Lesson 1.6: Compound Assignment Operators | AP CSA

Unit 1 · Lesson 1.6 · Code Mechanics

Lesson 1.6: Compound Assignment Operators

Reading time: 5–7 min · Practice: 6 exercises · Mastery: applied scenario

What you'll learn in this lesson

  • 1.6.A. Use compound assignment operators +=, -=, *=, /=, and %= as shorthand for full assignment statements, and predict the value stored after each.
  • 1.6.A. Use the post-increment (++) and post-decrement (--) operators to add or subtract 1 from a variable.

This lesson is part of the AP CSA Course and Exam Description (effective May 2027).

AP exam weight: Compound operators appear most often inside loops. The exam tests whether you can trace n += n, n *= 2, and sequences mixing ++ with arithmetic. Prefix form (++x) and operators inside expressions (arr[x++]) are explicitly excluded from the AP exam.

Key Vocabulary

Term Definition
compound assignment operator A shorthand combining arithmetic with assignment: +=, -=, *=, /=, %=.
post-increment The ++ operator as a standalone statement; adds 1 to a variable. x++ means x = x + 1.
post-decrement The -- operator as a standalone statement; subtracts 1 from a variable. x-- means x = x - 1.

Compound assignment operators: shorthand that still assigns

A compound assignment operator combines an arithmetic operation with assignment into one statement. Each one is exactly equivalent to its expanded form.

The Five Operators

x += y means x = x + y
x -= y means x = x - y
x *= y means x = x * y
x /= y means x = x / y
x %= y means x = x % y

The operator performs the arithmetic using the current value of x and the value of y, then stores the result back into x.

Tracing a Sequence

int n = 10;
n += 5;    // n = 10 + 5 = 15
n *= 2;    // n = 15 * 2 = 30
n -= 8;    // n = 30 - 8 = 22
n /= 2;    // n = 22 / 2 = 11 (int division)
n %= 4;    // n = 11 % 4 = 3

AP Trap: n += n

n += n is equivalent to n = n + n, which doubles n. Students sometimes read it as “add nothing” or confuse it with n++. If n = 6, then after n += n, n = 12.

Increment and decrement operators

The post-increment operator ++ adds 1 to the stored value of a variable and assigns the result back. The post-decrement operator -- subtracts 1.

Post-form Only on the AP Exam

x++ means x = x + 1 (post-increment)
x-- means x = x - 1 (post-decrement)

AP Exclusion: prefix form (++x) and using either operator inside another expression (e.g., arr[x++]) are out of scope. On the AP exam, x++ always appears as a standalone statement.

Example in Context

int count = 0;
count++;      // count = 1
count++;      // count = 2
count--;      // count = 1
System.out.println(count); // prints 1
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Tier 2 · AP Practice

Practice Questions

What value is stored in k after the following code segment executes?

int k = 8;
k -= 3;
k *= k;
k /= 5;
Trace each line. Write k's value after each statement before choosing.
A. 1
B. 25
C. 5
D. 4
C. k=8. After k -= 3: k=5. After k *= k: k=5*5=25. After k /= 5: k=25/5=5 (int division, exact here). A results from 5/5 then subtracting 4. B stops after the multiplication. D uses wrong intermediate values.
Consider the following code segment.

int num = 5;
num += num;
num *= num;
System.out.println(num);
Carefully evaluate num += num first. What does that give you? Then apply the next operator.
A. 100
B. 25
C. 50
D. 10
A. num += num means num = num + num = 5 + 5 = 10. Then num *= num means num = 10 * 10 = 100. B stops after the first operation. C multiplies 10 by 5. D stops after the first operation without squaring.
Assume int x = 12. Which of the following are equivalent to x -= 4?

I. x = x - 4;
II. x = 4 - x;
III. x--; x--; x--; x--;
A. I only
B. II only
C. I and II only
D. I and III only
D. x -= 4 subtracts 4, giving 8. Statement I: x = x - 4 = 12 - 4 = 8—identical. Statement II: x = 4 - x = 4 - 12 = -8—wrong sign. Statement III: four decrements each subtract 1, total subtraction of 4, giving 8—equivalent. I and III both produce 8; II produces -8.
A student writes the following code intending total to hold exactly half its current value as a decimal.

double total = 9.0;
total /= 2;
System.out.println(total);

The student expects 4.5. Which of the following best describes the actual output?
A. A compile error because /= requires both sides to be the same type.
B. The program prints 4.5 because total is double, so total / 2 performs double division.
C. The program prints 4.0 because /= always performs integer division when the right side is an int.
D. A run-time error because you cannot divide a double by an int.
B. total /= 2 expands to total = total / 2. Since total is double, this is 9.0 / 2—double divided by int = double = 4.5. The student’s expectation is correct. A is wrong: /= works with mixed types. C is wrong: the type of the left operand determines whether the result is double or int. D is wrong: dividing double by int is valid.
What is printed when the following code segment executes?

int j = 3;
j++;
j++;
j--;
j += j;
System.out.println(j);
Trace each operator one at a time. What is j after each line?
A. 8
B. 6
C. 10
D. 4
A. j=3. After j++: j=4. After j++: j=5. After j--: j=4. After j += j: j=4+4=8. B stops before the last step. C applies an extra increment. D stops after the decrement.
What value does n hold after the following code executes?

int n = 100;
n /= 4;
n %= 7;
n += n;
Trace each line in order. Write n after each statement before choosing.
A. 4
B. 8
C. 25
D. 50
B. n=100. After n /= 4: n=25 (int division exact). After n %= 7: 25 ÷ 7 = 3 remainder 4, so n=4. After n += n: n=4+4=8. A stops after the %= step. C stops after /=. D skips the %= step entirely.
What does the program print?
Trace every step before choosing. Write shelf after each line.
A. 0
B. 2
C. 1
D. 4
A. Trace: shelf=20. After -= 3: shelf=17. After += shelf: shelf=17+17=34. After /= 5: shelf=34/5=6 (int division). After %= 3: shelf=6%3=0. Prints 0. B and C result from arithmetic errors mid-trace. D results from stopping before the final %=.

Part B: Identify the operator

After shelf -= 3 runs with shelf = 20, the next line is shelf += shelf. Which of the following single statements produces the same result as shelf += shelf?
A. shelf += 17;
B. shelf++;
C. shelf *= 2;
D. shelf = shelf + 1;
C. shelf += shelf means shelf = shelf + shelf, which doubles shelf. shelf *= 2 also doubles shelf and always produces the same result regardless of shelf's value. A uses the hardcoded value 17 so it only works for that specific value. B adds 1. D also adds 1.

Part C: Structured response

A new requirement says the final shelf count must be at least 1 (to avoid an empty shelf). Write a version of the last line (shelf %= 3) that ensures shelf is never 0 after the operation, and explain in two sentences why your change works.

Extension · Beyond the Exam

Why post-increment matters in real code

In a loop like for (int i = 0; i < 10; i++) (defined in the Java Language Specification §15.14), the i++ is post-increment used as a standalone update statement—exactly the AP-tested form. The difference between i++ and ++i only matters when the expression’s return value is used (e.g., arr[i++] vs arr[++i]), which is why those forms are excluded from the AP exam. In most real loops, i++ and ++i behave identically.

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