at for which `IsMatchingSublist(` list`,` sub`,` at`)` is true, use a construction like `PositionSublist(` list`,` sub`)` `fail` instead (see PositionSublist); it's more efficient.

  Note: A list that contains mutable objects (like lists or records) cannot store attribute values that depend on the values of its entries, such as whether it is homogeneous, sorted, or strictly sorted, as changes in any of its entries could change such property values, like the following example shows.

      gap> l:=[[1],[2]];
      [ [ 1 ], [ 2 ] ]
      gap> IsSSortedList(l);
      true
      gap> l[1][1]:=3;
      3
      gap> IsSSortedList(l);
      false

  For such lists these property values must be computed anew each time the property is asked for. For example, if list is a list of mutable row vectors then the call of `Position` (see Position) with list as first argument cannot take advantage of the fact that list is in fact sorted. One solution is to call explicitly `PositionSorted` (see PositionSorted) in such a situation, another solution is to replace list by an immutable copy using `Immutable` (see Mutability and Copyability).

- `IsDuplicateFree(` obj`) P`
- `IsDuplicateFreeList(` obj`) P`

  `IsDuplicateFree(`obj`);` returns `true` if obj is both a list or collection, and it is duplicate free; otherwise it returns `false`. `IsDuplicateFreeList` is a synonym for `IsDuplicateFree and IsList`.

  A list is duplicate free if it is dense and does not contain equal entries in different positions. Every domain (see Domains) is duplicate free.

- `IsSortedList(` obj`) P`

  returns `true` if obj is a list and it is sorted, or `false` otherwise.

  A list list is sorted if it is dense (see IsDenseList) and satisfies the relation list [i] ≤ list [j] whenever i < j. Note that a sorted list is not necessarily duplicate free (see IsDuplicateFree and IsSSortedList).

  Many sorted lists are in fact homogeneous (see IsHomogeneousList), but also non-homogeneous lists may be sorted (see Comparison Operations for Elements).

- `IsSSortedList(` obj`) P`
- `IsSet(` obj`) P`

  returns `true` if obj is a list and it is strictly sorted, or `false` otherwise. `IsSSortedList` is short for ``is strictly sorted list''; `IsSet` is just a synonym for `IsSSortedList`.

  A list list is strictly sorted if it is sorted (see IsSortedList) and satisfies the relation list [i] < list [j] whenever i < j. In particular, such lists are duplicate free (see IsDuplicateFree).

  In sorted lists, membership test and computing of positions can be done by binary search, see Sorted Lists and Sets.

  (Currently there is little special treatment of lists that are sorted but not strictly sorted. In particular, internally represented lists will not store that they are sorted but not strictly sorted.)

- `Length(` list`) A`

  returns the length of the list list, which is defined to be the index of the last bound entry in list.

- `ConstantTimeAccessList(` list`) A`

  `ConstantTimeAccessList` returns an immutable list containing the same elements as the list list (which may have holes) in the same order. If list is already a constant time access list, `ConstantTimeAccessList` returns an immutable copy of list directly. Otherwise it puts all elements and holes of list into a new list and makes that list immutable.

  ## 21.18 Sorting Lists

- `Sort(` list`) O`
- `Sort(` list`,` func`) O`

  sorts the list list in increasing order. In the first form `Sort` uses the operator `<` to compare the elements. (If the list is not homogeneous it is the users responsibility to ensure that `<` is defined for all element pairs, see Comparison Operations for Elements) In the second form `Sort` uses the function func to compare elements. func must be a function taking two arguments that returns `true` if the first is regarded as strictly smaller than the second, and `false` otherwise.

  `Sort` does not return anything, it just changes the argument list. Use `ShallowCopy` (see ShallowCopy) if you want to keep list. Use `Reversed` (see Reversed) if you want to get a new list sorted in decreasing order.

  It is possible to sort lists that contain multiple elements which compare equal. It is not guaranteed that those elements keep their relative order, i.e., `Sort` is not stable.

      gap> list := [ 5, 4, 6, 1, 7, 5 ];; Sort( list ); list;
      [ 1, 4, 5, 5, 6, 7 ]
      gap> list := [ [0,6], [1,2], [1,3], [1,5], [0,4], [3,4] ];;
      gap> Sort( list, function(v,w) return v*v < w*w; end );
      gap> list;  # sorted according to the Euclidian distance from [0,0]
      [ [ 1, 2 ], [ 1, 3 ], [ 0, 4 ], [ 3, 4 ], [ 1, 5 ], [ 0, 6 ] ]
      gap> list := [ [0,6], [1,3], [3,4], [1,5], [1,2], [0,4], ];;
      gap> Sort( list, function(v,w) return v[1] < w[1]; end );
      gap> list;  # note the random order of the elements with equal first component
      [ [ 0, 6 ], [ 0, 4 ], [ 1, 3 ], [ 1, 5 ], [ 1, 2 ], [ 3, 4 ] ]

- `SortParallel(` list`,` list2`) O`
- `SortParallel(` list`,` list2`,` func`) O`

  sorts the list list1 in increasing order just as `Sort` (see Sort) does. In parallel it applies the same exchanges that are necessary to sort list1 to the list list2, which must of course have at least as many elements as list1 does.

