Get indices of the first element in a python list of lists

Question

I have a list like this one:

[[21], [22, 1], [23, 2], [24, 1, 0, 4], [25, 2, 3]]

and I want to append to a new list the indices of the first item of each sublist, while counting all elements in all the sublists. Each index of the new list depends on the length of each sublist. The result I am looking for is this one:

[0, 1, 3, 5, 9].

I don't want to use numpy or any other library. Thanks!

question from:https://stackoverflow.com/questions/65853400/get-indices-of-the-first-element-in-a-python-list-of-lists

Answer 1

You need something like this:

l = [[21], [22, 1], [23, 2], [24, 1, 0, 4], [25, 2, 3]]

n = []
p = 0
for s1 in l:
    n.append(p)
    p += len(s1)

print(n)