@c DO NOT EDIT!  Generated automatically by munge-texi.

@c Copyright (C) 1996, 1997 John W. Eaton
@c This is part of the Octave manual.
@c For copying conditions, see the file gpl.texi.

@node Error Handling
@chapter Error Handling

Octave includes several functions for printing error and warning
messages.  When you write functions that need to take special action
when they encounter abnormal conditions, you should print the error
messages using the functions described in this chapter.

@anchor{doc-error}
@deftypefn {Built-in Function} {} error (@var{template}, @dots{})
Format the optional arguments under the control of the template string
@var{template} using the same rules as the @code{printf} family of
functions (@pxref{Formatted Output}) and print the resulting message
on the @code{stderr} stream.  The message is prefixed by the character
string @samp{error: }.

Calling @code{error} also sets Octave's internal error state such that
control will return to the top level without evaluating any more
commands.  This is useful for aborting from functions or scripts.

If the error message does not end with a new line character, Octave will
print a traceback of all the function calls leading to the error.  For
example, given the following function definitions:

@example
@group
function f () g () end
function g () h () end
function h () nargin == 1 || error ("nargin != 1"); end
@end group
@end example

@noindent
calling the function @code{f} will result in a list of messages that
can help you to quickly locate the exact location of the error:

@example
@group
f ()
error: nargin != 1
error: evaluating index expression near line 1, column 30
error: evaluating binary operator `||' near line 1, column 27
error: called from `h'
error: called from `g'
error: called from `f'
@end group
@end example

If the error message ends in a new line character, Octave will print the
message but will not display any traceback messages as it returns
control to the top level.  For example, modifying the error message
in the previous example to end in a new line causes Octave to only print
a single message:

@example
@group
function h () nargin == 1 || error ("nargin != 1\n"); end
f ()
error: nargin != 1
@end group
@end example
@end deftypefn


@anchor{doc-beep}
@deftypefn {Function File} {} beep ()
Produce a beep from the speaker (or visual bell).
@seealso{puts, fputs, printf, fprintf}
@end deftypefn


@anchor{doc-beep_on_error}
@deftypefn {Built-in Function} {@var{val} =} beep_on_error ()
@deftypefnx {Built-in Function} {@var{old_val} =} beep_on_error (@var{new_val})
Query or set the internal variable that controls whether Octave will try
to ring the terminal bell before printing an error message.
@end deftypefn


@anchor{doc-warning}
@deftypefn {Built-in Function} {} warning (@var{template}, @dots{})
@deftypefnx {Built-in Function} {} warning (@var{id}, @var{template})
Format the optional arguments under the control of the template string
@var{template} using the same rules as the @code{printf} family of
functions (@pxref{Formatted Output}) and print the resulting message
on the @code{stderr} stream.  The message is prefixed by the character
string @samp{warning: }.
You should use this function when you want to notify the user
of an unusual condition, but only when it makes sense for your program
to go on.

The optional message identifier allows users to enable or disable
warnings tagged by @var{id}.  The special identifier @samp{"all"} may
be used to set the state of all warnings.

@deftypefnx {Built-in Function} {} warning ("on", @var{id})
@deftypefnx {Built-in Function} {} warning ("off", @var{id})
@deftypefnx {Built-in Function} {} warning ("error", @var{id})
@deftypefnx {Built-in Function} {} warning ("query", @var{id})
Set or query the state of a particular warning using the identifier
@var{id}.  If the identifier is omitted, a value of @samp{"all"} is
assumed.  If you set the state of a warning to @samp{"error"}, the
warning named by @var{id} is handled as if it were an error instead.
@seealso{warning_ids}
@end deftypefn


@anchor{doc-print_usage}
@deftypefn {Loadable Function} {} print_usage ()
Print the usage message for the currently executing function.  The
@code{print_usage} function is only intended to work inside a
user-defined function.
@seealso{help}
@end deftypefn


@anchor{doc-usage}
@deftypefn {Built-in Function} {} usage (@var{msg})
Print the message @var{msg}, prefixed by the string @samp{usage: }, and
set Octave's internal error state such that control will return to the
top level without evaluating any more commands.  This is useful for
aborting from functions.

After @code{usage} is evaluated, Octave will print a traceback of all
the function calls leading to the usage message.

You should use this function for reporting problems errors that result
from an improper call to a function, such as calling a function with an
incorrect number of arguments, or with arguments of the wrong type.  For
example, most functions distributed with Octave begin with code like
this

@example
@group
if (nargin != 2)
  usage ("foo (a, b)");
endif
@end group
@end example

@noindent
to check for the proper number of arguments.
@end deftypefn


@anchor{doc-lasterror}
@deftypefn {Built-in Function} {@var{err} =} lasterror (@var{err})
@deftypefnx {Built-in Function} {} lasterror ('reset')
Returns or sets the last error message. Called without any arguments
returns a structure containing the last error message, as well as other
information related to this error. The elements of this structure are:

@table @asis
@item 'message'
The text of the last error message
@item 'identifier'
The message identifier of this error message
@item 'stack'
A structure containing information on where the message occured. This might
be an empty structure if this in the case where this information can not
be obtained. The fields of this structure are:

@table @asis
@item 'file'
The name of the file where the error occurred
@item 'name'
The name of function in which the error occured
@item 'line'
The line number at which the error occured
@item 'column'
An optional field with the column number at which the error occurred
@end table
@end table

The @var{err} structure may also be passed to @code{lasterror} to set the
information about the last error. The only constraint on @var{err} in that
case is that it is a scalar structure. Any fields of @var{err} that match
the above are set to the value passed in @var{err}, while other fields are
set to their default values.

