calc/help/assign

NAME
    =

SYNOPSIS
    a = b
    a = {e_1, e_2, ...[ { ... } ] }

TYPES
    a			lvalue, current value a structure in { } case

    b			expression

    e_0, e_1, ...	expressions, blanks, or initializer lists


    return		lvalue (a)

DESCRIPTION
    Here an lvalue is either a simple variable specified by an identifier,
    or an element of an existing structure specified by one or more
    qualifiers following an identifier.

    An initializer list is a comma-separated list enclosed in braces as in

		{e_0, e_1, ... }

    where each e_i is an expression, blank or initializer list.

    a = b evaluates b, assigns its value to a, and returns a.

    a = {e_0, e_1, ... } where the e_i are expressions or blanks,
    requires the current value of a to be a matrix, list
    or object with at least as many elements as listed e_i.  Each non-blank
    e_i is evaluated and its value is assigned to a[[i]]; elements a[[i]]
    corresponding to blank e_i are unchanged.

    If, in a = {e_0, e_1, ...}, e_i is an initializer list, as in
    {e_i_0, e_1_1, ...}, the corresponding a[[i]] is to be a matrix, list
    or object with at least as many elements as listed e_i_j.  Depending on
    whether e_i_j is an expression, blank, or initializer list, one, no, or
    possibly more than one assignment, is made to a[[i]][[j]] or, if
    relevant and possible, its elements.

    In simple assignments, = associates from right to left so that, for
    example,

		a = b = c

    has the effect of a = (b = c) and results in assigning the value of c
    to both a and b.  The expression (a = b) = c is acceptable, but has the
    effect of a = b; a = c; in which the first assignment is superseded by
    the second.

    In initializations, = { ...} associates from left to right so that,
    for example,

		a = {e_0, ... } = {v_0, ...}

    first assigns e_0, ... to the elements of a, and then assigns v_0, ...
    to the result.

    If there are side effects in the evaluations involved in executing a = b,
    it should be noted that the order of evaluations is: first the address
    for a, then the value of b, and finally the assignment.  For example if
    A is a matrix and i = 0, then the assignment in A[i++] = A[i] is
    that of A[0] = A[1].

    If, in execution of a = b, a is changed by the evaluation of b, the
    value of b may be stored in an unintended or inaccessible location.	 For
    example,
		mat A[2]= {1,2};
		A[0] = (A = 3);

    results in the value 3 being stored not only as the new value for A
    but also at the now unnamed location earlier used for A[0].


EXAMPLE
    ; b = 3+1
    ; a = b
    ; print a, b
    4 4

    ; obj point {x,y}
    ; mat A[3] = {1, list(2,3), obj point = {4,5}}

    ; A[1][[0]] = 6; A[2].x = 7
    ; print A[1]

    list (2 elements, 2 nonzero):
	[[0]] = 6
	[[1]] = 3

    ; print A[2]
    obj point {7, 5}

    ; A = {A[2], , {9,10}}
    ; print A[0]
    obj point {7, 5}

    ; print A[2]
    obj point {9, 10}

    ; A = {, {2}}
    print A[1]

    list (2 elements, 2 nonzero):
	[[0]] = 2
	[[1]] = 3

LIMITS
    none

LINK LIBRARY
    none

SEE ALSO
    swap, quomod

## Copyright (C) 1999  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: 29.3 $
## @(#) $Id: assign,v 29.3 2006/05/07 07:25:46 chongo Exp $
## @(#) $Source: /usr/local/src/cmd/calc/help/RCS/assign,v $
##
## Under source code control:	1995/05/11 21:03:23
## File existed as early as:	1995
##
## chongo <was here> /\oo/\	http://www.isthe.com/chongo/
## Share and enjoy!  :-)	http://www.isthe.com/chongo/tech/comp/calc/