2.6 Assignment Operators
[This section corresponds to K&R Sec. 2.10]
The assignment operator = assigns a value to a variable. For example,
x = 1
sets x to 1, and
a = b
sets a to whatever b's value is. The expression
i = i + 1
is, as we've mentioned elsewhere, the standard programming idiom for increasing a variable's value by 1: this expression takes i's old value, adds 1 to it, and stores it back into i. (C provides several ``shortcut'' operators for modifying variables in this and similar ways, which we'll meet later.)
We've called the = sign the ``assignment operator'' and referred to ``assignment expressions'' because, in fact, = is an operator just like + or -. C does not have ``assignment statements''; instead, an assignment like a = b is an expression and can be used wherever any expression can appear. Since it's an expression, the assignment a = b has a value, namely, the same value that's assigned to a. This value can then be used in a larger expression; for example, we might write
c = a = b
which is equivalent to
c = (a = b)
and assigns b's value to both a and c. (The assignment operator, therefore, groups from right to left.) Later we'll see other circumstances in which it can be useful to use the value of an assignment expression.
It's usually a matter of style whether you initialize a variable with an initializer in its declaration or with an assignment expression near where you first use it. That is, there's no particular difference between
int a = 10;
and
int a;
/* later... */
a = 10;
