(octave.info)Persistent Variables

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7.2 Persistent Variables
========================

A variable that has been declared “persistent” 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.

   The following example uses a persistent variable to create a function
that prints the number of times it has been called.

     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

   As the example shows, a variable may be declared persistent using a
‘persistent’ declaration statement.  The following statements are all
persistent declarations.

     persistent a
     persistent a b
     persistent c = 2
     persistent d = 3 e f = 5

   The behavior of persistent variables is equivalent to the behavior of
static variables in C.

   One restriction for persistent variables is, that neither input nor
output arguments of a function can be persistent:

     function y = foo ()
       persistent y = 0;  # Not allowed!
     endfunction

     foo ()
     ⊣ error: can't make function parameter y persistent

   Like global variables, a persistent variable may only be initialized
once.  For example, after executing the following code

     persistent pvar = 1
     persistent pvar = 2

the value of the persistent variable ‘pvar’ is 1, not 2.

   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.

     function count_calls ()
       persistent calls;
       if (isempty (calls))
         calls = 0;
       endif
       printf ("'count_calls' has been called %d times\n",
               ++calls);
     endfunction

This implementation behaves in exactly the same way as the previous
implementation of ‘count_calls’.

   The value of a persistent variable is kept in memory until it is
explicitly cleared.  Assuming that the implementation of ‘count_calls’
is saved on disk, we get the following behavior.

     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

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 ‘clear’ 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 ‘mlock’ function (Note: Function Locking).