Compound Assignment Operators: Revision Notes (Computer Science A 1.6)

Short on time? This page is the recap. For explanations and worked examples, use the full study guide.

Recap

• A compound assignment operator performs the arithmetic operation between the value on the left and the value on the right, then assigns the result to the variable on the left.
• The operators are +=, -=, *=, /= and %=; they can be used in place of = in numeric expressions.
• The post-increment ++ and post-decrement -- add 1 to, or subtract 1 from, a numeric variable and store the new value.
• Prefix forms (++x) and uses inside other expressions (arr[x++]) are outside the course and exam.

Key relationships

Statement	Equivalent	Result for int x = 8
x += 3;	x = x + 3;	11
x -= 3;	x = x - 3;	5
x *= 3;	x = x * 3;	24
x /= 3;	x = x / 3;	2 (integer division)
x %= 3;	x = x % 3;	2
x++;	x += 1;	9
x--;	x -= 1;	7
x *= 2 + 3;	x = x * (2 + 3);	40
x /= 2 * 2;	x = x / (2 * 2);	2

For double d = 9; d /= 2; the result is 4.5.

The three jobs compound assignment does

• Accumulate: sum += price; keeps a running total.
• Count: count++; goes up by one each time; remaining--; counts down.
• Scale or wrap: price *= 2; doubles; pot /= 4; shares out whole units; position %= 10; keeps a non-negative value in the range 0 to 9.

Statement order matters: from 10, x += 5; x *= 2; leaves 30, but x *= 2; x += 5; leaves 25.

Assumptions and conventions

• The right side is evaluated in full before it is combined with the variable.
• Use ++ and -- as separate statements, after the variable.
• If an int variable is updated with a double value (n *= 1.5;), Java converts the result back to int, truncating the fraction.
• No calculator is allowed on the exam, so trace by hand with a table, one row per statement.

Mistakes to avoid

1. Reading x *= 2 + 3 as x = x * 2 + 3.
2. Writing =+ instead of +=. x =+ 5 just stores 5.
3. Forgetting integer division when /= is applied to an int.
4. Using the original value of a variable instead of the value left by the previous statement.
5. Using ++ inside an expression or in prefix form, which the course does not use.
6. Treating %= as a percentage. It stores the remainder.

Quick self-check

1. After int a = 12; a /= 5; what is a? (2)
2. After int b = 6; b *= 1 + 2; what is b? (18)
3. After double d = 7; d /= 2; what is d? (3.5)
4. After int c = 9; c %= 4; c++; what is c? (1 then 2, so 2)
5. What does int n = 3; n += 2.9; leave in n? (5, because 5.9 is truncated when stored)

Next: practice questions.