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<h4 class="subsection">12.2.1 Issuing Warnings</h4>
<p>It is possible to issue warnings from any code using the <code>warning</code>
function. In its most simple form, the <code>warning</code> function takes a
string describing the warning as its input argument. As an example,
the following code controls if the variable ‘<samp><span class="samp">a</span></samp>’ is non-negative,
and if not issues a warning and sets ‘<samp><span class="samp">a</span></samp>’ to zero.
<pre class="example"> a = -1;
if (a < 0)
warning ("'a' must be non-negative. Setting 'a' to zero.");
a = 0;
endif
-| 'a' must be non-negative. Setting 'a' to zero.
</pre>
<p>Since warnings aren't fatal to a running program, it is not possible
to catch a warning using the <code>try</code> statement or something similar.
It is however possible to access the last warning as a string using the
<code>lastwarn</code> function.
<p>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</code> 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 "NAMESPACE:WARNING-NAME". The namespace
"Octave" is used for Octave's own warnings. Any other string is available
as a namespace for user's own warnings.
<!-- warning src/error.cc -->
<p><a name="doc_002dwarning"></a>
<div class="defun">
— Built-in Function: <b>warning</b> (<var>template, <small class="dots">...</small></var>)<var><a name="index-warning-822"></a></var><br>
— Built-in Function: <b>warning</b> (<var>id, template, <small class="dots">...</small></var>)<var><a name="index-warning-823"></a></var><br>
— Built-in Function: <b>warning</b> (<var>"on", id</var>)<var><a name="index-warning-824"></a></var><br>
— Built-in Function: <b>warning</b> (<var>"off", id</var>)<var><a name="index-warning-825"></a></var><br>
— Built-in Function: <b>warning</b> (<var>"query", id</var>)<var><a name="index-warning-826"></a></var><br>
— Built-in Function: <b>warning</b> (<var>"error", id</var>)<var><a name="index-warning-827"></a></var><br>
<blockquote><p>Format the optional arguments under the control of the template string
<var>template</var> using the same rules as the <code>printf</code> family of
functions (see <a href="Formatted-Output.html#Formatted-Output">Formatted Output</a>) and print the resulting message
on the <code>stderr</code> stream. The message is prefixed by the character
string ‘<samp><span class="samp">warning: </span></samp>’.
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.
<p>The optional message identifier allows users to enable or disable
warnings tagged by <var>id</var>. A message identifier is of the form
"NAMESPACE:WARNING-NAME". Octave's own warnings use the "Octave"
namespace (see <a href="doc_002dwarning_005fids.html#doc_002dwarning_005fids">doc-warning_ids</a>). The special identifier ‘<samp><span class="samp">"all"</span></samp>’
may be used to set the state of all warnings.
<p>If the first argument is ‘<samp><span class="samp">"on"</span></samp>’ or ‘<samp><span class="samp">"off"</span></samp>’, set the state
of a particular warning using the identifier <var>id</var>. If the first
argument is ‘<samp><span class="samp">"query"</span></samp>’, query the state of this warning instead.
If the identifier is omitted, a value of ‘<samp><span class="samp">"all"</span></samp>’ is assumed. If
you set the state of a warning to ‘<samp><span class="samp">"error"</span></samp>’, the warning named by
<var>id</var> is handled as if it were an error instead. So, for example, the
following handles all warnings as errors:
<pre class="example"> warning ("error");
</pre>
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<p class="noindent"><strong>See also:</strong> <a href="doc_002dwarning_005fids.html#doc_002dwarning_005fids">warning_ids</a>.
</p></blockquote></div>
<!-- lastwarn src/error.cc -->
<p><a name="doc_002dlastwarn"></a>
<div class="defun">
— Built-in Function: [<var>msg</var>, <var>msgid</var>] = <b>lastwarn</b> (<var>msg, msgid</var>)<var><a name="index-lastwarn-828"></a></var><br>
<blockquote><p>Without any arguments, return the last warning message. With one
argument, set the last warning message to <var>msg</var>. With two arguments,
also set the last message identifier.
</p></blockquote></div>
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