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<a name="Variables"></a>
<div class="header">
<p>
Next: <a href="Expressions.html#Expressions" accesskey="n" rel="next">Expressions</a>, Previous: <a href="Data-Containers.html#Data-Containers" accesskey="p" rel="prev">Data Containers</a>, Up: <a href="index.html#Top" accesskey="u" rel="up">Top</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
</div>
<hr>
<a name="Variables-1"></a>
<h2 class="chapter">7 Variables</h2>
<a name="index-variables_002c-user_002ddefined"></a>
<a name="index-user_002ddefined-variables"></a>

<p>Variables let you give names to values and refer to them later.  You have
already seen variables in many of the examples.  The name of a variable
must be a sequence of letters, digits and underscores, but it may not begin
with a digit.  Octave does not enforce a limit on the length of variable
names, but it is seldom useful to have variables with names longer than
about 30 characters.  The following are all valid variable names
</p>
<div class="example">
<pre class="example">x
x15
__foo_bar_baz__
fucnrdthsucngtagdjb
</pre></div>

<p>However, names like <code>__foo_bar_baz__</code> that begin and end with two
underscores are understood to be reserved for internal use by Octave.
You should not use them in code you write, except to access Octave&rsquo;s
documented internal variables and built-in symbolic constants.
</p>
<p>Case is significant in variable names.  The symbols <code>a</code> and
<code>A</code> are distinct variables.
</p>
<p>A variable name is a valid expression by itself.  It represents the
variable&rsquo;s current value.  Variables are given new values with
<em>assignment operators</em> and <em>increment operators</em>.
See <a href="Assignment-Ops.html#Assignment-Ops">Assignment Expressions</a>.
</p>
<p>There is one built-in variable with a special meaning.  The <code>ans</code> variable
always contains the result of the last computation, where the output wasn&rsquo;t
assigned to any variable.  The code <code>a = cos (pi)</code> will assign the value
-1 to the variable <code>a</code>, but will not change the value of <code>ans</code>.
However, the code <code>cos (pi)</code> will set the value of <code>ans</code> to -1.
</p>
<p>Variables in Octave do not have fixed types, so it is possible to first
store a numeric value in a variable and then to later use the same name
to hold a string value in the same program.  Variables may not be used
before they have been given a value.  Doing so results in an error.
</p>
<a name="index-ans"></a>
<a name="XREFans"></a><dl>
<dt><a name="index-ans-1"></a>Automatic Variable: <strong>ans</strong></dt>
<dd><p>The most recently computed result that was not explicitly assigned to a
variable.
</p>
<p>For example, after the expression
</p>
<div class="example">
<pre class="example">3^2 + 4^2
</pre></div>

<p>is evaluated, the value returned by <code>ans</code> is 25.
</p></dd></dl>


<a name="XREFisvarname"></a><dl>
<dt><a name="index-isvarname"></a>: <em></em> <strong>isvarname</strong> <em>(<var>name</var>)</em></dt>
<dd><p>Return true if <var>name</var> is a valid variable name.
</p>
<p><strong>See also:</strong> <a href="Keywords.html#XREFiskeyword">iskeyword</a>, <a href="Status-of-Variables.html#XREFexist">exist</a>, <a href="Status-of-Variables.html#XREFwho">who</a>.
</p></dd></dl>


<a name="XREFgenvarname"></a><dl>
<dt><a name="index-genvarname"></a>: <em><var>varname</var> =</em> <strong>genvarname</strong> <em>(<var>str</var>)</em></dt>
<dt><a name="index-genvarname-1"></a>: <em><var>varname</var> =</em> <strong>genvarname</strong> <em>(<var>str</var>, <var>exclusions</var>)</em></dt>
<dd><p>Create valid unique variable name(s) from <var>str</var>.
</p>
<p>If <var>str</var> is a cellstr, then a unique variable is created for each cell
in <var>str</var>.
</p>
<div class="example">
<pre class="example">genvarname ({&quot;foo&quot;, &quot;foo&quot;})
  &rArr;
     {
       [1,1] = foo
       [1,2] = foo1
     }
</pre></div>

