AP CSP Practice: Procedures & Return Values

Big Idea 3: Algorithms and Programming
Day 7 Practice • AP CSP Daily Question
🎯 Focus: Procedures and Return Values

Practice Question

Consider the following procedure:
PROCEDURE mystery(a, b)
{
    IF (a > b)
    {
        RETURN a - b
    }
    ELSE
    {
        RETURN b - a
    }
}
What value is returned by the call mystery(8, 15)?
What This Tests: Big Idea 3 covers procedures (functions) that take parameters and return values. This question tests your ability to trace through conditional logic with given inputs.

Step-by-Step Trace

Call: mystery(8, 15)

Parameters: a = 8, b = 15

Step Code Evaluation
1 IF (a > b) IF (8 > 15) → FALSE
2 Go to ELSE branch —
3 RETURN b - a RETURN 15 - 8 = 7

What Does This Procedure Do?

This procedure calculates the absolute difference between two numbers:

  • If a > b, return a - b (positive result)
  • If b ≥ a, return b - a (positive result)

The result is always positive because it always subtracts the smaller from the larger!

// Examples:
mystery(10, 3)  → 10 - 3 = 7   // a > b, use IF branch
mystery(3, 10)  → 10 - 3 = 7   // b > a, use ELSE branch
mystery(5, 5)   → 5 - 5 = 0    // equal, use ELSE branch

Common Mistakes

Mistake: Answer A (-7) - Wrong branch

If you got -7, you calculated a - b (8 - 15) instead of b - a. Since 8 is NOT greater than 15, we use the ELSE branch, which computes b - a = 15 - 8 = 7.

Mistake: Answer C (23) - Adding instead of subtracting

The procedure subtracts values, not adds them. 8 + 15 = 23, but the code says RETURN b - a.

Mistake: Confusing parameters

In mystery(8, 15), the first value (8) becomes 'a' and the second value (15) becomes 'b'. Parameter order matters!

💡 AP Exam Tip

When tracing procedures: (1) Write down what each parameter equals, (2) Evaluate conditions with those values, (3) Follow ONLY the branch that executes. Don't try to hold everything in your head—write it out!

Difficulty: Medium • Time: 2-3 minutes • Topic: 3.12 Calling Procedures & 3.13 Developing Procedures

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