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<h3 class="section">7.2 Persistent Variables</h3>
<p><a name="index-persistent-variables-488"></a><a name="index-g_t_0040code_007bpersistent_007d-statement-489"></a><a name="index-variables_002c-persistent-490"></a><a name="doc_002dpersistent"></a>A variable that has been declared <dfn>persistent</dfn> within a function
will retain its contents in memory between subsequent calls to the
same function. The difference between persistent variables and global
variables is that persistent variables are local in scope to a
particular function and are not visible elsewhere.
<p>The following example uses a persistent variable to create a function
that prints the number of times it has been called.
<pre class="example"> function count_calls ()
persistent calls = 0;
printf ("'count_calls' has been called %d times\n",
++calls);
endfunction
for i = 1:3
count_calls ();
endfor
-| 'count_calls' has been called 1 times
-| 'count_calls' has been called 2 times
-| 'count_calls' has been called 3 times
</pre>
<p>As the example shows, a variable may be declared persistent using a
<code>persistent</code> declaration statement. The following statements are
all persistent declarations.
<pre class="example"> persistent a
persistent a b
persistent c = 2
persistent d = 3 e f = 5
</pre>
<p>The behavior of persistent variables is equivalent to the behavior of
static variables in C. The command <code>static</code> in Octave is also
recognized and is equivalent to <code>persistent</code>.
<p>Like global variables, a persistent variable may only be initialized once.
For example, after executing the following code
<pre class="example"> persistent pvar = 1
persistent pvar = 2
</pre>
<p class="noindent">the value of the persistent variable <code>pvar</code> is 1, not 2.
<p>If a persistent variable is declared but not initialized to a specific
value, it will contain an empty matrix. So, it is also possible to
initialize a persistent variable by checking whether it is empty, as the
following example illustrates.
<pre class="example"> function count_calls ()
persistent calls;
if (isempty (calls))
calls = 0;
endif
printf ("'count_calls' has been called %d times\n",
++calls);
endfunction
</pre>
<p class="noindent">This implementation behaves in exactly the same way as the previous
implementation of <code>count_calls</code>.
<p>The value of a persistent variable is kept in memory until it is
explicitly cleared. Assuming that the implementation of <code>count_calls</code>
is saved on disk, we get the following behavior.
<pre class="example"> for i = 1:2
count_calls ();
endfor
-| 'count_calls' has been called 1 times
-| 'count_calls' has been called 2 times
clear
for i = 1:2
count_calls();
endfor
-| 'count_calls' has been called 3 times
-| 'count_calls' has been called 4 times
clear all
for i = 1:2
count_calls();
endfor
-| 'count_calls' has been called 1 times
-| 'count_calls' has been called 2 times
clear count_calls
for i = 1:2
count_calls();
endfor
-| 'count_calls' has been called 1 times
-| 'count_calls' has been called 2 times
</pre>
<p class="noindent">That is, the persistent variable is only removed from memory when the
function containing the variable is removed. Note that if the function
definition is typed directly into the Octave prompt, the persistent
variable will be cleared by a simple <code>clear</code> command as the entire
function definition will be removed from memory. If you do not want
a persistent variable to be removed from memory even if the function is
cleared, you should use the <code>mlock</code> function as described in
See <a href="Function-Locking.html#Function-Locking">Function Locking</a>.
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