      gap> list1 := [ 5, 4, 6, 1, 7, 5 ];;
      gap> list2 := [ 2, 3, 5, 7, 8, 9 ];;
      gap> SortParallel( list1, list2 );
      gap> list1;
      [ 1, 4, 5, 5, 6, 7 ]
      gap> list2;  # note: [ 7, 3, 2, 9, 5, 8 ] or [ 7, 3, 9, 2, 5, 8 ] are possible results
      [ 7, 3, 2, 9, 5, 8 ]

- `Sortex(` list`) O`

  sorts the list list via the operator`<` and returns a permutation that can be applied to list to obtain the sorted list. (If the list is not homogeneous it is the user's responsibility to ensure that `<` is defined for all element pairs, see Comparison Operations for Elements)

  `Permuted` (see Permuted) allows you to rearrange a list according to a given permutation.

      gap> list1 := [ 5, 4, 6, 1, 7, 5 ];;
      gap> list2 := ShallowCopy( list1 );;
      gap> perm := Sortex( list1 );
      (1,3,5,6,4)
      gap> list1;
      [ 1, 4, 5, 5, 6, 7 ]
      gap> Permuted( list2, perm );
      [ 1, 4, 5, 5, 6, 7 ]

- `SortingPerm(` list`) A`

  `SortingPerm` returns the same as `Sortex(` list`)` (see Sortex) but does not change the argument.

      gap> list1 := [ 5, 4, 6, 1, 7, 5 ];;
      gap> list2 := ShallowCopy( list1 );;
      gap> perm := SortingPerm( list1 );
      (1,3,5,6,4)
      gap> list1;
      [ 5, 4, 6, 1, 7, 5 ]
      gap> Permuted( list2, perm );
      [ 1, 4, 5, 5, 6, 7 ]

  Currently GAP uses shellsort.

  ## 21.19 Sorted Lists and Sets

  Searching objects in a list works much quicker if the list is known to be sorted. Currently GAP exploits the sortedness of a list automatically only if the list is strictly sorted, which is indicated by the property `IsSSortedList`, see IsSSortedList.

  Remember that a list of mutable objects cannot store that it is strictly sorted but has to test it anew whenever it is asked whether it is sorted, see the remark in Properties and Attributes for Lists. Therefore GAP cannot take advantage of the sortedness of a list if this list has mutable entries. Moreover, if a sorted list list with mutable elements is used as an argument of a function that expects this argument to be sorted, for example `UniteSet` or `RemoveSet` (see UniteSet, RemoveSet), then it is checked whether list is in fact sorted; this check can have the effect actually to slow down the computations, compared to computations with sorted lists of immutable elements or computations that do not involve functions that do automatically check sortedness.

  Strictly sorted lists are used to represent sets in GAP. More precisely, a strictly sorted list is called a proper set in the following, in order to avoid confusion with domains (see Domains) which also represent sets.

  In short proper sets are represented by sorted lists without holes and duplicates in GAP. Note that we guarantee this representation, so you may make use of the fact that a set is represented by a sorted list in your functions.

  In some contexts (for example see Combinatorics), we also want to talk about multisets. A multiset is like a set, except that an element may appear several times in a multiset. Such multisets are represented by sorted lists without holes that may have duplicates.

  This section lists only those functions that are defined exclusively for proper sets. Set theoretic functions for general collections, such as `Intersection` and `Union`, are described in Chapter Collections. In particular, for the construction of proper sets, see SSortedList and AsSSortedList. For finding positions in sorted lists, see PositionSorted.

- ``obj `in` list``

  The element test for strictly sorted lists uses binary search.

  The following functions, if not explicitly stated differently, take two arguments, set and obj, where set must be a proper set, otherwise an error is signalled; If the second argument obj is a list that is not a proper set then `Set` (see Set) is silently applied to it first (see Set).

- `IsEqualSet(` list1`,` list2`) O`

  tests whether list1 and list2 are equal when viewed as sets, that is if every element of list1 is an element of list2 and vice versa. Either argument of `IsEqualSet` may also be a list that is not a proper set, in which case `Set` (see Set) is applied to it first.

  If both lists are proper sets then they are of course equal if and only if they are also equal as lists. Thus `IsEqualSet(` list1`,` list2`)` is equivalent to `Set(` list1`) = Set(` list2`)` (see Set), but the former is more efficient.

      gap> IsEqualSet( [2,3,5,7,11], [11,7,5,3,2] );
      true
      gap> IsEqualSet( [2,3,5,7,11], [2,3,5,7,11,13] );
      false

- `IsSubsetSet(` list1`,` list2`) O`

  tests whether every element of list2 is contained in list1. Either argument of `IsSubsetSet` may also be a list that is not a proper set, in which case `Set` (see Set) is applied to it first.

- `AddSet(` set`,` obj`) O`

  adds the element obj to the proper set set. If obj is already contained in set then set is not changed. Otherwise obj is inserted at the correct position such that set is again a proper set afterwards.

