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)
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3 Answers
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.
answered Dec 14, 2016 at 1:24
Dimitris Fasarakis Hilliard
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2 Comments
Jerrybibo
I guess you could also use the
range(len(midterms)) function.Dimitris Fasarakis Hilliard
@Jerrybibo you could be you really shouldn't. This is exactly why
enumerate exists.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)
Comments
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]
