list index out of range,python

Question

I have a question, that is, I have 1D list like this:

List_A = []

and I also have another list, something like this:

List_B = [(1,2,3),(2,3,4),(3,4,5)]

Now I try to use:

for i in range(3):
List_A[i] = List_B[i][2]

which means, I want take every last (3rd) number from List_B to List_A, but it is said "index out of range", so what is the problem? Thank you very much for your helping

I know the append method but I think I can't use it, because my code is under a loop, the value in List_B changes in every step,something like:

List_A = []
for i in range(5):
List_B = [(i*1,i*2,i*3),(i*2,i*3,i*4),(i*3,i*4,i*5)] 
 for j in range(3):
 List_A[j] = List_B[j][2]

that is to say, if I use append method, the List_A will enlarge to 15 element size, but I need refresh value in every i loop.

• Cory Kramer: List_A = [3] doesn't create a list with a length of 3, if that's what you think it did. It creates a list with a single element, whose value is 3.
• Bhargav Rao: Please edit your post to include the complete questions and not answers. Please add relevant examples too

Answer 1

The problem is with List_A which has only one value. Here you are trying to change the values for the indexes 1 and 2 of the list where there is none. So you get the error.

Use append instead after declaring List_A as []

for i in range(3):
 List_A.append(List_B[i][2])

This can be done in a single list-comp as

List_A = [i[2] for i in List_B]

---

Post edit-

Put the initialization right after the first loop

for i in range(5):
 List_A = [] # Here 
 List_B = [(1,2,3),(2,3,4),(3,4,5)] 
 for j in range(3):
 List_A.append(List_B[j][2])
 # Do other stuff

Answer 2

This loop is the problem

for i in range(3):
 List_A[i] = List_B[i][2]

You can only access List_A[0], the rest of the values of i are out of range. If you are trying to populate the list, you should use append

List_A = []
for i in range(3):
 List_A.append(List_B[i][2])

Or more Pythonic use a list comprehension

List_A = [i[2] for i in List_B]