  Note that obj must be in the same family as each element of set.

      gap> s := [2,3,7,11];;
      gap> AddSet( s, 5 );  s;
      [ 2, 3, 5, 7, 11 ]
      gap> AddSet( s, 13 );  s;
      [ 2, 3, 5, 7, 11, 13 ]
      gap> AddSet( s, 3 );  s;
      [ 2, 3, 5, 7, 11, 13 ]

- `RemoveSet(` set`,` obj`) O`

  removes the element obj from the proper set set. If obj is not contained in set then set is not changed. If obj is an element of set it is removed and all the following elements in the list are moved one position forward.

      gap> s := [ 2, 3, 4, 5, 6, 7 ];;
      gap> RemoveSet( s, 6 ); s;
      [ 2, 3, 4, 5, 7 ]
      gap> RemoveSet( s, 10 ); s;
      [ 2, 3, 4, 5, 7 ]

- `UniteSet(` set`,` list`) O`

  unites the proper set set with list. This is equivalent to adding all elements of list to set (see AddSet).

      gap> set := [ 2, 3, 5, 7, 11 ];;
      gap> UniteSet( set, [ 4, 8, 9 ] );  set;
      [ 2, 3, 4, 5, 7, 8, 9, 11 ]
      gap> UniteSet( set, [ 16, 9, 25, 13, 16 ] );  set;
      [ 2, 3, 4, 5, 7, 8, 9, 11, 13, 16, 25 ]

- `IntersectSet(` set`,` list`) O`

  intersects the proper set set with list. This is equivalent to removing from set all elements of set that are not contained in list.

      gap> set := [ 2, 3, 4, 5, 7, 8, 9, 11, 13, 16 ];;
      gap> IntersectSet( set, [ 3, 5, 7, 9, 11, 13, 15, 17 ] );  set;
      [ 3, 5, 7, 9, 11, 13 ]
      gap> IntersectSet( set, [ 9, 4, 6, 8 ] );  set;
      [ 9 ]

- `SubtractSet(` set`,` list`) O`

  subtracts list from the proper set set. This is equivalent to removing from set all elements of list.

      gap> set := [ 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 ];;
      gap> SubtractSet( set, [ 6, 10 ] );  set;
      [ 2, 3, 4, 5, 7, 8, 9, 11 ]
      gap> SubtractSet( set, [ 9, 4, 6, 8 ] );  set;
      [ 2, 3, 5, 7, 11 ]

  There are nondestructive counterparts of the functions `UniteSet`, `IntersectSet`, and `SubtractSet` available for proper sets. These are `UnionSet`, `IntersectionSet`, and `Difference`. The former two are methods for the more general operations `Union` and `Intersection` (see Union, Intersection), the latter is itself an operation (see Difference).

  The result of `IntersectionSet` and `UnionSet` is always a new list, that is not identical to any other list. The elements of that list however are identical to the corresponding elements of the first argument set. If set is not a proper set it is not specified to which of a number of equal elements in set the element in the result is identical (see Identical Lists).

  ## 21.20 Operations for Lists

  Several of the following functions expect the first argument to be either a list or a collection (see Collections), with possibly slightly different meaning for lists and non-list collections. For these functions, the list case is indicated by an argument named list, and the collection case by one named C.

- `Concatenation(` list1`,` list2`, ... ) F`
- `Concatenation(` list`) F`

  In the first form `Concatenation` returns the concatenation of the lists list1, list2, etc. The concatenation is the list that begins with the elements of list1, followed by the elements of list2, and so on. Each list may also contain holes, in which case the concatenation also contains holes at the corresponding positions.

  In the second form list must be a dense list of lists list1, list2, etc., and `Concatenation` returns the concatenation of those lists.

  The result is a new mutable list, that is not identical to any other list. The elements of that list however are identical to the corresponding elements of list1, list2, etc. (see Identical Lists).

  Note that `Concatenation` creates a new list and leaves its arguments unchanged, while `Append` (see Append) changes its first argument. For computing the union of proper sets, `Union` can be used, see Union and Sorted Lists and Sets.

      gap> Concatenation( [ 1, 2, 3 ], [ 4, 5 ] );
      [ 1, 2, 3, 4, 5 ]
      gap> Concatenation( [2,3,,5,,7], [11,,13,,,,17,,19] );
      [ 2, 3,, 5,, 7, 11,, 13,,,, 17,, 19 ]
      gap> Concatenation( [ [1,2,3], [2,3,4], [3,4,5] ] );
      [ 1, 2, 3, 2, 3, 4, 3, 4, 5 ]

- `Compacted(` list`) O`

  returns a new mutable list that contains the elements of list in the same order but omitting the holes.

      gap> l:=[,1,,,3,,,4,[5,,,6],7];;  Compacted( l );
      [ 1, 3, 4, [ 5,,, 6 ], 7 ]

- `Collected(` list`) O`

  returns a new list new that contains for each element elm of the list list a list of length two, the first element of this is elm itself and the second element is the number of times elm appears in list. The order of those pairs in new corresponds to the ordering of the elements elm, so that the result is sorted.

