IndexError: list assignment index out of range.

Question

The following code raises an IndexError , can anyone explain why the logic of this code does not work?

midterms = [90, 80, 89, 87, 97, 100]
for mark in midterms:
    newMark = mark + 2
    midterms[mark] = newMark
print(midterms)

Answer 1

Because you're using the values contained in the list as indices; mark takes the values 90, 80, ..., 100 . The subscription midterm[90] is obviously out of bounds.

To iterate through the items while also having a handle on the position, Python offers enumerate which provides an index along with the current value:

midterms = [90, 80, 89, 87, 97, 100]
for ind, mark in enumerate(midterms):
    newMark = mark + 2
    midterms[ind] = newMark
print(midterms)

This, in effect, allows iterating through the list and changing it effortlessly.

Answer 2

Another way to think of this if you're new to python (I didn't know about enumerate as a beginner) is to use Jerrybibo's suggestion. Code for that would look like this:

midterms = [90, 80, 89, 87, 97, 100]
for i in range(len(midterms)):
    newMark = midterms[i] + 2
    midterms[i] = newMark
print(midterms)

Answer 3

Jim Fasarakis-Hilliard and I share the same answer. Another workaround for your code is this:

midterms = [90, 80, 89, 87, 97, 100]
print [mark+2 for mark in midterms]

which shall also yield

[92, 82, 91, 89, 99, 102]

python