<p>If <var>exclusions</var> is given, then the variable(s) will be unique to each
other and to <var>exclusions</var> (<var>exclusions</var> may be either a string or a
cellstr).
</p>
<div class="example">
<pre class="example">x = 3.141;
genvarname (&quot;x&quot;, who ())
  &rArr; x1
</pre></div>

<p>Note that the result is a char array or cell array of strings, not the
variables themselves.  To define a variable, <code>eval()</code> can be used.
The following trivial example sets <code>x</code> to <code>42</code>.
</p>
<div class="example">
<pre class="example">name = genvarname (&quot;x&quot;);
eval ([name &quot; = 42&quot;]);
  &rArr; x =  42
</pre></div>

<p>This can be useful for creating unique struct field names.
</p>
<div class="example">
<pre class="example">x = struct ();
for i = 1:3
  x.(genvarname (&quot;a&quot;, fieldnames (x))) = i;
endfor
  &rArr; x =
     {
       a =  1
       a1 =  2
       a2 =  3
     }
</pre></div>

<p>Since variable names may only contain letters, digits, and underscores,
<code>genvarname</code> will replace any sequence of disallowed characters with
an underscore.  Also, variables may not begin with a digit; in this case
an &lsquo;<samp>x</samp>&rsquo; is added before the variable name.
</p>
<p>Variable names beginning and ending with two underscores <code>&quot;__&quot;</code> are
valid, but they are used internally by Octave and should generally be
avoided; therefore, <code>genvarname</code> will not generate such names.
</p>
<p><code>genvarname</code> will also ensure that returned names do not clash with
keywords such as <code>&quot;for&quot;</code> and <code>&quot;if&quot;</code>.  A number will be
appended if necessary.  Note, however, that this does <strong>not</strong> include
function names such as <code>&quot;sin&quot;</code>.  Such names should be included in
<var>exclusions</var> if necessary.
</p>
<p><strong>See also:</strong> <a href="#XREFisvarname">isvarname</a>, <a href="Keywords.html#XREFiskeyword">iskeyword</a>, <a href="Status-of-Variables.html#XREFexist">exist</a>, <a href="Status-of-Variables.html#XREFwho">who</a>, <a href="Temporary-Files.html#XREFtempname">tempname</a>, <a href="Evaluation.html#XREFeval">eval</a>.
</p></dd></dl>


<a name="XREFnamelengthmax"></a><dl>
<dt><a name="index-namelengthmax"></a>: <em></em> <strong>namelengthmax</strong> <em>()</em></dt>
<dd><p>Return the <small>MATLAB</small> compatible maximum variable name length.
</p>
<p>Octave is capable of storing strings up to <em>2^{31} - 1</em> in length.
However for <small>MATLAB</small> compatibility all variable, function, and structure
field names should be shorter than the length returned by
<code>namelengthmax</code>.  In particular, variables stored to a <small>MATLAB</small> file
format (<samp>*.mat</samp>) will have their names truncated to this length.
</p></dd></dl>


<table class="menu" border="0" cellspacing="0">
<tr><td align="left" valign="top">&bull; <a href="Global-Variables.html#Global-Variables" accesskey="1">Global Variables</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Persistent-Variables.html#Persistent-Variables" accesskey="2">Persistent Variables</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Status-of-Variables.html#Status-of-Variables" accesskey="3">Status of Variables</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">
</td></tr>
</table>

<hr>
<div class="header">
<p>
Next: <a href="Expressions.html#Expressions" accesskey="n" rel="next">Expressions</a>, Previous: <a href="Data-Containers.html#Data-Containers" accesskey="p" rel="prev">Data Containers</a>, Up: <a href="index.html#Top" accesskey="u" rel="up">Top</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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