PCEP Control Flow, Loops, Lists and Logic Practice Question

A programmer is building a simple inventory tracker using a Python list named stock that contains integer quantities. They need to add 10 to every quantity in the list, modifying the list in place. Which code correctly performs this operation?

⚠ Common exam trap

The trap here is assuming that modifying the loop variable modifies the list, when actually it only rebinds the variable to a new value.

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

✓

for i in range(len(stock)): stock[i] = stock[i] + 10

To modify a list in place, you must assign to individual elements using their indices. Using range(len(stock)) provides those indices, and stock[i] = stock[i] + 10 updates each element. The other options either leave the list unchanged, create a new list, or cause an error, so they do not satisfy the in-place requirement.

Answer analysis

Option-by-option breakdown

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

✗
stock = [qty + 10 for qty in stock]
Why it's wrong here
This list comprehension creates a new list and rebinds the name stock to it, rather than modifying the original list in place. If other references to the original list exist, they will still see the old values. The requirement is to modify the list in place, so this approach does not meet the specification.
✗
stock = stock + 10
Why it's wrong here
Adding an integer to a list with the + operator is not valid in Python and raises a TypeError. The + operator concatenates lists, but only with another list. This code would fail at runtime, so it cannot be used to update quantities. It is a common mistake when trying to apply arithmetic to all elements.
✗
for qty in stock: qty = qty + 10
Why it's wrong here
This loop assigns a new value to the loop variable qty, but that variable is a separate name bound to each element's value. Reassigning qty does not affect the list because integers are immutable and the loop variable is not a reference to the list slot. The list stock remains unchanged after this loop.
✓
for i in range(len(stock)): stock[i] = stock[i] + 10
Why this is correct
Iterating over indices with range(len(stock)) allows direct assignment to each element via stock[i], which modifies the list in place. This is the standard pattern for updating every element when you need to change the original list. The other approaches either do not modify the list or produce incorrect results.

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PCEP Control Flow, Loops, Lists and Logic Practice Question

Why each option matters

Option-by-option breakdown

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