If @code{lasterror} is called with the argument 'reset', all values take
their default values.
@end deftypefn


@anchor{doc-lasterr}
@deftypefn {Built-in Function} {[@var{msg}, @var{msgid}] =} lasterr (@var{msg}, @var{msgid})
Without any arguments, return the last error message.  With one
argument, set the last error message to @var{msg}.  With two arguments,
also set the last message identifier.
@end deftypefn


@anchor{doc-lastwarn}
@deftypefn {Built-in Function} {[@var{msg}, @var{msgid}] =} lastwarn (@var{msg}, @var{msgid})
Without any arguments, return the last warning message.  With one
argument, set the last warning message to @var{msg}.  With two arguments,
also set the last message identifier.
@end deftypefn


@anchor{doc-rethrow}
@deftypefn {Built-in Function} {} rethrow (@var{err})
Reissues a previous error as defined by @var{err}. @var{err} is a structure
that must contain at least the 'message' and 'identifier' fields. @var{err}
can also contain a field 'stack' that gives information on the assumed
location of the error. Typically @var{err} is returned from
@code{lasterror}.
@seealso{lasterror, lasterr, error}
@end deftypefn


@anchor{doc-warning_ids}
@table @code
@item Octave:array-to-scalar
If the @code{Octave:array-to-scalar} warning is enabled, Octave will
warn when an implicit conversion from an array to a scalar value is
attempted.  By default, the @code{Octave:array-to-scalar} warning is
disabled.

@item Octave:array-to-vector
If the @code{Octave:array-to-vector} warning is enabled, Octave will
warn when an implicit conversion from an array to a vector value is
attempted.  By default, the @code{Octave:array-to-vector} warning is
disabled.

@item Octave:assign-as-truth-value
If the @code{Octave:assign-as-truth-value} warning is
enabled, a warning is issued for statements like

@example
if (s = t)
  ...
@end example

@noindent
since such statements are not common, and it is likely that the intent
was to write

@example
if (s == t)
  ...
@end example

@noindent
instead.

There are times when it is useful to write code that contains
assignments within the condition of a @code{while} or @code{if}
statement.  For example, statements like

@example
while (c = getc())
  ...
@end example

@noindent
are common in C programming.

It is possible to avoid all warnings about such statements by
disabling the @code{Octave:assign-as-truth-value} warning,
but that may also let real errors like

@example
if (x = 1)  # intended to test (x == 1)!
  ...
@end example

@noindent
slip by.

In such cases, it is possible suppress errors for specific statements by
writing them with an extra set of parentheses.  For example, writing the
previous example as

@example
while ((c = getc()))
  ...
@end example

@noindent
will prevent the warning from being printed for this statement, while
allowing Octave to warn about other assignments used in conditional
contexts.

By default, the @code{Octave:assign-as-truth-value} warning is enabled.

@item Octave:associativity-change
If the @code{Octave:associativity-change} warning is
enabled, Octave will warn about possible changes in the meaning of
some code due to changes in associativity for some operators.
Associativity changes have typically been made for @sc{Matlab}
compatibility.  By default, the @code{Octave:associativity-change}
warning is enabled.

@item Octave:divide-by-zero
If the @code{Octave:divide-by-zero} warning is enabled, a
warning is issued when Octave encounters a division by zero.  By
default, the @code{Octave:divide-by-zero} warning is enabled.

@item Octave:empty-list-elements
If the @code{Octave:empty-list-elements} warning is enabled, a
warning is issued when an empty matrix is found in a matrix list.
For example,

@example
a = [1, [], 3, [], 5]
@end example

@noindent
By default, the @code{Octave:empty-list-elements} warning is enabled.

@item Octave:fortran-indexing
If the @code{Octave:fortran-indexing} warning is enabled, a warning is
printed for expressions which select elements of a two-dimensional matrix
using a single index.  By default, the @code{Octave:fortran-indexing}
warning is disabled.

@item Octave:function-name-clash
If the @code{Octave:function-name-clash} warning is enabled, a
warning is issued when Octave finds that the name of a function
defined in a function file differs from the name of the file.  (If
the names disagree, the name declared inside the file is ignored.)
By default, the @code{Octave:function-name-clash} warning is enabled.

