📓1.6: Compound Operators

Table of Contents

• Compound Assignment Operators
  • Operator Shortcuts
    • Code Tracing
• Summary

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📖 This page is a condensed version of CSAwesome Topic 1.6

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Compound Assignment Operators

💬 As a class, brainstorm some examples of how people use shortcuts.

Compound assignment operators +=, -=, *=, /=, and %= are shortcuts that do a math operation and an assignment in one step.

For example, x += 1 adds 1 to x and stores the result back in x (same as x = x + 1).

If a mnemonic helps: the operator happens first, then the result is assigned back. So it’s += (not =+).

Since changing a variable by 1 is so common, Java also has ++ (increment) and -- (decrement) extra concise operator.

• x++ is even shorter than x += 1 or x = x + 1.
• y-- is the same idea for subtracting 1.

Operator Shortcuts

Meaning	Written out	Compound	Extra Concise
add	x = x + 1	x += 1	x++
subtract	x = x - 1	x -= 1	x--
multiply	x = x * 2	x *= 2	—
divide	x = x / 2	x /= 2	—
remainder	x = x % 2	x %= 2	—

Type each of these lines, run, and observe. Then complete the two TODOs.

int score = 0;
System.out.println(score); // 0

score++;                   // +1
System.out.println(score); // 1

score *= 2;                // ×2
System.out.println(score); // 2

int penalty = 5;
score -= penalty / 2;      // 2 - (5/2) -> 2 - 2 -> 0 (integer division)
System.out.println(score); // 0

// 1) Add 3 to score using a compound operator
// score ...

// 2) Divide score by 2 using a compound operator
// score ...

Code Tracing

Code tracing means simulating a run through the program line by line, as if you are the computer.

Use a trace table (or bullet points) to track variable values as they change and predict output.

🔮 Predict: What are the values of x, y, and z after this code?

int x = 0;
int y = 1;
int z = 2;
x--;
y++;
z += y;

🔮 Predict: What are the values of x, y, and z after this code?

int x = 3;
int y = 5;
int z = 2;
x = z * 2;
y = y / 2;
z++;

Try the Operators Maze game: Operators Maze game.

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Summary

• (AP 1.6.A.1) Compound assignment (+=, -=, *=, /=, %=) performs the operation using the current value on the left and the value on the right, then assigns the result back to the left variable.
• (AP 1.6.A.2) Increment (++) and decrement (--) add or subtract 1 from a numeric variable and store the new value.

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Acknowledgement

Content on this page is adapted from Runestone Academy - Barb Ericson, Beryl Hoffman, Peter Seibel.