  For all pairs of elements in list the comparison via `<` must be defined.

      gap> Factors( Factorial( 10 ) );
      [ 2, 2, 2, 2, 2, 2, 2, 2, 3, 3, 3, 3, 5, 5, 7 ]
      gap> Collected( last );
      [ [ 2, 8 ], [ 3, 4 ], [ 5, 2 ], [ 7, 1 ] ]
      gap> Collected( last );
      [ [ [ 2, 8 ], 1 ], [ [ 3, 4 ], 1 ], [ [ 5, 2 ], 1 ], [ [ 7, 1 ], 1 ] ]

- `DuplicateFreeList(` list`) O`
- `Unique(` list`) O`

  returns a new mutable list whose entries are the elements of the list list with duplicates removed. `DuplicateFreeList` only uses the `=` comparison and will not sort the result. Therefore `DuplicateFreeList` can be used even if the elements of list do not lie in the same family. `Unique` is an alias for `DuplicateFreeList`.

      gap> l:=[1,Z(3),1,"abc",Group((1,2,3),(1,2)),Z(3),Group((1,2),(2,3))];;
      gap> DuplicateFreeList( l );
      [ 1, Z(3), "abc", Group([ (1,2,3), (1,2) ]) ]

- `AsDuplicateFreeList(` list`) A`

  returns the same result as `DuplicateFreeList` (see DuplicateFreeList), except that the result is immutable.

- `Flat(` list`) O`

  returns the list of all elements that are contained in the list list or its sublists. That is, `Flat` first makes a new empty list new. Then it loops over the elements elm of list. If elm is not a list it is added to new, otherwise `Flat` appends `Flat(` elm`)` to new.

      gap> Flat( [ 1, [ 2, 3 ], [ [ 1, 2 ], 3 ] ] );
      [ 1, 2, 3, 1, 2, 3 ]
      gap> Flat( [ ] );
      [  ]

  (To reconstruct a matrix from a `Flat`tened list, the sublist operator can be used:

      gap> l:=[9..14];;w:=2;; # w is the length of each row
      gap> sub:=[1..w];;List([1..Length(l)/w],i->l{(i-1)*w+sub});
      [ [ 9, 10 ], [ 11, 12 ], [ 13, 14 ] ]

  )

- `Reversed(` list`) F`

  returns a new mutable list, containing the elements of the dense list list in reversed order.

  The argument list is unchanged. The result list is a new list, that is not identical to any other list. The elements of that list however are identical to the corresponding elements of the argument list (see Identical Lists).

  `Reversed` implements a special case of list assignment, which can also be formulated in terms of the `` operator (see List Assignment).

      gap> Reversed( [ 1, 4, 9, 5, 6, 7 ] );
      [ 7, 6, 5, 9, 4, 1 ]

- `IsLexicographicallyLess(` list1`,` list2`) F`

  Let list1 and list2 be two dense lists, but not necessarily homogeneous (see IsDenseList, IsHomogeneousList), such that for each i, the entries in both lists at position i can be compared via `<`. `IsLexicographicallyLess` returns `true` if list1 is smaller than list2 w.r.t. lexicographical ordering, and `false` otherwise.

- `Apply(` list`,` func`) F`

  `Apply` applies the function func to every element of the dense and mutable list list, and replaces each element entry by the corresponding return value.

  `Apply` changes its argument. The nondestructive counterpart of `Apply` is `List` (see List).

      gap> l:= [ 1, 2, 3 ];;  Apply( l, i -> i^2 );  l;
      [ 1, 4, 9 ]

- `Perform(` list`,` func`) O`

  `Perform(` list`,` func`)` applies func to every element of list, discarding any return values. It does not return a value.

      gap> l := [1, 2, 3];; Perform(l,
      > function(x) if IsPrimeInt(x) then Print(x,"\n"); fi; end);
      2
      3

- `PermListList(` list1`,` list2`) F`

  returns a permutation p of `[ 1 .. Length(` list1`) ]` such that ``list1`[i^p] =` list2`[i]`. It returns `fail` if there is no such permutation.

      gap> list1 := [ 5, 4, 6, 1, 7, 5 ];;
      gap> list2 := [ 4, 1, 7, 5, 5, 6 ];;
      gap> perm := PermListList(list1, list2);
      (1,2,4)(3,5,6)
      gap> Permuted( list2, perm );
      [ 5, 4, 6, 1, 7, 5 ]

- `Maximum(` obj1`,` obj2`... ) F`
- `Maximum(` list`) F`

  In the first form `Maximum` returns the maximum of its arguments, i.e., one argument obj for which obj ≥ obj1 , obj ≥ obj2 etc. In the second form `Maximum` takes a homogeneous list list and returns the maximum of the elements in this list.

      gap> Maximum( -123, 700, 123, 0, -1000 );
      700
      gap> Maximum( [ -123, 700, 123, 0, -1000 ] );
      700
      gap> Maximum( [1,2], [0,15], [1,5], [2,-11] );  # lists are compared elementwise
      [ 2, -11 ]

- `Minimum(` obj1`,` obj2`... ) F`
- `Minimum(` list`) F`

  In the first form `Minimum` returns the minimum of its arguments, i.e., one argument obj for which obj ≤ obj1 , obj ≤ obj2 etc. In the second form `Minimum` takes a homogeneous list list and returns the minimum of the elements in this list.

