section 2.8: Increment and Decrement Operators

The distinction between the prefix and postfix forms of ++ and -- will probably seem strained at first, but it will make more sense once we begin using these operators in more realistic situations.

The authors point out that an expression like (i+j)++ is illegal, and it's worth thinking for a moment about why. The ++ operator doesn't just mean ``add one''; it means ``add one to a variable'' or ``make a variable's value one more than it was before.'' But (i+j) is not a variable, it's an expression; so there's no place for ++ to store the incremented result. If you were bound and determined to use ++ here, you'd have to introduce another variable:

	int k = i + j;
	k++;

But really, when you want to add one to an expression, just use

	i + j + 1

Another unfortunate (and utterly meaningless) example is

	i = i++;

If you want to increment i (that is, add one to it, and store the result back in i), either use

	i = i + 1;

	i++;

Don't try to combine the two.

page 47

Deep sentence:

In a context where no value is wanted, just the incrementing effect, as in
	if(c == '\n')
		nl++;
prefix and postfix are the same.

In other words, when you're just incrementing some variable, you can use either the nl++ or ++nl form. But when you're immediately using the result, as in the examples we'll look at later, using one or the other makes a big difference.

In that light, study one of the examples on this page--squeeze, the modified getline, or strcat--and convince yourself that it would not work if the wrong form of increment (++i or ++j) were used.

You may note that all three examples on pages 47-48 use the postfix form. Postfix increment is probably more common, though prefix definitely has its uses, too.

You may notice the keyword void popping up in a few code examples. void is a type we haven't met yet; it's a type with no values and no operations. When a function is declared as ``returning'' void, as in the squeeze and strcat examples on pages 47 and 48, it means that the function does not return a value. (This was briefly mentioned on page 30 in chapter 1.)

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