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@node Errors and Warnings
@chapter Errors and Warnings

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.

Since many of Octave's functions use these functions, it is also useful
to understand them, so that errors and warnings can be handled.

@menu
* Handling Errors::
* Handling Warnings::
@end menu

@node Handling Errors
@section Handling Errors

An error is something that occurs when a program is in a state where
it doesn't make sense to continue.  An example is when a function is
called with too few input arguments.  In this situation the function
should abort with an error message informing the user of the lacking
input arguments.

Since an error can occur during the evaluation of a program, it is
very convenient to be able to detect that an error occurred, so that
the error can be fixed.  This is possible with the @code{try} statement
described in @ref{The try Statement}.

@menu
* Raising Errors::
* Catching Errors::
* Recovering From Errors::
@end menu

@node Raising Errors
@subsection Raising Errors

The most common use of errors is for checking input arguments to
functions.  The following example calls the @code{error} function if
the function @code{f} is called without any input arguments.

@example
@group
function f (arg1)
  if (nargin == 0)
    error ("not enough input arguments");
  endif
endfunction
@end group
@end example

When the @code{error} function is called, it prints the given message
and returns to the Octave prompt.  This means that no code following
a call to @code{error} will be executed.

@c error libinterp/corefcn/error.cc
@anchor{XREFerror}
@deftypefn  {Built-in Function} {} error (@var{template}, @dots{})
@deftypefnx {Built-in Function} {} error (@var{id}, @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 newline 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: called from:
error:   error at line -1, column -1
error:   h at line 1, column 27
error:   g at line 1, column 15
error:   f at line 1, column 15
@end group
@end example

If the error message ends in a newline 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 newline 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

A null string ("") input to @code{error} will be ignored and the code
will continue running as if the statement were a NOP@.  This is for
compatibility with @sc{matlab}.  It also makes it possible to write code such
as

@example
@group
err_msg = "";
if (CONDITION 1)
  err_msg = "CONDITION 1 found";
elseif (CONDITION2)
  err_msg = "CONDITION 2 found";
@dots{}
endif
error (err_msg);
@end group
@end example

@noindent
which will only stop execution if an error has been found.

Implementation Note: For compatibility with @sc{matlab}, escape
sequences in @var{template} (e.g., "\n" => newline) are processed
regardless of whether @var{template} has been defined with single quotes,
as long as there are two or more input arguments.  To disable escape sequence
expansion use a second backslash before the sequence (e.g., "\\n") or
use the @code{regexptranslate} function.
@seealso{@ref{XREFwarning,,warning}, @ref{XREFlasterror,,lasterror}}
@end deftypefn


Since it is common to use errors when there is something wrong with
the input to a function, Octave supports functions to simplify such code.
When the @code{print_usage} function is called, it reads the help text
of the function calling @code{print_usage}, and presents a useful error.
If the help text is written in Texinfo it is possible to present an
error message that only contains the function prototypes as described
by the @code{@@deftypefn} parts of the help text.  When the help text
isn't written in Texinfo, the error message contains the entire help
message.

Consider the following function.

@example
@group
## -*- texinfo -*-
## @@deftypefn @{Function File@} f (@@var@{arg1@})
## Function help text goes here@dots{}
## @@end deftypefn
function f (arg1)
  if (nargin == 0)
    print_usage ();
  endif
endfunction
@end group
@end example

@noindent
When it is called with no input arguments it produces the following
error.