  Note that for both `Maximum` and `Minimum` the comparison of the objects obj1, obj2 etc. must be defined; for that, usually they must lie in the same family (see Families).

      gap> Minimum( -123, 700, 123, 0, -1000 );
      -1000
      gap> Minimum( [ -123, 700, 123, 0, -1000 ] );
      -1000
      gap> Minimum( [ 1, 2 ], [ 0, 15 ], [ 1, 5 ], [ 2, -11 ] );
      [ 0, 15 ]

- `MaximumList(` list`) O`
- `MinimumList(` list`) O`

  return the maximum resp. the minimum of the elements in the list list. They are the operations called by `Maximum` resp. `Minimum`. Methods can be installed for special kinds of lists. For example, there are special methods to compute the maximum resp. the minimum of a range (see Ranges).

- `Cartesian(` list1`,` list2`... ) F`
- `Cartesian(` list`) F`

  In the first form `Cartesian` returns the cartesian product of the lists list1, list2, etc.

  In the second form list must be a list of lists list1, list2, etc., and `Cartesian` returns the cartesian product of those lists.

  The cartesian product is a list cart of lists tup, such that the first element of tup is an element of list1, the second element of tup is an element of list2, and so on. The total number of elements in cart is the product of the lengths of the argument lists. In particular cart is empty if and only if at least one of the argument lists is empty. Also cart contains duplicates if and only if no argument list is empty and at least one contains duplicates.

  The last index runs fastest. That means that the first element tup1 of cart contains the first element from list1, from list2 and so on. The second element tup2 of cart contains the first element from list1, the first from list2, an so on, but the last element of tup2 is the second element of the last argument list. This implies that cart is a proper set if and only if all argument lists are proper sets (see Sorted Lists and Sets).

  The function `Tuples` (see Tuples) computes the k-fold cartesian product of a list.

      gap> Cartesian( [1,2], [3,4], [5,6] );
      [ [ 1, 3, 5 ], [ 1, 3, 6 ], [ 1, 4, 5 ], [ 1, 4, 6 ], [ 2, 3, 5 ],
        [ 2, 3, 6 ], [ 2, 4, 5 ], [ 2, 4, 6 ] ]
      gap> Cartesian( [1,2,2], [1,1,2] );
      [ [ 1, 1 ], [ 1, 1 ], [ 1, 2 ], [ 2, 1 ], [ 2, 1 ], [ 2, 2 ], [ 2, 1 ],
        [ 2, 1 ], [ 2, 2 ] ]

- `Permuted(` list`,` perm`) O`

  returns a new list new that contains the elements of the list list permuted according to the permutation perm. That is ``new`[`i`^` perm`] =` list`[`i`]`.

  `Sortex` (see Sortex) allows you to compute a permutation that must be applied to a list in order to get the sorted list.

      gap> Permuted( [ 5, 4, 6, 1, 7, 5 ], (1,3,5,6,4) );
      [ 1, 4, 5, 5, 6, 7 ]

- `List(` list`) F`
- `List(` C`) F`
- `List(` list`,` func`) F`

  In the first form, where list is a list (not necessarily dense or homogeneous), `List` returns a new mutable list new that contains the elements (and the holes) of list in the same order; thus `List` does the same as `ShallowCopy` (see ShallowCopy) in this case.

  In the second form, where C is a collection (see Collections) that is not a list, `List` returns a new mutable list new such that `Length(` new`)` is the number of different elements of C, and new contains the different elements of C in an unspecified order which may change for repeated calls. ``new`[`pos`]` executes in constant time (see IsConstantTimeAccessList), and the size of new is proportional to its length. The generic method for this case is `ShallowCopy( Enumerator(` C`) )`.

  In the third form, for a dense list list and a function func, which must take exactly one argument, `List` returns a new mutable list new given by new [i] = func ( list [i] ).

      gap> List( [1,2,3], i -> i^2 );
      [ 1, 4, 9 ]
      gap> List( [1..10], IsPrime );
      [ false, true, true, false, true, false, true, false, false, false ]

- `Filtered(` list`,` func`) F`
- `Filtered(` C`,` func`) F`

  returns a new list that contains those elements of the list list or collection C (see Collections), respectively, for which the unary function func returns `true`.

  If the first argument is a list, the order of the elements in the result is the same as the order of the corresponding elements of list. If an element for which func returns `true` appears several times in list it will also appear the same number of times in the result. list may contain holes, they are ignored by `Filtered`.

  For each element of list resp. C, func must return either `true` or `false`, otherwise an error is signalled.

  The result is a new list that is not identical to any other list. The elements of that list however are identical to the corresponding elements of the argument list (see Identical Lists).

  List assignment using the operator `` (see List Assignment) can be used to extract elements of a list according to indices given in another list.

      gap> Filtered( [1..20], IsPrime );
      [ 2, 3, 5, 7, 11, 13, 17, 19 ]
      gap> Filtered( [ 1, 3, 4, -4, 4, 7, 10, 6 ], IsPrimePowerInt );
      [ 3, 4, 4, 7 ]
      gap> Filtered( [ 1, 3, 4, -4, 4, 7, 10, 6 ],
      >              n -> IsPrimePowerInt(n) and n mod 2 <> 0 );
      [ 3, 7 ]
      gap> Filtered( Group( (1,2), (1,2,3) ), x -> Order( x ) = 2 );
      [ (2,3), (1,2), (1,3) ]

- `Number(` list`) F`
- `Number(` list`,` func`) F`
- `Number(` C`,` func`) F`

  In the first form, `Number` returns the number of bound entries in the list list. For dense lists `Number`, `Length` (see Length), and `Size` (see Size) return the same value; for lists with holes `Number` returns the number of bound entries, `Length` returns the largest index of a bound entry, and `Size` signals an error.

