AP CSP Day 56: Creative Commons
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Practice Question
What is displayed after the following code runs?
x ← 1
total ← 0
REPEAT UNTIL x > 100
{
total ← total + x
x ← x * 2
}
DISPLAY(total)Trace: x=1,total=0. Iteration 1: total=1,x=2. Iteration 2: total=3,x=4. Iteration 3: total=7,x=8. Iteration 4: total=15,x=16. Iteration 5: total=31,x=32. Iteration 6: total=63,x=64. Iteration 7: total=127,x=128. Now x=128>100, loop stops. total=127.
A) 63 is total after 6 iterations, but x=64 is not > 100, so one more iteration occurs. C) 100 is the threshold, not the sum. B) 255 would require one additional iteration (x=256), but x=128 already exceeds 100.
Students stop one iteration too early (at total=63 when x=64) because 64 seems close to 100. The condition x > 100 is not met until x=128, so the iteration where x goes from 64 to 128 does execute.
REPEAT UNTIL checks the condition BEFORE each iteration. If the condition is still false (x ≤ 100), the loop body executes. Continue iterating until the condition is checked and found to be true.
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