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To get 

[[1, 1], [2, 2], [3, 3, 3], [4, 4, 4, 4], [5, 5, 5, 5, 5]]

from old_list = 

[1,1,2,2,3,3,3,4,4,4,4,5,5,5,5,5]

I found this answer:

new_list = []
for value in old_list:
    if new_list and new_list[-1][0] == value:
        new_list[-1].append(value)
    else:
        new_list.append([value])

I am trying to understand what is meaning of new_list[-1][0] confusion came when I tried to run

new_list = []
new_list[-1] # shows index out of bounds

and 

new_list and new_list[-1][0]
does not produce any error under if statement

What 

new_list[-1][0]
is doing here.

Anonymous Asked question May 14, 2021