@item Octave:future-time-stamp
If the @code{Octave:future-time-stamp} warning is enabled, Octave
will print a warning if it finds a function file with a time stamp
that is in the future.  By default, the
@code{Octave:future-time-stamp} warning is enabled.

@item Octave:imag-to-real
If the @code{Octave:imag-to-real} warning is enabled, a warning is
printed for implicit conversions of complex numbers to real numbers.
By default, the @code{Octave:imag-to-real} warning is disabled.

@item Octave:matlab-incompatible
Print warnings for Octave language features that may cause
compatibility problems with @sc{Matlab}.

@item Octave:missing-semicolon
If the @code{Octave:missing-semicolon} warning is enabled, Octave
will warn when statements in function definitions don't end in
semicolons.  By default the @code{Octave:missing-semicolon} warning
is disabled.

@item Octave:neg-dim-as-zero
If the @code{Octave:neg-dim-as-zero} warning is enabled, print a warning
for expressions like

@example
eye (-1)
@end example

@noindent
By default, the @code{Octave:neg-dim-as-zero} warning is disabled.

@item Octave:num-to-str
If the @code{Octave:num-to-str} warning is enable, a warning is
printed for implicit conversions of numbers to their ASCII character
equivalents when strings are constructed using a mixture of strings and
numbers in matrix notation.  For example,

@example
@group
[ "f", 111, 111 ]
@result{} "foo"
@end group
@end example
elicits a warning if the @code{Octave:num-to-str} warning is
enabled.  By default, the @code{Octave:num-to-str} warning is enabled.

@item Octave:precedence-change
If the @code{Octave:precedence-change} warning is enabled, Octave
will warn about possible changes in the meaning of some code due to
changes in precedence for some operators.  Precedence changes have
typically been made for @sc{Matlab} compatibility.  By default, the
@code{Octave:precedence-change} warning is enabled.

@item Octave:reload-forces-clear
If several functions have been loaded from the same file, Octave must
clear all the functions before any one of them can be reloaded.  If
the @code{Octave:reload-forces-clear} warning is enabled, Octave will
warn you when this happens, and print a list of the additional
functions that it is forced to clear.  By default, the
@code{Octave:reload-forces-clear} warning is enabled.

@item Octave:resize-on-range-error
If the @code{Octave:resize-on-range-error} warning is enabled, print a
warning when a matrix is resized by an indexed assignment with
indices outside the current bounds.  By default, the
@code{Octave:resize-on-range-error} warning is disabled.

@item Octave:separator-insert
Print warning if commas or semicolons might be inserted
automatically in literal matrices.

@item Octave:single-quote-string
Print warning if a signle quote character is used to introduce a
string constant.

@item Octave:str-to-num
If the @code{Octave:str-to-num} warning is enabled, a warning is printed
for implicit conversions of strings to their numeric ASCII equivalents.
For example,
@example
@group
"abc" + 0
@result{} 97 98 99
@end group
@end example
elicits a warning if the @code{Octave:str-to-num} warning is enabled.
By default, the @code{Octave:str-to-num} warning is disabled.

@item Octave:string-concat
If the @code{Octave:string-concat} warning is enabled, print a
warning when concatenating a mixture of double and single quoted strings.
By default, the @code{Octave:string-concat} warning is disabled.

@item Octave:undefined-return-values
If the @code{Octave:undefined-return-values} warning is disabled,
print a warning if a function does not define all the values in
the return list which are expected.  By default, the
@code{Octave:undefined-return-values} warning is enabled.

@item Octave:variable-switch-label
If the @code{Octave:variable-switch-label} warning is enabled, Octave
will print a warning if a switch label is not a constant or constant
expression.  By default, the @code{Octave:variable-switch-label}
warning is disabled.
@end table


@anchor{doc-errno}
@deftypefn {Built-in Function} {@var{err} =} errno ()
@deftypefnx {Built-in Function} {@var{err} =} errno (@var{val})
@deftypefnx {Built-in Function} {@var{err} =} errno (@var{name})
Return the current value of the system-dependent variable errno,
set its value to @var{val} and return the previous value, or return
the named error code given @var{name} as a character string, or -1
if @var{name} is not found.
@end deftypefn


@anchor{doc-errno_list}
@deftypefn {Built-in Function} {} errno_list ()
Return a structure containing the system-dependent errno values.
@end deftypefn


The following pair of functions are of limited usefulness, and may be
removed from future versions of Octave.

@anchor{doc-perror}
@deftypefn {Function File} {} perror (@var{name}, @var{num})
Print the error message for function @var{name} corresponding to the
error number @var{num}.  This function is intended to be used to print
useful error messages for those functions that return numeric error
codes.
@seealso{strerror}
@end deftypefn


@anchor{doc-strerror}
@deftypefn {Function File} {} strerror (@var{name}, @var{num})
Return the text of an error message for function @var{name}
corresponding to the error number @var{num}.  This function is intended
to be used to print useful error messages for those functions that
return numeric error codes.
@end deftypefn