@example
@group
f ()

@print{}  error: Invalid call to f.  Correct usage is:
@print{}
@print{}   -- Function File: f (ARG1)
@print{}
@print{}
@print{}  Additional help for built-in functions and operators is
@print{}  available in the online version of the manual.  Use the command
@print{}  `doc <topic>' to search the manual index.
@print{}
@print{}  Help and information about Octave is also available on the WWW
@print{}  at http://www.octave.org and via the help@@octave.org
@print{}  mailing list.
@end group
@end example

@c print_usage scripts/help/print_usage.m
@anchor{XREFprint_usage}
@deftypefn  {Function File} {} print_usage ()
@deftypefnx {Function File} {} print_usage (@var{name})
Print the usage message for a function.  When called with no input arguments
the @code{print_usage} function displays the usage message of the currently
executing function.
@seealso{@ref{XREFhelp,,help}}
@end deftypefn


@c beep scripts/io/beep.m
@anchor{XREFbeep}
@deftypefn {Function File} {} beep ()
Produce a beep from the speaker (or visual bell).
@seealso{@ref{XREFputs,,puts}, @ref{XREFfputs,,fputs}, @ref{XREFprintf,,printf}, @ref{XREFfprintf,,fprintf}}
@end deftypefn


@c beep_on_error libinterp/corefcn/error.cc
@anchor{XREFbeep_on_error}
@deftypefn  {Built-in Function} {@var{val} =} beep_on_error ()
@deftypefnx {Built-in Function} {@var{old_val} =} beep_on_error (@var{new_val})
@deftypefnx {Built-in Function} {} beep_on_error (@var{new_val}, "local")
Query or set the internal variable that controls whether Octave will try
to ring the terminal bell before printing an error message.

When called from inside a function with the @qcode{"local"} option, the
variable is changed locally for the function and any subroutines it calls.  
The original variable value is restored when exiting the function.
@end deftypefn


@node Catching Errors
@subsection Catching Errors

When an error occurs, it can be detected and handled using the
@code{try} statement as described in @ref{The try Statement}.
As an example, the following piece of code counts the number of errors
that occurs during a @code{for} loop.

@example
@group
number_of_errors = 0;
for n = 1:100
  try
    @dots{}
  catch
    number_of_errors++;
  end_try_catch
endfor
@end group
@end example

The above example treats all errors the same.  In many situations it
can however be necessary to discriminate between errors, and take
different actions depending on the error.  The @code{lasterror}
function returns a structure containing information about the last
error that occurred.  As an example, the code above could be changed
to count the number of errors related to the @samp{*} operator.

@example
@group
number_of_errors = 0;
for n = 1:100
  try
    @dots{}
  catch
    msg = lasterror.message;
    if (strfind (msg, "operator *"))
      number_of_errors++;
    endif
  end_try_catch
endfor
@end group
@end example

@noindent
Alternatively, the output of the @code{lasterror} function can be found
in a variable indicated immediately after the @code{catch} keyword, as
in the example below showing how to redirect an error as a warning:

@example
@group
try
  @dots{}
catch err
  warning(err.identifier, err.message);
  @dots{}
end_try_catch
@end group
@end example

@c lasterror libinterp/corefcn/error.cc
@anchor{XREFlasterror}
@deftypefn  {Built-in Function} {@var{lasterr} =} lasterror ()
@deftypefnx {Built-in Function} {} lasterror (@var{err})
@deftypefnx {Built-in Function} {} lasterror ("reset")
Query or set the last error message structure.  When called without
arguments, return a structure containing the last error message and other
information related to this error.  The elements of the structure are:

@table @code
@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 occurred.  This may
be an empty structure if the information cannot
be obtained.  The fields of the structure are:

@table @code
@item file
The name of the file where the error occurred

@item name
The name of function in which the error occurred

@item line
The line number at which the error occurred

@item column
An optional field with the column number at which the error occurred
@end table
@end table

The last error structure may be set by passing a scalar structure, @var{err},
as input.  Any fields of @var{err} that match those above are set while any
unspecified fields are initialized with default values.

