Variable declarations

    Variables can be declared as either being global, local, or static.
    Global variables are visible to all functions and on the command
    line, and are permanent.  Local variables are visible only within
    a single function or command sequence.  When the function or command
    sequence returns, the local variables are deleted.	Static variables
    are permanent like global variables, but are only visible within the
    same input file or function where they are defined.

    To declare one or more variables, the 'local', 'global', or 'static'
    keywords are used, followed by the desired list of variable names,
    separated by commas.  The definition is terminated with a semicolon.
    Examples of declarations are:

	    local	x, y, z;
	    global	fred;
	    local	foo, bar;
	    static	var1, var2, var3;

    Variables may have initializations applied to them.	 This is done
    by following the variable name by an equals sign and an expression.
    Global and local variables are initialized each time that control
    reaches them (e.g., at the entry to a function which contains them).
    Static variables are initialized once only, at the time that control
    first reaches them (but in future releases the time of initialization
    may change).  Unlike in C, expressions for static variables may
    contain function calls and refer to variables.  Examples of such
    initializations are:

	    local	a1 = 7, a2 = 3;
	    static	b = a1 + sin(a2);

    Within function declarations, all variables must be defined.
    But on the top level command line, assignments automatically define
    global variables as needed.	 For example, on the top level command
    line, the following defines the global variable x if it had not
    already been defined:

	    x = 7

    The static keyword may be used at the top level command level to
    define a variable which is only accessible interactively, or within
    functions defined interactively.

    Variables have no fixed type, thus there is no need or way to
    specify the types of variables as they are defined.	 Instead, the
    types of variables change as they are assigned to or are specified
    in special statements such as 'mat' and 'obj'.  When a variable is
    first defined using 'local', 'global', or 'static', it has the
    value of zero.

    If a procedure defines a local or static variable name which matches
    a global variable name, or has a parameter name which matches a
    global variable name, then the local variable or parameter takes
    precedence within that procedure, and the global variable is not
    directly accessible.

    The MAT and OBJ keywords may be used within a declaration statement
    in order to initially define variables as that type.  Initialization
    of these variables are also allowed.  Examples of such declarations
    are:

	    static mat table[3] = {5, 6, 7};
	    local obj point p1, p2;

    When working with user-defined functions, the syntax for passing an
    lvalue by reference rather than by value is to precede an expression
    for the lvalue by a backquote. For example, if the function invert is
    defined by:

	    define invert(x) {x = inverse(x)}

    then invert(`A) achieves the effect of A = inverse(A).  In other
    words, passing and argument of `variable (with a back-quote)
    will cause and changes to the function argument to be applied to
    the calling variable.  Calling invert(A) (without the ` backquote)
    assigns inverse(A) to the temporary function parameter x and leaves
    A unchanged.

    In an argument, a backquote before other than an lvalue is ignored.
    Consider, for example:

	    ; define logplus(x,y,z) {return log(++x + ++y + ++z);}

	    ; eh = 55;
	    ; mi = 25;
	    ; answer = logplus(eh, `mi, `17);

	    ; print eh, mi, answer;
	    55 26 2

    The value of eh is was not changed because eh was used as
    an argument without a back-quote (`).  However, mi was incremented
    because it was passed as `mi (with a back-quote).  Passing 17
    (not an lvalue) as `17 has not effect on the value 17.

    The back-quote should only be used before arguments to a function.
    In all other contexts, a backquote causes a compile error.

    Another method is to pass the address of the lvalue explicitly and
    use the indirection operator * (star) to refer to the lvalue in the
    function body.  Consider the following function:

	    ; define ten(a) { *a = 10; }

	    ; n = 17;
	    ; ten(n);
	    ; print n;
	    17

	    ; ten(`n);
	    ; print n;
	    17

	    ; ten(&n);
	    ; print n;
	    10

    Passing an argument with a & (ampersand) allows the tenmore()
    function to modify the calling variable:

	    ; wa = tenmore(&vx);
	    ; print vx, wa;
	    65 65

    Great care should be taken when using a pointer to a local variable
    or element of a matrix, list or object, since the lvalue pointed to
    is deleted when evaluation of the function is completed or the lvalue
    whose value is the matrix, list or object is assigned another value.

    As both of the above methods (using & arguments (ampersand) *value
    (star) function values or by using ` arguments (back quote) alone)
    copy the address rather than the value of the argument to the function
    parameter, they allow for faster calls of functions when the memory
    required for the value is huge (such as for a large matrix).

    As the built-in functions and object functions always accept their
    arguments as addresses, there is no gain in using the backquote when
    calling these functions.

## Copyright (C) 1999-2006  Landon Curt Noll
##
## Calc is open software; you can redistribute it and/or modify it under
## the terms of the version 2.1 of the GNU Lesser General Public License
## as published by the Free Software Foundation.
##
## Calc is distributed in the hope that it will be useful, but WITHOUT
## ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
## or FITNESS FOR A PARTICULAR PURPOSE.	 See the GNU Lesser General
## Public License for more details.
##
## A copy of version 2.1 of the GNU Lesser General Public License is
## distributed with calc under the filename COPYING-LGPL.  You should have
## received a copy with calc; if not, write to Free Software Foundation, Inc.
## 59 Temple Place, Suite 330, Boston, MA  02111-1307, USA.
##
## @(#) $Revision: 29.5 $
## @(#) $Id: variable,v 29.5 2006/09/17 20:54:07 chongo Exp $
## @(#) $Source: /usr/local/src/cmd/calc/help/RCS/variable,v $
##
## Under source code control:	1991/07/21 04:37:25
## File existed as early as:	1991
##
## chongo <was here> /\oo/\	http://www.isthe.com/chongo/
## Share and enjoy!  :-)	http://www.isthe.com/chongo/tech/comp/calc/
