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<p>
Next: <a href="Recovering-From-Errors.html" accesskey="n" rel="next">Recovering From Errors</a>, Previous: <a href="Raising-Errors.html" accesskey="p" rel="prev">Raising Errors</a>, Up: <a href="Handling-Errors.html" accesskey="u" rel="up">Handling Errors</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html" title="Index" rel="index">Index</a>]</p>
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<hr>
<h4 class="subsection" id="Catching-Errors-1"><span>12.1.2 Catching Errors<a class="copiable-link" href="#Catching-Errors-1"> ¶</a></span></h4>
<p>When an error occurs, it can be detected and handled using the
<code class="code">try</code> statement as described in <a class="ref" href="The-try-Statement.html">The try Statement</a>.
As an example, the following piece of code counts the number of errors
that occurs during a <code class="code">for</code> loop.
</p>
<div class="example">
<div class="group"><pre class="example-preformatted">number_of_errors = 0;
for n = 1:100
try
...
catch
number_of_errors++;
end_try_catch
endfor
</pre></div></div>
<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 class="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 class="samp">*</samp>’ operator.
</p>
<div class="example">
<div class="group"><pre class="example-preformatted">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></div></div>
<p>Alternatively, the output of the <code class="code">lasterror</code> function can be found
in a variable indicated immediately after the <code class="code">catch</code> keyword, as
in the example below showing how to redirect an error as a warning:
</p>
<div class="example">
<div class="group"><pre class="example-preformatted">try
...
catch err
warning(err.identifier, err.message);
...
end_try_catch
</pre></div></div>
<a class="anchor" id="XREFlasterror"></a><span style="display:block; margin-top:-4.5ex;"> </span>
<dl class="first-deftypefn">
<dt class="deftypefn" id="index-lasterror"><span><code class="def-type"><var class="var">lasterr</var> =</code> <strong class="def-name">lasterror</strong> <code class="def-code-arguments">()</code><a class="copiable-link" href="#index-lasterror"> ¶</a></span></dt>
<dt class="deftypefnx def-cmd-deftypefn" id="index-lasterror-1"><span><strong class="def-name">lasterror</strong> <code class="def-code-arguments">(<var class="var">err</var>)</code><a class="copiable-link" href="#index-lasterror-1"> ¶</a></span></dt>
<dt class="deftypefnx def-cmd-deftypefn" id="index-lasterror-2"><span><strong class="def-name">lasterror</strong> <code class="def-code-arguments">("reset")</code><a class="copiable-link" href="#index-lasterror-2"> ¶</a></span></dt>
<dd><p>Query or set the last error message structure.
</p>
<p>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:
</p>
<dl class="table">
<dt><code class="code">message</code></dt>
<dd><p>The text of the last error message
</p>
</dd>
<dt><code class="code">identifier</code></dt>
<dd><p>The message identifier of this error message
</p>
</dd>
<dt><code class="code">stack</code></dt>
<dd><p>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:
</p>
<dl class="table">
<dt><code class="code">file</code></dt>
<dd><p>The name of the file where the error occurred
</p>
</dd>
<dt><code class="code">name</code></dt>
<dd><p>The name of function in which the error occurred
</p>
</dd>
<dt><code class="code">line</code></dt>
<dd><p>The line number at which the error occurred
</p>
</dd>
<dt><code class="code">column</code></dt>
<dd><p>An optional field with the column number at which the error occurred
</p></dd>
</dl>
</dd>
</dl>
<p>The last error structure may be set by passing a scalar structure,
<var class="var">err</var>, as input. Any fields of <var class="var">err</var> that match those above are
set while any unspecified fields are initialized with default values.
</p>
<p>If <code class="code">lasterror</code> is called with the argument <code class="code">"reset"</code>, all
fields are set to their default values.
</p>
<p><strong class="strong">See also:</strong> <a class="ref" href="#XREFlasterr">lasterr</a>, <a class="ref" href="Raising-Errors.html#XREFerror">error</a>, <a class="ref" href="Issuing-Warnings.html#XREFlastwarn">lastwarn</a>.
</p></dd></dl>
<a class="anchor" id="XREFlasterr"></a><span style="display:block; margin-top:-4.5ex;"> </span>
<dl class="first-deftypefn">
<dt class="deftypefn" id="index-lasterr"><span><code class="def-type">[<var class="var">msg</var>, <var class="var">msgid</var>] =</code> <strong class="def-name">lasterr</strong> <code class="def-code-arguments">()</code><a class="copiable-link" href="#index-lasterr"> ¶</a></span></dt>
<dt class="deftypefnx def-cmd-deftypefn" id="index-lasterr-1"><span><strong class="def-name">lasterr</strong> <code class="def-code-arguments">(<var class="var">msg</var>)</code><a class="copiable-link" href="#index-lasterr-1"> ¶</a></span></dt>
<dt class="deftypefnx def-cmd-deftypefn" id="index-lasterr-2"><span><strong class="def-name">lasterr</strong> <code class="def-code-arguments">(<var class="var">msg</var>, <var class="var">msgid</var>)</code><a class="copiable-link" href="#index-lasterr-2"> ¶</a></span></dt>
<dd><p>Query or set the last error message.
</p>
<p>When called without input arguments, return the last error message and
message identifier.
</p>
<p>With one argument, set the last error message to <var class="var">msg</var>.
</p>
<p>With two arguments, also set the last message identifier.
</p>
<p><strong class="strong">See also:</strong> <a class="ref" href="#XREFlasterror">lasterror</a>, <a class="ref" href="Raising-Errors.html#XREFerror">error</a>, <a class="ref" href="Issuing-Warnings.html#XREFlastwarn">lastwarn</a>.
