AP CSP 3.8 Exercise 2: Iteration Applied Challenge

Big Idea 3: Algorithms and Programming · Topic 3.8 · Exercise 2

Iteration: Applied Challenge

Six loops. Count the iterations before you count anything else.

Back to the Topic 3.8 lesson All Big Idea 3 topics

How this one is different

Off by one is the single most common loop error on the exam. Every question here can be settled by counting passes carefully.

Applied Practice

6 questions · scenario driven · every answer is recorded for your teacher

Question 1 of 6Trace

What does this segment display?

i <- 1 total <- 0 REPEAT UNTIL (i > 4) { total <- total + i i <- i + 1 } DISPLAY total

Incorrect. 15 would include 5, but the loop exits before adding it.

Incorrect. 6 stops one iteration early, at 1 plus 2 plus 3.

Incorrect. 4 is the last value added, not the running total.

Correct. The loop adds 1, 2, 3, and 4 before i reaches 5 and the condition stops it, giving 10.

Question 2 of 6Trace

How many times does the body of this loop execute?

count <- 0 REPEAT 5 TIMES { count <- count + 1 }

Incorrect. The body changing count has no effect on how many times a REPEAT n TIMES loop runs.

Incorrect. 4 undercounts. REPEAT n TIMES runs exactly n passes.

Correct. REPEAT 5 TIMES is an exact count, independent of anything in the body.

Incorrect. 6 overcounts by one, which is the usual result of counting the check that ends the loop as a pass.

Question 3 of 6Debug

A loop never ends.

Which change fixes it?

i <- 1 REPEAT UNTIL (i > 3) { DISPLAY i }

Incorrect. The condition is fine. Nothing in the body ever changes i, so no comparison would ever become true.

Correct. A condition based loop needs the body to make progress toward the exit condition. Incrementing i is that progress.

Incorrect. Removing the display leaves an empty loop that still never ends.

Incorrect. This does terminate, but it is a different loop rather than a fix to the described bug, and it still never advances i.

Question 4 of 6Apply

Which loop displays exactly the values 1, 2, and 3?

Correct. Starting at 1 and stopping once i passes 3 displays exactly 1, 2, and 3.

Incorrect. The loop exits as soon as i reaches 3, so 3 is never displayed.

Incorrect. This continues through 4.

Incorrect. Starting at 0 displays 0, 1, 2, and 3.

Question 5 of 6Trace

What does this segment display?

n <- 16 steps <- 0 REPEAT UNTIL (n = 1) { n <- n / 2 steps <- steps + 1 } DISPLAY steps

Incorrect. 8 is the value after one halving, not the step count.

Incorrect. 16 is the starting value.

Incorrect. 2 stops after reaching 4.

Correct. 16 halves to 8, 4, 2, and 1, which is four steps.

Question 6 of 6Analyze

A loop processes every element of a list whose length changes at runtime.

Which loop form is appropriate, and why?

Incorrect. Loops handle unknown counts routinely, which is much of what they are for.

Incorrect. A fixed count either runs past the end of a short list or misses part of a long one.

Correct. FOR EACH is defined in terms of the list itself, so the iteration count follows the data rather than a constant written in the code.

Incorrect. A loop with no exit condition never terminates.

Where to go next

Rework the Topic 3.8 lesson and quiz Sit the Big Idea 3 unit test part A Sit the Big Idea 3 unit test part B

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