PCEP Control Flow, Loops, Lists and Logic Practice Question

Exhibit

>>> nums = [0, 1, 2, 3, 4, 5]
>>> for i in range(len(nums)):
...     if nums[i] % 2 == 0:
...         nums.remove(nums[i])
...
>>> nums
[1, 3, 5]

Refer to the exhibit. Why does the code successfully remove all even numbers but the output is [1, 3, 5]?

⚠ Common exam trap

The PCEP exam often tests the subtle bug of modifying a list while iterating forward, where candidates mistakenly believe the code works correctly because the output happens to be correct in this specific case, overlooking that the logic is flawed and would fail with a different arrangement of numbers.

Answer choices

Why each option matters

Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.

Correct answer & explanation

✓

Because removing elements while iterating forward causes skipping of elements, but in this case the even numbers are at indices 0,2,4 and the removal shifts subsequent elements; however, the loop variable i increments and skips the next odd number, leading to correct removal by chance.

When iterating forward over a list while removing elements, the removal shifts subsequent elements to lower indices, causing the loop variable i to skip the next element. In this specific case, the even numbers happen to be at indices 0, 2, and 4, and after each removal the next element (which is odd) shifts into the current index, but i increments past it, so the odd numbers are never checked and remain, resulting in the correct output by coincidence rather than by correct logic.

Answer analysis

Option-by-option breakdown

For each option: why learners choose it and why it is or isn't the right answer here.

✗
Because the condition checks for even numbers and removes them correctly without side effects.
Why it's wrong here
Modifying a list while iterating forward generally causes side effects.
✓
Because removing elements while iterating forward causes skipping of elements, but in this case the even numbers are at indices 0,2,4 and the removal shifts subsequent elements; however, the loop variable i increments and skips the next odd number, leading to correct removal by chance.
Why this is correct
The code works because the even numbers are at even indices and after removal, the next element becomes at the same index, but since i increments, it skips the next element; however, the skipped elements are odd and remain, so the result is all odds.
✗
Because the range is computed once and the list is shortened, so the loop ends early.
Why it's wrong here
The loop goes through all indices initially, but removal reduces list length.
✗
Because the remove() method removes only the first occurrence, and the loop iterates backwards.
Why it's wrong here
The loop does not iterate backwards.

Go deeper

PCEP Control Flow, Loops, Lists and Logic Practice Question

Exhibit

Why each option matters

Option-by-option breakdown

Related to this question