1.6 Compound Assignment Operators

1. The five compound assignment operators

The compound assignment operators +=, -=, *=, /= and %= can be used in place of the assignment operator in numeric expressions. A compound assignment operator takes the current value of the variable on the left, does the arithmetic with the value on the right, and stores the result back in that variable 1.6.A.1.

Compound form	Same as
x += 3;	x = x + 3;
x -= 3;	x = x - 3;
x *= 3;	x = x * 3;
x /= 3;	x = x / 3;
x %= 3;	x = x % 3;

Each line uses the value left by the line before:

int k = 7;
k += 3;    // 10
k *= 2;    // 20
k -= 5;    // 15
k /= 4;    // 3 (int division)
k %= 3;    // 0
System.out.println(k);

Output:

0

k /= 4 follows the same int division rule as /: 15 / 4 is 3.

2. The right side comes first

The whole right side is worked out first. If x is 4, x *= 2 + 3 means x = x * (2 + 3), so x becomes 20, not 11 1.6.A.1.

int x = 4;
x *= 2 + 3;
System.out.println(x);
double price = 10.0;
price -= 2.5 * 2;
System.out.println(price);

Output:

20
5.0

Watch out: /= and %= on an int variable keep int arithmetic, so the decimal part is lost. On a double variable, /= keeps the decimals.

3. Increment and decrement

The post-increment operator ++ adds 1 to the stored value of a numeric variable, and the post-decrement operator -- subtracts 1. The new value is assigned to the variable 1.6.A.2.

int lives = 3;
lives++;   // 4
lives++;   // 5
lives--;   // 4
System.out.println("Lives: " + lives);

Output:

Lives: 4

Outside the course: the prefix form ++x, and using ++ or -- inside another expression, such as arr[x++]. On the exam, x++ and x-- appear as statements on their own.

4. Tracing a full sequence

Trace one line at a time and write the new value after each line.

int score = 12;
score += 8;
score /= 3;
score *= 4;
score %= 7;
score--;
System.out.println(score);

Line	Code	score
1	int score = 12;	12
2	score += 8;	20
3	score /= 3;	6 (int division: 20 / 3)
4	score *= 4;	24
5	score %= 7;	3
6	score--;	2

Output:

2