How do i add element from one list into another list?

Question

I am trying to create a new list from the 4th element of preexisting lists that are read in from a text file in order to get the sum of all elements. This is what I currently have, but I can't get it working.

mainlist = []
newlist = []
openfile = open('filename.txt', 'r')

for line in openfile:
    line = line.rstrip().split()
    mainlist.append(line)

for i in mainlist:
    newlist.append(mainlist[i][4])

I am a complete novice so any help would be greatly appreciated.

Answer 1

Here we are

with  open('source.txt', 'r') as openfile:
    mainlist = [line.rstrip().split(', ') for line in openfile]
    newlist = [item[3:] for item in mainlist]

    print(newlist)

Outputs

['30', '34', '80']

In case you want all "counts" from the third column

openfile = open('source.txt', 'r')

mainlist = [line.rstrip().split(', ') for line in openfile]
newlist = [item[3:] for item in mainlist]

print(newlist)

Outputs

[['30', '30', '30'], ['34', '32', '23'], ['80', '30', '32']]