CODEWITHSUNDEEP
pythonlistnested-loopsnested-lists
cooldood3490
cooldood3490

Reputation: 2488

Why am I getting "list assignment index out of range" error?

I keep getting the same error:

Traceback (most recent call last):
  File "<stdin>", line 3, in <module>
IndexError: list assignment index out of range

What's wrong with the following code?

myList = [([None] * 8) for x in range(16)]

for i in range(0,7):
    for j in range(0,15):
        myList[i][j] = 2 * i

Upvotes: 1

Views: 615

Answers (2)

thefourtheye
thefourtheye

Reputation: 239443

The list comprehension

myList = [([None] * 8) for x in range(16)]

can be understood like this

mylist = []
for x in range(16):
    mylist.append([None] * 8)

So, you are creating a list of 16 lists, each contains 8 Nones. But with the loops

for i in range(0,7):
    for j in range(0,15):

you are trying to access 15 elements from the first 7 lists. That is why it is failing. Instead, you might want to do

for i in range(16):
    for j in range(8):
        ...

Actually you can do the same in list comprehension, like this

[[2 * i for j in range(8)] for i in range(16)]

Demo:

>>> from pprint import pprint
>>> pprint([[2 * i for j in range(8)] for i in range(16)])
[[0, 0, 0, 0, 0, 0, 0, 0],
 [2, 2, 2, 2, 2, 2, 2, 2],
 [4, 4, 4, 4, 4, 4, 4, 4],
 [6, 6, 6, 6, 6, 6, 6, 6],
 [8, 8, 8, 8, 8, 8, 8, 8],
 [10, 10, 10, 10, 10, 10, 10, 10],
 [12, 12, 12, 12, 12, 12, 12, 12],
 [14, 14, 14, 14, 14, 14, 14, 14],
 [16, 16, 16, 16, 16, 16, 16, 16],
 [18, 18, 18, 18, 18, 18, 18, 18],
 [20, 20, 20, 20, 20, 20, 20, 20],
 [22, 22, 22, 22, 22, 22, 22, 22],
 [24, 24, 24, 24, 24, 24, 24, 24],
 [26, 26, 26, 26, 26, 26, 26, 26],
 [28, 28, 28, 28, 28, 28, 28, 28],
 [30, 30, 30, 30, 30, 30, 30, 30]]

Note: I have used 16 and 8 in the range function because it, by default, starts from 0 and iterates till the parameter passed to it - 1. So, 0 to 15 and 0 to 8 respectively.

Upvotes: 1

Martijn Pieters
Martijn Pieters

Reputation: 1121524

You have your indices reversed; the outer list has 16 elements, but you are trying to index the inner list past the 8 elements that are there.

That's because your list comprehension built 16 lists of each 8 values, so len(myList) is 16, and len(myList[0]) is 8.

Reverse the ranges:

for i in range(15):
    for j in range(7):
        myList[i][j] = 2 * i

or reverse the use of i and j when trying to index against myList:

for i in range(7):
    for j in range(15):
        myList[j][i] = 2 * i

Note that the outer myList is using j now and each nested inner list is indexed with i.

Both i and j omit the last element, if that wasn't intentional, use 8 and 16 rather than 7 and 15.

Upvotes: 1

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