  In the last two forms, `Number` returns the number of elements of the list list resp. the collection C for which the unary function func returns `true`. If an element for which func returns `true` appears several times in list it will also be counted the same number of times.

  For each element of list resp. C, func must return either `true` or `false`, otherwise an error is signalled.

  `Filtered` (see Filtered) allows you to extract the elements of a list that have a certain property.

      gap> Number( [ 2, 3, 5, 7 ] );
      4
      gap> Number( [, 2, 3,, 5,, 7,,,, 11 ] );
      5
      gap> Number( [1..20], IsPrime );
      8
      gap> Number( [ 1, 3, 4, -4, 4, 7, 10, 6 ], IsPrimePowerInt );
      4
      gap> Number( [ 1, 3, 4, -4, 4, 7, 10, 6 ],
      >            n -> IsPrimePowerInt(n) and n mod 2 <> 0 );
      2
      gap> Number( Group( (1,2), (1,2,3) ), x -> Order( x ) = 2 );
      3

- `First(` list`,` func`) F`

  `First` returns the first element of the list list for which the unary function func returns `true`. list may contain holes. func must return either `true` or `false` for each element of list, otherwise an error is signalled. If func returns `false` for all elements of list then `First` returns `fail`.

  `PositionProperty` (see PositionProperty) allows you to find the position of the first element in a list that satisfies a certain property.

      gap> First( [10^7..10^8], IsPrime );
      10000019
      gap> First( [10^5..10^6],
      >      n -> not IsPrime(n) and IsPrimePowerInt(n) );
      100489
      gap> First( [ 1 .. 20 ], x -> x < 0 );
      fail
      gap> First( [ fail ], x -> x = fail );
      fail

- `ForAll(` list`,` func`) F`
- `ForAll(` C`,` func`) F`

  tests whether the unary function func returns `true` for all elements in the list list resp. the collection C.

      gap> ForAll( [1..20], IsPrime );
      false
      gap> ForAll( [2,3,4,5,8,9], IsPrimePowerInt );
      true
      gap> ForAll( [2..14], n -> IsPrimePowerInt(n) or n mod 2 = 0 );
      true
      gap> ForAll( Group( (1,2), (1,2,3) ), i -> SignPerm(i) = 1 );
      false

- `ForAny(` list`,` func`) F`
- `ForAny(` C`,` func`) F`

  tests whether the unary function func returns `true` for at least one element in the list list resp. the collection C.

      gap> ForAny( [1..20], IsPrime );
      true
      gap> ForAny( [2,3,4,5,8,9], IsPrimePowerInt );
      true
      gap> ForAny( [2..14],
      >    n -> IsPrimePowerInt(n) and n mod 5 = 0 and not IsPrime(n) );
      false
      gap> ForAny( Integers, i ->     i > 0
      >                           and ForAll( [0,2..4], j -> IsPrime(i+j) ) );
      true

- `Product(` list`[,` init`] ) F`
- `Product(` C`[,` init`] ) F`
- `Product(` list`,` func`[,` init`] ) F`
- `Product(` C`,` func`[,` init`] ) F`

  In the first two forms `Product` returns the product of the elements of the dense list list resp. the collection C (see Collections). In the last two forms `Product` applies the function func, which must be a function taking one argument, to the elements of the dense list list resp. the collection C, and returns the product of the results. In either case `Product` returns `1` if the first argument is empty.

  The general rules for arithmetic operations apply (see Mutability Status and List Arithmetic), so the result is immutable if and only if all summands are immutable.

  If list or C contains exactly one element then this element (or its image under func if applicable) itself is returned, not a shallow copy of this element.

  If an additional initial value init is given, `Product` returns the product of init and the elements of the first argument resp. of their images under the function func. This is useful for example if the first argument is empty and a different identity than `1` is desired, in which case init is returned.

      gap> Product( [ 2, 3, 5, 7, 11, 13, 17, 19 ] );
      9699690
      gap> Product( [1..10], x->x^2 );
      13168189440000
      gap> Product( [ (1,2), (1,3), (1,4), (2,3), (2,4), (3,4) ] );
      (1,4)(2,3)
      gap> Product( GF(8) );
      0*Z(2)

- `Sum(` list`[,` init`] ) F`
- `Sum(` C`[,` init`] ) F`
- `Sum(` list`,` func`[,` init`] ) F`
- `Sum(` C`,` func`[,` init`] ) F`

  In the first two forms `Sum` returns the sum of the elements of the dense list list resp. the collection C (see Collections). In the last two forms `Sum` applies the function func, which must be a function taking one argument, to the elements of the dense list list resp. the collection C, and returns the sum of the results. In either case `Sum` returns `0` if the first argument is empty.

  The general rules for arithmetic operations apply (see Mutability Status and List Arithmetic), so the result is immutable if and only if all summands are immutable.

  If list or C contains exactly one element then this element (or its image under func if applicable) itself is returned, not a shallow copy of this element.

