@c Copyright (C) 1996-2011 John W. Eaton
@c
@c This file is part of Octave.
@c
@c Octave is free software; you can redistribute it and/or modify it
@c under the terms of the GNU General Public License as published by the
@c Free Software Foundation; either version 3 of the License, or (at
@c your option) any later version.
@c 
@c Octave is distributed in the hope that it will be useful, but WITHOUT
@c ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
@c FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
@c for more details.
@c 
@c You should have received a copy of the GNU General Public License
@c along with Octave; see the file COPYING.  If not, see
@c <http://www.gnu.org/licenses/>.

@node Variables
@chapter Variables
@cindex variables, user-defined
@cindex user-defined variables

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

@cindex job hunting
@cindex getting a good job
@cindex flying high and fast

@example
@group
x
x15
__foo_bar_baz__
fucnrdthsucngtagdjb
@end group
@end example

@noindent
However, names like @code{__foo_bar_baz__} 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's
documented internal variables and built-in symbolic constants.

Case is significant in variable names.  The symbols @code{a} and
@code{A} are distinct variables.

A variable name is a valid expression by itself.  It represents the
variable's current value.  Variables are given new values with
@dfn{assignment operators} and @dfn{increment operators}.
@xref{Assignment Ops, ,Assignment Expressions}.

There is one built-in variable with a special meaning.  The @code{ans} variable
always contains the result of the last computation, where the output wasn't
assigned to any variable.  The code @code{a = cos (pi)} will assign the value -1
to the variable @code{a}, but will not change the value of @code{ans}.  However,
the code @code{cos (pi)} will set the value of @code{ans} to -1.

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.

@cindex @code{ans}
@DOCSTRING(ans)

@DOCSTRING(isvarname)

@DOCSTRING(genvarname)

@DOCSTRING(namelengthmax)

@menu
* Global Variables::            
* Persistent Variables::        
* Status of Variables::         
@end menu

@node Global Variables
@section Global Variables
@cindex global variables
@cindex @code{global} statement
@cindex variables, global

A variable that has been declared @dfn{global} may be accessed from
within a function body without having to pass it as a formal parameter.

A variable may be declared global using a @code{global} declaration
statement.  The following statements are all global declarations.

@example
@group
global a
global a b
global c = 2
global d = 3 e f = 5
@end group
@end example

A global variable may only be initialized once in a @code{global}
statement.  For example, after executing the following code

@example
@group
global gvar = 1
global gvar = 2
@end group
@end example

@noindent
the value of the global variable @code{gvar} is 1, not 2.  Issuing a
@samp{clear gvar} command does not change the above behavior, but
@samp{clear all} does.

It is necessary declare a variable as global within a function body in
order to access it.  For example,

@example
@group
global x
function f ()
  x = 1;
endfunction
f ()
@end group
@end example

@noindent
does @emph{not} set the value of the global variable @code{x} to 1.  In
order to change the value of the global variable @code{x}, you must also
declare it to be global within the function body, like this

@example
@group
function f ()
  global x;
  x = 1;
endfunction
@end group
@end example

Passing a global variable in a function parameter list will
make a local copy and not modify the global value.  For example, given
the function

@example
@group
function f (x)
  x = 0
endfunction
@end group
@end example

@noindent
and the definition of @code{x} as a global variable at the top level,

@example
global x = 13
@end example

@noindent
the expression

@example
f (x)
@end example

@noindent
will display the value of @code{x} from inside the function as 0,
but the value of @code{x} at the top level remains unchanged, because
the function works with a @emph{copy} of its argument.

@DOCSTRING(isglobal)

@node Persistent Variables
@section Persistent Variables
@cindex persistent variables
@cindex @code{persistent} statement
@cindex variables, persistent
@anchor{doc-persistent}

A variable that has been declared @dfn{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.

@example
@group
function count_calls ()
  persistent calls = 0;
  printf ("'count_calls' has been called %d times\n",
          ++calls);
endfunction

for i = 1:3
  count_calls ();
endfor

@print{} 'count_calls' has been called 1 times
@print{} 'count_calls' has been called 2 times
@print{} 'count_calls' has been called 3 times
@end group
@end example

As the example shows, a variable may be declared persistent using a
@code{persistent} declaration statement.  The following statements are
all persistent declarations.

@example
@group
persistent a
persistent a b
persistent c = 2
persistent d = 3 e f = 5
@end group
@end example

The behavior of persistent variables is equivalent to the behavior of
static variables in C@.  The command @code{static} in Octave is also
recognized and is equivalent to @code{persistent}.

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

@example
@group
persistent pvar = 1
persistent pvar = 2
@end group
@end example

@noindent
the value of the persistent variable @code{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.

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

@noindent
This implementation behaves in exactly the same way as the previous
implementation of @code{count_calls}.

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

@example
for i = 1:2
  count_calls ();
endfor
@print{} 'count_calls' has been called 1 times
@print{} 'count_calls' has been called 2 times

clear
for i = 1:2
  count_calls();
endfor
@print{} 'count_calls' has been called 3 times
@print{} 'count_calls' has been called 4 times

clear all
for i = 1:2
  count_calls();
endfor
@print{} 'count_calls' has been called 1 times
@print{} 'count_calls' has been called 2 times

clear count_calls
for i = 1:2
  count_calls();
endfor
@print{} 'count_calls' has been called 1 times
@print{} 'count_calls' has been called 2 times
@end example

@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} 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} function as described in
@xref{Function Locking}.

@node Status of Variables
@section Status of Variables

When creating simple one-shot programs it can be very convenient to
see which variables are available at the prompt.  The function @code{who}
and its siblings @code{whos} and @code{whos_line_format} will show
different information about what is in memory, as the following shows.

@example
@group
str = "A random string";
who -variables
     @print{} *** local user variables:
     @print{} 
     @print{} __nargin__  str
@end group
@end example

@DOCSTRING(who)

@DOCSTRING(whos)

@DOCSTRING(whos_line_format)

Instead of displaying which variables are in memory, it is possible
to determine if a given variable is available.  That way it is possible
to alter the behavior of a program depending on the existence of a
variable.  The following example illustrates this.

@example
@group
if (! exist ("meaning", "var"))
  disp ("The program has no 'meaning'");
endif
@end group
@end example

@DOCSTRING(exist)

Usually Octave will manage the memory, but sometimes it can be practical
to remove variables from memory manually.  This is usually needed when
working with large variables that fill a substantial part of the memory.
On a computer that uses the IEEE floating point format, the following
program allocates a matrix that requires around 128 MB memory.

@example
large_matrix = zeros (4000, 4000);
@end example

@noindent
Since having this variable in memory might slow down other computations,
it can be necessary to remove it manually from memory.  The @code{clear}
function allows this.

@DOCSTRING(clear)

Information about a function or variable such as its location in the
file system can also be acquired from within Octave.  This is usually
only useful during development of programs, and not within a program.

@DOCSTRING(type)

@DOCSTRING(which)

@DOCSTRING(what)
