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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 @code{try} Statement}.

@menu
* Raising Errors::
* Catching 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 src/error.cc
@anchor{doc-error}
@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 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: 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 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


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 on-line 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{doc-print_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{doc-help,,help}}
@end deftypefn


@c usage src/error.cc
@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


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


@c beep_on_error src/error.cc
@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


@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 @code{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

@c lasterror src/error.cc
@anchor{doc-lasterror}
@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.  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 occurred.  This might
be an empty structure if this in the case where this information cannot
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 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 @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


@c lasterr src/error.cc
@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


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 src/error.cc
@anchor{doc-rethrow}
@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 '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{@ref{doc-lasterror,,lasterror}, @ref{doc-lasterr,,lasterr}, @ref{doc-error,,error}}
@end deftypefn


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

@c errno src/utils.cc
@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


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

@c warning src/error.cc
@anchor{doc-warning}
@deftypefn  {Built-in Function} {} warning (@var{template}, @dots{})
@deftypefnx {Built-in Function} {} warning (@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{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{@ref{doc-warning_ids,,warning_ids}}
@end deftypefn


@c lastwarn src/error.cc
@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


@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 @code{"on"} or @code{"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 ("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", "non-negative-variable");
warning ("non-negative-variable", 
         "'a' must be non-negative.  Setting 'a' to zero.");
@end group
@end example

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

@c warning_ids scripts/miscellaneous/warning_ids.m
@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
@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: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

@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: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 single 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

@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: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



