AP CSP Day 34: Iteration Patterns | Cycle 2
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Loop termination errors occur when a REPEAT UNTIL condition is written incorrectly, causing infinite loops or premature exits. An off-by-one error in iteration count produces a result that is consistently one step short or long of the correct answer. AP CSP Cycle 2 iteration questions often show pseudocode with a subtle termination condition error and ask students to identify the incorrect output produced. Tracing the loop counter and accumulated value through the first two and last two iterations reveals whether the loop terminates correctly.
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Iteration Bugs: Wrong Termination
Infinite Loop
An infinite loop occurs when the REPEAT UNTIL condition can never become true given the loop's logic. If the loop body never moves the relevant variable toward satisfying the condition, the loop runs forever.
Early Termination
A loop that terminates one iteration too early produces a result that is one step short of correct. This often happens when the condition checks the variable before updating it instead of after, or when the count starts at 1 instead of 0.
Practice Question
What is displayed after the following code runs?
n ← 64
count ← 0
REPEAT UNTIL n < 2
{
n ← n / 2
count ← count + 1
}
DISPLAY(count)Trace: n=64→2=32(count=1), 32→2=16(count=2), 16→2=8(count=3), 8→2=4(count=4), 4→2=2(count=5), 2→2=1(count=6). Now n=1 < 2 is true, loop stops. count = 6.
A) 5 stops one iteration too early, when n=2 instead of halving once more. B) 7 counts one extra iteration that doesn't occur. D) 32 confuses 64/2 with the iteration count.
Students miscalculate the stopping point. The loop continues while n ≥ 2, so n=2 still triggers another iteration (n becomes 1, count becomes 6). The condition n < 2 is only checked before each iteration.
For REPEAT UNTIL loops, the condition is checked BEFORE each iteration. If the condition is false, the body executes. Trace carefully until the condition becomes true.
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