If @code{lasterror} is called with the argument @qcode{"reset"}, all
fields are set to their default values.
@seealso{@ref{XREFlasterr,,lasterr}, @ref{XREFerror,,error}, @ref{XREFlastwarn,,lastwarn}}
@end deftypefn


@c lasterr libinterp/corefcn/error.cc
@anchor{XREFlasterr}
@deftypefn  {Built-in Function} {[@var{msg}, @var{msgid}] =} lasterr ()
@deftypefnx {Built-in Function} {} lasterr (@var{msg})
@deftypefnx {Built-in Function} {} lasterr (@var{msg}, @var{msgid})
Query or set the last error message.  When called without input arguments,
return the last error message and message identifier.  With one
argument, set the last error message to @var{msg}.  With two arguments,
also set the last message identifier.
@seealso{@ref{XREFlasterror,,lasterror}, @ref{XREFerror,,error}, @ref{XREFlastwarn,,lastwarn}}
@end deftypefn


It is also possible to assign an identification string to an error.
If an error has such an ID the user can catch this error
as will be shown in the next example.  To assign an ID to an error,
simply call @code{error} with two string arguments, where the first
is the identification string, and the second is the actual error.  Note
that error IDs are in the format @qcode{"NAMESPACE:ERROR-NAME"}.  The namespace
@qcode{"Octave"} is used for Octave's own errors.  Any other string is available
as a namespace for user's own errors.

The next example counts indexing errors.  The errors are caught using the
field identifier of the structure returned by the function @code{lasterror}.

@example
@group
number_of_errors = 0;
for n = 1:100
  try
    @dots{}
  catch
    id = lasterror.identifier;
    if (strcmp (id, "Octave:invalid-indexing"))
      number_of_errors++;
    endif
  end_try_catch
endfor
@end group
@end example

The functions distributed with Octave can issue one of the following
errors.

@c error_ids scripts/miscellaneous/error_ids.m
@anchor{XREFerror_ids}
@cindex error ids

@table @code
@item Octave:invalid-context
Indicates the error was generated by an operation that cannot be executed in
the scope from which it was called.  For example, the function
@code{print_usage ()} when called from the Octave prompt raises this error.

@item Octave:invalid-input-arg
Indicates that a function was called with invalid input arguments.

@item Octave:invalid-fun-call
Indicates that a function was called in an incorrect way, e.g., wrong number
of input arguments.

@item Octave:invalid-indexing
Indicates that a data-type was indexed incorrectly, e.g., real-value index
for arrays, nonexistent field of a structure.

@item Octave:bad-alloc
Indicates that memory couldn't be allocated.

@item Octave:undefined-function
Indicates a call to a function that is not defined.  The function may
exist but Octave is unable to find it in the search path.

@end table



When an error has been handled it is possible to raise it again.  This
can be useful when an error needs to be detected, but the program should
still abort.  This is possible using the @code{rethrow} function.  The
previous example can now be changed to count the number of errors
related to the @samp{*} operator, but still abort if another kind of
error occurs.

@example
@group
number_of_errors = 0;
for n = 1:100
  try
    @dots{}
  catch
    msg = lasterror.message;
    if (strfind (msg, "operator *"))
      number_of_errors++;
    else
      rethrow (lasterror);
    endif
  end_try_catch
endfor
@end group
@end example

@c rethrow libinterp/corefcn/error.cc
@anchor{XREFrethrow}
@deftypefn {Built-in Function} {} rethrow (@var{err})
Reissue a previous error as defined by @var{err}.  @var{err} is a structure
that must contain at least the @qcode{"message"} and @qcode{"identifier"}
fields.  @var{err} can also contain a field @qcode{"stack"} that gives
information on the assumed location of the error.  Typically @var{err} is
returned from @code{lasterror}.
@seealso{@ref{XREFlasterror,,lasterror}, @ref{XREFlasterr,,lasterr}, @ref{XREFerror,,error}}
@end deftypefn


@c FIXME: I have no idea what the rest of the functions are used for...

