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<p>
Next: <a rel="next" accesskey="n" href="Recovering-From-Errors.html#Recovering-From-Errors">Recovering From Errors</a>,
Previous: <a rel="previous" accesskey="p" href="Raising-Errors.html#Raising-Errors">Raising Errors</a>,
Up: <a rel="up" accesskey="u" href="Handling-Errors.html#Handling-Errors">Handling Errors</a>
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<h4 class="subsection">12.1.2 Catching Errors</h4>
<p>When an error occurs, it can be detected and handled using the
<code>try</code> statement as described in <a href="The-try-Statement.html#The-try-Statement">The try Statement</a>.
As an example, the following piece of code counts the number of errors
that occurs during a <code>for</code> loop.
<pre class="example"> number_of_errors = 0;
for n = 1:100
try
...
catch
number_of_errors++;
end_try_catch
endfor
</pre>
<p>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</code>
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><span class="samp">*</span></samp>’ operator.
<pre class="example"> number_of_errors = 0;
for n = 1:100
try
...
catch
msg = lasterror.message;
if (strfind (msg, "operator *"))
number_of_errors++;
endif
end_try_catch
endfor
</pre>
<!-- lasterror src/error.cc -->
<p><a name="doc_002dlasterror"></a>
<div class="defun">
— Built-in Function: <var>lasterr</var> = <b>lasterror</b> ()<var><a name="index-lasterror-810"></a></var><br>
— Built-in Function: <b>lasterror</b> (<var>err</var>)<var><a name="index-lasterror-811"></a></var><br>
— Built-in Function: <b>lasterror</b> (<var>'reset'</var>)<var><a name="index-lasterror-812"></a></var><br>
<blockquote><p>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:
<dl>
<dt>'message'<dd>The text of the last error message
<br><dt>'identifier'<dd>The message identifier of this error message
<br><dt>'stack'<dd>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:
<dl>
<dt>'file'<dd>The name of the file where the error occurred
<br><dt>'name'<dd>The name of function in which the error occurred
<br><dt>'line'<dd>The line number at which the error occurred
<br><dt>'column'<dd>An optional field with the column number at which the error occurred
</dl>
</dl>
<p>The last error structure may be set by passing a scalar structure, <var>err</var>,
as input. Any fields of <var>err</var> that match those above are set while any
unspecified fields are initialized with default values.
<p>If <code>lasterror</code> is called with the argument 'reset', all fields are
set to their default values.
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<p class="noindent"><strong>See also:</strong> <a href="doc_002dlasterr.html#doc_002dlasterr">lasterr</a>.
</p></blockquote></div>
<!-- lasterr src/error.cc -->
<p><a name="doc_002dlasterr"></a>
<div class="defun">
— Built-in Function: [<var>msg</var>, <var>msgid</var>] = <b>lasterr</b> ()<var><a name="index-lasterr-813"></a></var><br>
— Built-in Function: <b>lasterr</b> (<var>msg</var>)<var><a name="index-lasterr-814"></a></var><br>
— Built-in Function: <b>lasterr</b> (<var>msg, msgid</var>)<var><a name="index-lasterr-815"></a></var><br>
<blockquote><p>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</var>. With two arguments,
also set the last message identifier.
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<p class="noindent"><strong>See also:</strong> <a href="doc_002dlasterror.html#doc_002dlasterror">lasterror</a>.
</p></blockquote></div>
<p>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</code> function. The
previous example can now be changed to count the number of errors
related to the ‘<samp><span class="samp">*</span></samp>’ operator, but still abort if another kind of
error occurs.
<pre class="example"> number_of_errors = 0;
for n = 1:100
try
...
catch
msg = lasterror.message;
if (strfind (msg, "operator *"))
number_of_errors++;
else
rethrow (lasterror);
endif
end_try_catch
endfor
</pre>
<!-- rethrow src/error.cc -->
<p><a name="doc_002drethrow"></a>
<div class="defun">
— Built-in Function: <b>rethrow</b> (<var>err</var>)<var><a name="index-rethrow-816"></a></var><br>
<blockquote><p>Reissue a previous error as defined by <var>err</var>. <var>err</var> is a structure
that must contain at least the 'message' and 'identifier' fields. <var>err</var>
can also contain a field 'stack' that gives information on the assumed
location of the error. Typically <var>err</var> is returned from
<code>lasterror</code>.
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<p class="noindent"><strong>See also:</strong> <a href="doc_002dlasterror.html#doc_002dlasterror">lasterror</a>, <a href="doc_002dlasterr.html#doc_002dlasterr">lasterr</a>, <a href="doc_002derror.html#doc_002derror">error</a>.
</p></blockquote></div>
<!-- FIXME: I have no idea what the rest of the functions are used for... -->
<!-- errno src/utils.cc -->
<p><a name="doc_002derrno"></a>
<div class="defun">
— Built-in Function: <var>err</var> = <b>errno</b> ()<var><a name="index-errno-817"></a></var><br>
— Built-in Function: <var>err</var> = <b>errno</b> (<var>val</var>)<var><a name="index-errno-818"></a></var><br>
— Built-in Function: <var>err</var> = <b>errno</b> (<var>name</var>)<var><a name="index-errno-819"></a></var><br>
<blockquote><p>Return the current value of the system-dependent variable errno,
set its value to <var>val</var> and return the previous value, or return
the named error code given <var>name</var> as a character string, or -1
if <var>name</var> is not found.
</p></blockquote></div>
<!-- errno_list src/utils.cc -->
<p><a name="doc_002derrno_005flist"></a>
<div class="defun">
— Built-in Function: <b>errno_list</b> ()<var><a name="index-errno_005flist-820"></a></var><br>
<blockquote><p>Return a structure containing the system-dependent errno values.
</p></blockquote></div>
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