APCSA 1.6 — Compound Assignment Operators

In AP Computer Science A, compound assignment operators are shorthand ways to update the value of a variable. They let you do math on a variable and then store the result back into the same variable in one step.

1. What Do Compound Assignment Operators Do?

Normally, if you want to add 5 to a variable x, you might write:


x = x + 5;

With a compound assignment operator, you can write the same idea as:


x += 5;

This means: take the current value of x, add 5, and store it back into x.

Key idea: A compound assignment operator combines an operation (like + or -) with assignment (=) into one step.

2. Common Compound Assignment Operators

Operator	Example	Equivalent To	Meaning
+=	x += 3;	x = x + 3;	Add and assign
-=	x -= 2;	x = x - 2;	Subtract and assign
*=	x *= 4;	x = x * 4;	Multiply and assign
/=	x /= 2;	x = x / 2;	Divide and assign
%=	x %= 10;	x = x % 10;	Take remainder and assign

3. Why Are These Useful?

Less typing: You don’t have to repeat the variable name.
Easier to read: Makes it obvious that you are updating the same variable.
Common in loops: You’ll see these in counters, totals, and accumulators.

Example:


int total = 0;
total += 10;  // total is now 10
total -= 3;   // total is now 7
total *= 2;   // total is now 14

4. Integer Math Still Applies

If a variable is an int, it still behaves like an integer even when you use compound assignment. That means integer division cuts off the decimal.


int slices = 11;
slices /= 2;   // slices becomes 5, NOT 5.5

Because slices is an int, it cannot keep the .5 part. The decimal is thrown away.

5. Type Conversions and Truncation

Compound assignment can also force a value back into the original variable’s type.


int x = 5;
x *= 2.5;   // Allowed. x becomes 12

Here’s what happened:

5 * 2.5 is 12.5
But x is an int, so it can’t store decimals
Java automatically drops the decimal part → 12

This silent cut-off (called truncation) is something the AP exam likes to test.

6. How This Shows Up in APCS A

You need to be able to read and write statements like:


i += 2;         // counting by 2
sum += value;   // running total
count %= 10;    // keep last digit only

The College Board expects you to know what the operators +=, -=, *=, /=, and %= do and be able to predict the new value of the variable after they run.

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