2007-08-30 |  RatheeshTR  | Viewed: 442  |    0

Assignment Operators and Expressions:

Expressions such as

             i  =  i + 2;

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

            i    +=   2

The operator +- is called an assignment operator.

we don't ``evaluate'' the i on the left hand side of the = at all, we assign to it. The distinction becomes important, however, when the left hand side (expr₁) is more complicated than a simple variable. For example, we could add 2 to each of the n cells of an array a with code like

	int i = 0;
	while(i < n)
		a[i++] += 2;

If we tried to use the expanded form, we'd get

	int i = 0;
	while(i < n)
		a[i++] = a[i++] + 2;

and by trying to increment i twice within the same expression we'd get (as we'll see) undesired, unpredictable, and in fact undefined results. (Of course, a more natural form of this loop would be

	for(i = 0; i < n; i++)
		a[i] += 2;

and with the increment of i moved out of the array subscript, it wouldn't matter so much whether we used a[i] += 2 or a[i] = a[i] + 2.)

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


eg,
int bitcount (unsigned x)
{
          int   b;
          for ( b=0;  x !=  0;  x>>=1)
                if ( x &  01)
                     b++;
         
           return b;
}