1.6 Compound Assignment Operators

What you need to know

• x += 5 means x = x + 5. Same for -=, *=, /=, %=.
• Compound operators include an implicit cast back to the variable's type: if x is an int, x /= 2.0 compiles and stores an int. x = x / 2.0 would not compile.
• x++ and ++x both add 1 to x. x-- and --x subtract 1. On the exam these are used as standalone statements, where the pre/post distinction doesn't matter.
• The AP exam does not test x++ inside larger expressions (like y = x++). Keep increments on their own line.
• x *= 2 + 3 means x = x * (2 + 3) — the whole right side is computed first.
• These operators work on doubles too: total += 0.5;

Worked example

int x = 10;
x += 3;        // 13
x -= 5;        // 8
x *= 2;        // 16
x /= 3;        // 5   (integer division)
x %= 4;        // 1
x++;           // 2
x *= 1 + 1;    // 4   (x * 2, not x * 1 + 1)

Going deeper

The nuance, edge cases, and connections that turn a 3 into a 5.

• x op= expr means x = x op (expr) — the entire right side is parenthesized. x *= 2 + 3 is x = x * 5, not x * 2 + 3.
• The hidden cast: compound operators cast the result back to the variable's type. int x = 10; x /= 4.0; compiles and leaves x = 2 (10 / 4.0 = 2.5, cast to int). The long form x = x / 4.0 would be a compile error (double to int). This asymmetry is occasionally tested.
• x++ and ++x are identical as standalone statements. The exam only uses them that way — inside larger expressions (y = x++) is explicitly out of scope, so you won't see it.
• x-- and --x likewise. x += 1, x++, x = x + 1 are all the same statement.
• Compound operators work on doubles (total += 0.25) and, for += only, on Strings (s += "x" concatenates).
• Loop updates almost always use these: i++, i += 2, i--. Reading them fluently speeds up every loop question.

Mistakes that cost points

• Forgetting the implied parentheses. x -= 2 * 3 subtracts 6, not 2 then times 3.
• Misreading x -= 5 as x = -5. It subtracts 5 from x.
• Thinking x++ inside a loop body increments twice with the loop header. Each increment is one. If both the header and body increment, i goes up by 2 per iteration.

Practice questions

Written in the style of the real exam. Try each one before revealing the answer.

int n = 7;
n += 3;
n /= 2;
n *= n;