Topic 1.6 Notes – Compound Assignment Operators

What Compound Assignment Operators Do

You already know a basic assignment:

x = x + 4;

A compound assignment operator compresses that idea:

x += 4;

General pattern:

variable op= expression;

means the same as:

variable = variable op expression;

Here’s the flow when you run something like x += 4;:

Execution flow of a compound assignment statement

Important details:

• The right side is evaluated first.
• The operation uses the variable’s current value.
• The result replaces the old value in that variable.
• The variable must already be declared and initialized.

If you try this:

int total;
total += 5;   // error

Java complains because total has no starting value to use in the calculation.

The Five Compound Assignment Operators

All five follow the same structure. What changes is the operation.

+= Add and assign

sum += 7;

• Adds 7 to the current value of sum.
• Common for totals, counters, accumulating values in loops.
• Also works with Strings:

String name = "AP";
name += " CSA";   // "AP CSA"

With strings, it concatenates.

-= Subtract and assign

balance -= 20;

• Decreases a value.
• Used for score deductions, removing quantities, etc.

*= Multiply and assign

price *= 2;

• Scales a number.
• Often seen in growth or repeated multiplication.

/= Divide and assign

count /= 3;

• Divides the current value.
• If count is an int, integer division rules apply.

Example:

int x = 7;
x /= 2;   // x becomes 3

Decimals are truncated.

%= Modulus and assign

num %= 4;

• Stores the remainder.
• Helpful for wrapping values or checking cycles.

Type Rules You Need to Know

• Work with int and double.
• += works with String.
• Automatic casting can happen:

int x = 5;
x += 2.9;   // x becomes 7

The decimal is truncated when stored back into an int.

That truncation is a common multiple-choice trap.

Increment and Decrement Operators

These are specialized compound assignments.

Post-increment x++

Equivalent to:

x += 1;

Adds 1 to the variable and stores it back.

Post-decrement x--

Equivalent to:

x -= 1;

Subtracts 1 and stores it back.

On the AP exam:

• You will only see post form (x++, x--).
• You will not see ++x.
• You will not see it embedded in expressions like arr[i++].

They are usually standalone statements, especially in loops.

Example:

for (int i = 0; i < 5; i++) {
    System.out.println(i);
}

Each loop iteration increases i by 1.

Tracing Compound Assignments Correctly

When you see code like this on a quiz:

int a = 4;
int b = 3;
a *= b + 2;
b -= 1;
a += b;

Trace it line by line:

Start: a = 4, b = 3

a *= b + 2
b + 2 = 5
a = 4 * 5 = 20

b -= 1
b = 2

a += b
a = 20 + 2 = 22

Final values: a = 22, b = 2.

Students often mess up by forgetting that a was already changed before the last line.

Where This Shows Up

You’ll see compound assignment constantly in:

• Running totals → sum += arr[i];
• Counting matches → count++;
• Updating instance variables inside methods
• Loop control variables → i++

If you’re writing a loop on an FRQ, you’re almost guaranteed to use ++ or +=.