</p></dd></dl>
<p>The next example counts indexing errors. The errors are caught using the
field identifier of the structure returned by the function <code class="code">lasterror</code>.
</p>
<div class="example">
<div class="group"><pre class="example-preformatted">number_of_errors = 0;
for n = 1:100
try
...
catch
id = lasterror.identifier;
if (strcmp (id, "Octave:invalid-indexing"))
number_of_errors++;
endif
end_try_catch
endfor
</pre></div></div>
<p>The functions distributed with Octave can issue one of the following
errors.
</p>
<a class="anchor" id="XREFerror_005fids"></a><span style="display:block; margin-top:-4.5ex;"> </span>
<a class="index-entry-id" id="index-error-ids"></a>
<dl class="table">
<dt><code class="code">Octave:bad-alloc</code></dt>
<dd><p>Indicates that memory couldn’t be allocated.
</p>
</dd>
<dt><code class="code">Octave:invalid-context</code></dt>
<dd><p>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 class="code">print_usage ()</code> when called from the Octave prompt raises this error.
</p>
</dd>
<dt><code class="code">Octave:invalid-fun-call</code></dt>
<dd><p>Indicates that a function was called in an incorrect way, e.g., wrong number
of input arguments.
</p>
</dd>
<dt><code class="code">Octave:invalid-indexing</code></dt>
<dd><p>Indicates that a data-type was indexed incorrectly, e.g., real-value index
for arrays, nonexistent field of a structure.
</p>
</dd>
<dt><code class="code">Octave:invalid-input-arg</code></dt>
<dd><p>Indicates that a function was called with invalid input arguments.
</p>
</dd>
<dt><code class="code">Octave:parse-error</code></dt>
<dd><p>The interpreter failed to parse (read) specified code.
</p>
</dd>
<dt><code class="code">Octave:undefined-function</code></dt>
<dd><p>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.
</p>
</dd>
</dl>
<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 class="code">rethrow</code> function. The
previous example can now be changed to count the number of errors
related to the ‘<samp class="samp">*</samp>’ operator, but still abort if another kind of
error occurs.
</p>
<div class="example">
<div class="group"><pre class="example-preformatted">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></div></div>
<a class="anchor" id="XREFrethrow"></a><span style="display:block; margin-top:-4.5ex;"> </span>
<dl class="first-deftypefn">
<dt class="deftypefn" id="index-rethrow"><span><strong class="def-name">rethrow</strong> <code class="def-code-arguments">(<var class="var">err</var>)</code><a class="copiable-link" href="#index-rethrow"> ¶</a></span></dt>
<dd><p>Reissue a previous error as defined by <var class="var">err</var>.
</p>
<p><var class="var">err</var> is a structure that must contain at least the <code class="code">"message"</code>
and <code class="code">"identifier"</code> fields. <var class="var">err</var> can also contain a field
<code class="code">"stack"</code> that gives information on the assumed location of the
error. Typically <var class="var">err</var> is returned from <code class="code">lasterror</code>.
</p>
<p><strong class="strong">See also:</strong> <a class="ref" href="#XREFlasterror">lasterror</a>, <a class="ref" href="#XREFlasterr">lasterr</a>, <a class="ref" href="Raising-Errors.html#XREFerror">error</a>.
</p></dd></dl>
<a class="anchor" id="XREFerrno"></a><span style="display:block; margin-top:-4.5ex;"> </span>
<dl class="first-deftypefn">
<dt class="deftypefn" id="index-errno"><span><code class="def-type"><var class="var">err</var> =</code> <strong class="def-name">errno</strong> <code class="def-code-arguments">()</code><a class="copiable-link" href="#index-errno"> ¶</a></span></dt>
<dt class="deftypefnx def-cmd-deftypefn" id="index-errno-1"><span><code class="def-type"><var class="var">err</var> =</code> <strong class="def-name">errno</strong> <code class="def-code-arguments">(<var class="var">val</var>)</code><a class="copiable-link" href="#index-errno-1"> ¶</a></span></dt>
<dt class="deftypefnx def-cmd-deftypefn" id="index-errno-2"><span><code class="def-type"><var class="var">err</var> =</code> <strong class="def-name">errno</strong> <code class="def-code-arguments">(<var class="var">name</var>)</code><a class="copiable-link" href="#index-errno-2"> ¶</a></span></dt>
<dd><p>Query or set the system-dependent variable errno.
</p>
<p>When called with no inputs, return the current value of errno.
</p>
<p>When called with a numeric input <var class="var">val</var>, set the current value of errno
to the specified value. The previous value of errno is returned as <var class="var">err</var>.
</p>
<p>When called with a character string <var class="var">name</var>, return the numeric value of
errno which corresponds to the specified error code. If <var class="var">name</var> is not
a recognized error code then -1 is returned.
</p>
<p><strong class="strong">See also:</strong> <a class="ref" href="#XREFerrno_005flist">errno_list</a>.
</p></dd></dl>
<a class="anchor" id="XREFerrno_005flist"></a><span style="display:block; margin-top:-4.5ex;"> </span>
<dl class="first-deftypefn">
<dt class="deftypefn" id="index-errno_005flist"><span><code class="def-type"><var class="var">S</var> =</code> <strong class="def-name">errno_list</strong> <code class="def-code-arguments">()</code><a class="copiable-link" href="#index-errno_005flist"> ¶</a></span></dt>
<dd><p>Return a structure containing the system-dependent errno values.
</p>
<p><strong class="strong">See also:</strong> <a class="ref" href="#XREFerrno">errno</a>.
</p></dd></dl>
</div>
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