Assignment Operators and Expressions in C

By: Daniel Malcolm in C Tutorials on 2007-09-20

An expression such as

   i = i + 2

in which the variable on the left side is repeated immediately on the right, can be written in the compressed form

   i += 2

The operator += is called an assignment operator.

Most binary operators (operators like + that have a left and right operand) have a corresponding assignment operator op=, where op is one of

   +   -   *   /   %   <<   >>   &   ^   |

If expr₁ and expr₂ are expressions, then

   expr₁ op= expr₂

is equivalent to

   expr₁ = (expr₁) op (expr₂)

except that expr₁ is computed only once. Notice the parentheses around expr₂:

   x *= y + 1

means

   x = x * (y + 1)

rather than

   x = x * y + 1

As an example, the function bitcount counts the number of 1-bits in its integer argument.

   /* bitcount:  count 1 bits in x */
   int bitcount(unsigned x)
   {
       int b;

       for (b = 0; x != 0; x >>= 1)
           if (x & 01)
               b++;
       return b;
   }

Declaring the argument x to be an unsigned ensures that when it is right-shifted, vacated bits will be filled with zeros, not sign bits, regardless of the machine the program is run on.

Quite apart from conciseness, assignment operators have the advantage that they correspond better to the way people think. We say ``add 2 to i'' or ``increment i by 2'', not ``take i, add 2, then put the result back in i''. Thus the expression i += 2 is preferable to i = i+2. In addition, for a complicated expression like

   yyval[yypv[p3+p4] + yypv[p1]] += 2

the assignment operator makes the code easier to understand, since the reader doesn't have to check painstakingly that two long expressions are indeed the same, or to wonder why they're not. And an assignment operator may even help a compiler to produce efficient code.

We have already seen that the assignment statement has a value and can occur in expressions; the most common example is

   while ((c = getchar()) != EOF)
       ...

The other assignment operators (+=, -=, etc.) can also occur in expressions, although this is less frequent.

In all such expressions, the type of an assignment expression is the type of its left operand, and the value is the value after the assignment.

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