  If an additional initial value init is given, `Sum` returns the sum of init and the elements of the first argument resp. of their images under the function func. This is useful for example if the first argument is empty and a different zero than `0` is desired, in which case init is returned.

      gap> Sum( [ 2, 3, 5, 7, 11, 13, 17, 19 ] );
      77
      gap> Sum( [1..10], x->x^2 );
      385
      gap> Sum( [ [1,2], [3,4], [5,6] ] );
      [ 9, 12 ]
      gap> Sum( GF(8) );
      0*Z(2)

- `Iterated(` list`,` func`) O`

  returns the result of the iterated application of the function func, which must take two arguments, to the elements of the list list. More precisely `Iterated` returns the result of the following application, f (…f ( f ( list [1], list [2] ), list [3] ), …, list [n ] ).

      gap> Iterated( [ 126, 66, 105 ], Gcd );
      3

- `ListN(` list1`,` list2`, ...,` listn`,` f`) F`

  Applies the n-argument function func to the lists. That is, `ListN` returns the list whose ith entry is f (list1 [i ], list2 [i ], …, listn [i ]).

      gap> ListN( [1,2], [3,4], \+ );
      [ 4, 6 ]

  ## 21.21 Advanced List Manipulations

  The following functions are generalizations of `List` (see List), `Set` (see Set), `Sum` (see Sum), and `Product` (see Product).

- `ListX(` arg1`,` arg2`, ...` argn`,` func`) O`

  `ListX` returns a new list constructed from the arguments.

  Each of the arguments ``arg1`,` arg2`, ...` argn`` must be one of the following:

  a list or collection
    this introduces a new for-loop in the sequence of nested for-loops and if-statements;

  a function returning a list or collection
    this introduces a new for-loop in the sequence of nested for-loops and if-statements, where the loop-range depends on the values of the outer loop-variables; or

  a function returning `true` or `false`
    this introduces a new if-statement in the sequence of nested for-loops and if-statements.

  The last argument func must be a function, it is applied to the values of the loop-variables and the results are collected.

  Thus `ListX(` list`,` func`)` is the same as `List(` list`,` func`)`, and `ListX(` list`,` func`, x -> x )` is the same as `Filtered(` list`,` func`)`.

  As a more elaborate example, assume arg1 is a list or collection, arg2 is a function returning `true` or `false`, arg3 is a function returning a list or collection, and arg4 is another function returning `true` or `false`, then

  ``result`:= ListX(` arg1`,` arg2`,` arg3`,` arg4`,` func`);`

  is equivalent to

  ``result`:= [];`
  `for v1 in` arg1 `do`
  `if` arg2`( v1 ) then`
  `for v2 in` arg3`( v1 ) do`
  `if` arg4`( v1, v2 ) then`
  `Add(` result`,` func`( v1, v2 ) );`
  `fi;`
  `od;`
  `fi;`
  `od;`

  The following example shows how `ListX` can be used to compute all pairs and all strictly sorted pairs of elements in a list.

      gap> l:= [ 1, 2, 3, 4 ];;
      gap> pair:= function( x, y ) return [ x, y ]; end;;
      gap> ListX( l, l, pair );
      [ [ 1, 1 ], [ 1, 2 ], [ 1, 3 ], [ 1, 4 ], [ 2, 1 ], [ 2, 2 ], [ 2, 3 ],
        [ 2, 4 ], [ 3, 1 ], [ 3, 2 ], [ 3, 3 ], [ 3, 4 ], [ 4, 1 ], [ 4, 2 ],
        [ 4, 3 ], [ 4, 4 ] ]

  In the following example, `<` is the comparison operation:

      gap> ListX( l, l, \<, pair );
      [ [ 1, 2 ], [ 1, 3 ], [ 1, 4 ], [ 2, 3 ], [ 2, 4 ], [ 3, 4 ] ]

- `SetX(` arg1`,` arg2`, ...` func`) O`

  The only difference between `SetX` and `ListX` is that the result list of `SetX` is strictly sorted.

- `SumX(` arg1`,` arg2`, ...` func`) O`

  `SumX` returns the sum of the elements in the list obtained by `ListX` when this is called with the same arguments.

- `ProductX(` arg1`,` arg2`, ...` func`) O`

  `ProductX` returns the product of the elements in the list obtained by `ListX` when this is called with the same arguments.

  ## 21.22 Ranges

  A range is a dense list of integers in arithmetic progression (or degression). This is a list of integers such that the difference between consecutive elements is a nonzero constant. Ranges can be abbreviated with the syntactic construct `[` first`,` second`..` last`]` or, if the difference between consecutive elements is 1, as `[` first`..` last`]`.

  If ``first`>` last``, `[`first`..`last`]` is the empty list, which by definition is also a range; also, if ``second`>` first`>` last`` or ``second`<` first`<` last``, then `[`first`,`second`..`last`]` is the empty list. If ``first`=` last``, `[`first`,`second`..`last`]` is a singleton list, which is a range, too. Note that ``last`-` first`` must be divisible by the increment ``second`-` first``, otherwise an error is signalled.