@c errno libinterp/corefcn/utils.cc
@anchor{XREFerrno}
@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


@c errno_list libinterp/corefcn/utils.cc
@anchor{XREFerrno_list}
@deftypefn {Built-in Function} {} errno_list ()
Return a structure containing the system-dependent errno values.
@end deftypefn


@node Recovering From Errors
@subsection Recovering From Errors

Octave provides several ways of recovering from errors.  There are
@code{try}/@code{catch} blocks,
@code{unwind_protect}/@code{unwind_protect_cleanup} blocks,
and finally the @code{onCleanup} command.

The @code{onCleanup} command associates an ordinary Octave variable (the
trigger) with an arbitrary function (the action).  Whenever the Octave variable
ceases to exist---whether due to a function return, an error, or simply because
the variable has been removed with @code{clear}---then the assigned function
is executed.

The function can do anything necessary for cleanup such as closing open file
handles, printing an error message, or restoring global variables to their
initial values.  The last example is a very convenient idiom for Octave code.
For example:

@example
@group
function rand42
  old_state = rand ("state");
  restore_state = onCleanup (@@() rand ("state", old_state));
  rand ("state", 42);
  @dots{}
endfunction  # rand generator state restored by onCleanup
@end group
@end example

@c onCleanup libinterp/octave-value/ov-oncleanup.cc
@anchor{XREFonCleanup}
@deftypefn {Built-in Function} {@var{obj} =} onCleanup (@var{function})
Create a special object that executes a given function upon destruction.
If the object is copied to multiple variables (or cell or struct array
elements) or returned from a function, @var{function} will be executed after
clearing the last copy of the object.  Note that if multiple local onCleanup
variables are created, the order in which they are called is unspecified.
For similar functionality @xref{The unwind_protect Statement}.
@end deftypefn


@node Handling Warnings
@section Handling Warnings

Like an error, a warning is issued when something unexpected happens.
Unlike an error, a warning doesn't abort the currently running program.
A simple example of a warning is when a number is divided by zero.  In
this case Octave will issue a warning and assign the value @code{Inf}
to the result.

@example
@group
a = 1/0
     @print{} warning: division by zero
     @result{} a = Inf
@end group
@end example

@menu
* Issuing Warnings::
* Enabling and Disabling Warnings::
@end menu

@node Issuing Warnings
@subsection Issuing Warnings

It is possible to issue warnings from any code using the @code{warning}
function.  In its most simple form, the @code{warning} function takes a
string describing the warning as its input argument.  As an example,
the following code controls if the variable @samp{a} is non-negative,
and if not issues a warning and sets @samp{a} to zero.

@example
@group
a = -1;
if (a < 0)
  warning ("'a' must be non-negative.  Setting 'a' to zero.");
  a = 0;
endif
     @print{} 'a' must be non-negative.  Setting 'a' to zero.
@end group
@end example

Since warnings aren't fatal to a running program, it is not possible
to catch a warning using the @code{try} statement or something similar.
It is however possible to access the last warning as a string using the
@code{lastwarn} function.

It is also possible to assign an identification string to a warning.
If a warning has such an ID the user can enable and disable this warning
as will be described in the next section.  To assign an ID to a warning,
simply call @code{warning} with two string arguments, where the first
is the identification string, and the second is the actual warning.  Note
that warning IDs are in the format @qcode{"NAMESPACE:WARNING-NAME"}.  The
namespace @qcode{"Octave"} is used for Octave's own warnings.  Any other string
is available as a namespace for user's own warnings.

@c warning libinterp/corefcn/error.cc
@anchor{XREFwarning}
@deftypefn  {Built-in Function} {} warning (@var{template}, @dots{})
@deftypefnx {Built-in Function} {} warning (@var{id}, @var{template}, @dots{})
@deftypefnx {Built-in Function} {} warning ("on", @var{id})
@deftypefnx {Built-in Function} {} warning ("off", @var{id})
@deftypefnx {Built-in Function} {} warning ("query", @var{id})
@deftypefnx {Built-in Function} {} warning ("error", @var{id})
@deftypefnx {Built-in Function} {} warning (@var{state}, "backtrace")
@deftypefnx {Built-in Function} {} warning (@var{state}, @var{id}, "local")
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}.  A message identifier is of the form
"NAMESPACE:WARNING-NAME".  Octave's own warnings use the @qcode{"Octave"}
namespace (@pxref{XREFwarning_ids}).  The special identifier @qcode{"all"}
may be used to set the state of all warnings.

