AP CSA Unit 1 Day 24: Complex Return Tracing
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Complex Method Return Tracing
Section 1.8 — Methods: Return Values
Key Concept
Complex method return tracing involves following the flow of values through multiple method calls where return values feed into subsequent calculations. When methodA() calls methodB() which calls methodC(), you must trace the innermost call first, pass its return value back, and continue outward. The AP exam adds complexity by having methods modify instance variables as a side effect while also returning values, so you must track both the return value and the object state changes.
Consider the following methods.
What is returned by the call puzzle(3)?
Answer: (A) 6
Trace from the outside in:
puzzle(3): x = 3. Returns mystery(3, 3 * 3) = mystery(3, 9).
mystery(3, 9): a = 3, b = 9. Returns a + b / a = 3 + 9 / 3 = 3 + 3 = 6.
Note: b / a is evaluated first (division before addition), giving 9 / 3 = 3. Then 3 + 3 = 6.
Why Not the Others?
(B) This might come from computing (a + b) / a = (3 + 9) / 3 = 4. But division has higher precedence than addition, so b / a is computed first.
(C) 12 = a + b = 3 + 9, ignoring the division entirely.
(D) No valid computation path produces 5 with these values.
Common Mistake
In a + b / a, division happens before addition (operator precedence). So it is a + (b / a), not (a + b) / a. Many students incorrectly group left to right without considering precedence.
AP Exam Tip
When tracing nested method calls, start with the outermost call to determine the arguments, then trace the inner call. Always apply operator precedence within return expressions.