  Note also that a range is just a special case of a list. Thus you can access elements in a range (see List Elements), test for membership etc. You can even assign to such a range if it is mutable (see List Assignment). Of course, unless you assign ``last`+` second`-`first`` to the entry ``range`[Length(`range`)+1]`, the resulting list will no longer be a range.

      gap> r := [10..20];
      [ 10 .. 20 ]
      gap> Length( r );
      11
      gap> r[3];
      12
      gap> 17 in r;
      true
      gap> r[12] := 25;; r;  # r is no longer a range
      [ 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 25 ]
      gap> r := [1,3..17];
      [ 1, 3 .. 17 ]
      gap> Length( r );
      9
      gap> r[4];
      7
      gap> r := [0,-1..-9];
      [ 0, -1 .. -9 ]
      gap> r[5];
      -4
      gap> r := [ 1, 4 .. 32 ];
      Range: <last>-<first> (31) must be divisible by <inc> (3)

  Most often ranges are used in connection with the `for`-loop (see For). Here the construct

  `for` var `in [`first`..`last`] do` statements `od`

  replaces the

  `for` var `from` first `to` last `do` statements `od`

  which is more usual in other programming languages.

      gap> s := [];; for i in [10..20] do Add( s, i^2 ); od; s;
      [ 100, 121, 144, 169, 196, 225, 256, 289, 324, 361, 400 ]

  Note that a range with ``last`>=` first`` is at the same time also a proper set (see Sorted Lists and Sets), because it contains no holes or duplicates and is sorted, and also a row vector (see Row Vectors), because it contains no holes and all elements are integers.

- `IsRange(` obj`) C`

  tests if the object obj is a range, i.e. is a dense list of integers that is also a range (see Ranges for a definition of ``range'').

      gap> IsRange( [1,2,3] );  IsRange( [7,5,3,1] );
      true
      true
      gap> IsRange( [1,2,4,5] );  IsRange( [1,,3,,5,,7] );
      false
      false
      gap> IsRange( [] );  IsRange( [1] );
      true
      true

- `ConvertToRangeRep(` list`) F`

  For some lists the GAP kernel knows that they are in fact ranges. Those lists are represented internally in a compact way instead of the ordinary way.

  If list is a range then `ConvertToRangeRep` changes the representation of list to this compact representation.

  This is important since this representation needs only 12 bytes for the entire range while the ordinary representation needs 4 length bytes.

  Note that a list that is represented in the ordinary way might still be a range. It is just that GAP does not know this. The following rules tell you under which circumstances a range is represented in the compact way, so you can write your program in such a way that you make best use of this compact representation for ranges.

  Lists created by the syntactic construct `[` first`,` second`..` last`]` are of course known to be ranges and are represented in the compact way.

  If you call `ConvertToRangeRep` for a list represented the ordinary way that is indeed a range, the representation is changed from the ordinary to the compact representation. A call of `ConvertToRangeRep` for a list that is not a range is ignored.

  If you change a mutable range that is represented in the compact way, by assignment, `Add` or `Append`, the range will be converted to the ordinary representation, even if the change is such that the resulting list is still a proper range.

  Suppose you have built a proper range in such a way that it is represented in the ordinary way and that you now want to convert it to the compact representation to save space. Then you should call `ConvertToRangeRep` with that list as an argument. You can think of the call to `ConvertToRangeRep` as a hint to GAP that this list is a proper range.

      gap> r:= [ 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 ];
      [ 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 ]
      gap> ConvertToRangeRep( r );  r;
      [ 1 .. 10 ]
      gap> l:= [ 1, 2, 4, 5 ];;  ConvertToRangeRep( l );  l;
      [ 1, 2, 4, 5 ]

  ## 21.23 Enumerators

  An enumerator is an immutable list that need not store its elements explicitly but knows, from a set of basic data, how to determine the i-th element and the position of a given object. A typical example of this is a vector space over a finite field with q elements, say, for which it is very easy to enumerate all elements using q-adic expansions of integers.

  Using this enumeration can be even quicker than a binary search in a sorted list of vectors:

- `IsQuickPositionList(` list`) F`

  This filter indicates that a position test in list is quicker than about 5 or 6 element comparisons for ``smaller''. If this is the case it can be beneficial to use `Position` in list and a bit list than ordered lists to represent subsets of list.

  On the one hand, element access to an enumerator may take more time than element access to an internally represented list containing the same elements. On the other hand, an enumerator may save a vast amount of memory. Take for example a permutation group of size a few millions. Even for moderate degree it is unlikely that a list of all its elements will fit into memory whereas it is no problem to construct an enumerator from a stabilizer chain (see Stabilizer Chains).

  There are situations where one only wants to loop over the elements of a domain, without using the special facilities of an enumerator, namely the particular order of elements and the possibility to find the position of elements. For such cases, GAP provides iterators (see Iterators).

  The functions `Enumerator` and `EnumeratorSorted` (see Enumerator and EnumeratorSorted) return enumerators of domains. Most of the special implementations of enumerators in the GAP library are based on the general interface that is provided by `EnumeratorByFunctions` (see EnumeratorByFunctions); one generic example is `EnumeratorByBasis` (see EnumeratorByBasis), which can be used to get an enumerator of a finite dimensional free module.

  Also enumerators for non-domains can be implemented via `EnumeratorByFunctions`; for a discussion, see Example -- Constructing Enumerators in ``Programming in GAP''.

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  GAP 4 manual
  May 2005