If the first argument is @qcode{"on"} or @qcode{"off"},
set the state of a particular warning using the identifier @var{id}.  If the
first argument is @qcode{"query"}, query the state of this warning
instead.  If the identifier is omitted, a value of @qcode{"all"} is
assumed.  If you set the state of a warning to @qcode{"error"}, the
warning named by @var{id} is handled as if it were an error instead.  So,
for example, the following handles all warnings as errors:

@example
@group
warning ("error");
@end group
@end example

If the state is @qcode{"on"} or @qcode{"off"} and the third argument
is @qcode{"backtrace"}, then a stack trace is printed along with the
warning message when warnings occur inside function calls.  This option
is enabled by default.

If the state is @qcode{"on"}, @qcode{"off"}, or @qcode{"error"}
and the third argument is @qcode{"local"}, then the warning state
will be set temporarily, until the end of the current function.
Changes to warning states that are set locally affect the current
function and all functions called from the current scope.  The
previous warning state is restored on return from the current
function.  The @qcode{"local"} option is ignored if used in the top-level
workspace.

Implementation Note: For compatibility with @sc{matlab}, escape
sequences in @var{template} (e.g., "\n" => newline) are processed
regardless of whether @var{template} has been defined with single quotes,
as long as there are two or more input arguments.  To disable escape sequence
expansion use a second backslash before the sequence (e.g., "\\n") or
use the @code{regexptranslate} function.
@seealso{@ref{XREFwarning_ids,,warning_ids}, @ref{XREFlastwarn,,lastwarn}, @ref{XREFerror,,error}}
@end deftypefn


@c lastwarn libinterp/corefcn/error.cc
@anchor{XREFlastwarn}
@deftypefn  {Built-in Function} {[@var{msg}, @var{msgid}] =} lastwarn ()
@deftypefnx {Built-in Function} {} lastwarn (@var{msg})
@deftypefnx {Built-in Function} {} lastwarn (@var{msg}, @var{msgid})
Query or set the last warning message.  When called without input arguments,
return the last warning message and message identifier.  With one
argument, set the last warning message to @var{msg}.  With two arguments,
also set the last message identifier.
@seealso{@ref{XREFwarning,,warning}, @ref{XREFlasterror,,lasterror}, @ref{XREFlasterr,,lasterr}}
@end deftypefn


The functions distributed with Octave can issue one of the following
warnings.

@c warning_ids scripts/miscellaneous/warning_ids.m
@anchor{XREFwarning_ids}
@cindex warning ids

@table @code
@item Octave:abbreviated-property-match
By default, the @code{Octave:abbreviated-property-match} warning is enabled.

@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
@group
if (s = t)
  @dots{}
@end group
@end example

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

@example
@group
if (s == t)
  @dots{}
@end group
@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
@group
while (c = getc ())
  @dots{}
@end group
@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
@group
if (x = 1)  # intended to test (x == 1)!
  @dots{}
@end group
@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
@group
while ((c = getc ()))
  @dots{}
@end group
@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:autoload-relative-file-name
If the @code{Octave:autoload-relative-file-name} is enabled,
Octave will warn when parsing autoload() function calls with relative
paths to function files.  This usually happens when using autoload()
calls in PKG_ADD files, when the PKG_ADD file is not in the same
directory as the .oct file referred to by the autoload() command.
By default, the @code{Octave:autoload-relative-file-name} warning is enabled.

@item Octave:built-in-variable-assignment
By default, the @code{Octave:built-in-variable-assignment} warning is
enabled.

