AP CSP Day 37: Accumulator Pattern | Cycle 2
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Accumulator initialization errors cause systematic incorrect results: initializing a sum accumulator to 1 instead of 0 adds 1 to every computed total, and initializing a product accumulator to 0 instead of 1 makes every product zero. A subtler error is resetting the accumulator inside the loop rather than before it, restarting the accumulation on every iteration. AP CSP Cycle 2 accumulator questions present pseudocode with one of these initialization or placement errors and ask students to determine the incorrect output and explain the fix.
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Accumulator Bugs: Wrong Initialization
Wrong Starting Value
A sum accumulator initialized to 1 instead of 0 adds 1 to every computed sum. A product accumulator initialized to 0 instead of 1 makes every product 0 regardless of input. The initial value is as important as the accumulation logic.
Reset Inside Loop
Initializing the accumulator inside the loop body resets it to the starting value on every iteration, discarding all previous work. The accumulator initialization must appear before the loop.
Practice Question
What is displayed after the following code runs?
nums ← [4, 7, 2, 9, 1, 6]
result ← 0
FOR EACH n IN nums
{
IF n MOD 2 = 1
{
result ← result + n
}
}
DISPLAY(result)The code accumulates only odd numbers (n MOD 2 = 1). Checking each: 4 is even (skip), 7 is odd (add), 2 is even (skip), 9 is odd (add), 1 is odd (add), 6 is even (skip). Sum of odds: 7 + 9 + 1 = 17.
A) 29 is the sum of all elements (4+7+2+9+1+6), not just the odd ones. B) 12 is the sum of even elements (4+2+6). D) 22 does not match any valid subset sum.
Students either sum all elements without applying the MOD filter, or confuse MOD 2 = 1 (odd) with MOD 2 = 0 (even), computing the sum of even numbers instead.
When you see an accumulator inside a loop with a condition, identify two things separately: what gets added (the accumulator update) and when it gets added (the filter condition).
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