@item Octave:deprecated-keyword
If the @code{Octave:deprecated-keyword} warning is enabled, a
warning is issued when Octave encounters a keyword that is obsolete and
scheduled for removal from Octave.
By default, the @code{Octave:deprecated-keyword} 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:fopen-file-in-path
By default, the @code{Octave:fopen-file-in-path} warning is enabled.

@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:glyph-render
By default, the @code{Octave:glyph-render} 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:language-extension
Print warnings when using features that are unique to the Octave
language and may be still be missing in @sc{Matlab}.
By default, the @code{Octave:language-extension} warning is disabled.
The --traditional or --braindead startup options for Octave may also
be of use, @pxref{Command Line Options}.

@item Octave:load-file-in-path
By default, the @code{Octave:load-file-in-path} warning is enabled.

@item Octave:logical-conversion
By default, the @code{Octave:logical-conversion} warning is enabled.

@item Octave:md5sum-file-in-path
By default, the @code{Octave:md5sum-file-in-path} warning is enabled.

@item Octave:missing-glyph
By default, the @code{Octave:missing-glyph} warning is enabled.

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

@item Octave:noninteger-range-as-index
By default, the @code{Octave:noninteger-range-as-index} warning is enabled.

@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

@noindent
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:possible-matlab-short-circuit-operator
If the @code{Octave:possible-matlab-short-circuit-operator} warning
is enabled, Octave will warn about using the not short circuiting
operators @code{&} and @code{|} inside @code{if} or @code{while}
conditions.  They normally never short circuit, but @sc{matlab} always
short circuits if any logical operators are used in a condition.  You
can turn on the option

@example
@group
do_braindead_shortcircuit_evaluation (1)
@end group
@end example

@noindent
if you would like to enable this short-circuit evaluation in
Octave.  Note that the @code{&&} and @code{||} operators always short
circuit in both Octave and @sc{matlab}, so it's only necessary to
enable @sc{matlab}-style short-circuiting if it's too arduous to modify
existing code that relies on this behavior.
By default, the @code{Octave:possible-matlab-short-circuit-operator} 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:recursive-path-search
By default, the @code{Octave:recursive-path-search} warning is enabled.

@item Octave:remove-init-dir
The @code{path} function changes the search path that Octave uses
to find functions.  It is possible to set the path to a value which
excludes Octave's own built-in functions.  If the
@code{Octave:remove-init-dir} warning is enabled then Octave will warn
when the @code{path} function has been used in a way that may render
Octave unworkable.
By default, the @code{Octave:remove-init-dir} 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.
By default, the @code{Octave:separator-insert} warning is disabled.

@item Octave:shadowed-function
By default, the @code{Octave:shadowed-function} warning is enabled.

@item Octave:single-quote-string
Print warning if a single quote character is used to introduce a
string constant.
By default, the @code{Octave:single-quote-string} warning is disabled.

@item  Octave:nearly-singular-matrix
@itemx Octave:singular-matrix
By default, the @code{Octave:nearly-singular-matrix} and
@code{Octave:singular-matrix} warnings are enabled.

@item Octave:sqrtm:SingularMatrix
By default, the @code{Octave:sqrtm:SingularMatrix} warning is enabled.

@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

@noindent
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: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



@node Enabling and Disabling Warnings
@subsection Enabling and Disabling Warnings

The @code{warning} function also allows you to control which warnings
are actually printed to the screen.  If the @code{warning} function
is called with a string argument that is either @qcode{"on"} or @qcode{"off"}
all warnings will be enabled or disabled.

It is also possible to enable and disable individual warnings through
their string identifications.  The following code will issue a warning

@example
@group
warning ("example:non-negative-variable",
         "'a' must be non-negative.  Setting 'a' to zero.");
@end group
@end example

@noindent
while the following won't issue a warning

@example
@group
warning ("off", "example:non-negative-variable");
warning ("example:non-negative-variable",
         "'a' must be non-negative.  Setting 'a' to zero.");
@end group
@end example


