如何替换给定范围内列表列表中的元素？

Question

Let's say I've defined list of lists, eg:

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

What I want to do is to create a loop which will be iterating through every list in list a and replace elements in given range (increasing by 1 when moving to next list) with zeros, so the output will be:

a = [[1,2,1,1,1],[0,1,3,4,1],[0,0,2,5,1],[0,0,0,2,3],[0,0,0,0,1]]

I've tried different ideas, but finally none of them seem to be working and I feel like I'm misunderstanding something.

I've tried:

k = 0
for i in range(len(a)):
    a[i][:k] = 0
    k += 1

But it didn't work.

Answer 1

I assume that the last element of the last sublist should be zero.

a = [[1,2,1,1,1],[1,1,3,4,1],[2,1,2,5,1],[1,1,2,2,3],[1,1,1,1,1]]
for i, row in enumerate(a):
    row[:i+1] = [0] * (i+1)

The 3rd line uses the Python's slice subscription, which we use to assign an iterable (list of zeros in particular) to it.

Answer 2

Here's the basic way to do it:

a = [[1,2,1,1,1],[1,1,3,4,1],[2,1,2,5,1],[1,1,2,2,3],[1,1,1,1,1]]
for i in range(len(a)):
    for j in range(i):
      a[i][j]=0
print(a)

Assuming that the first element of the first list should not be a Zero because it is "(increasing by 1 when moving to next list)"

Answer 3

b = []
for i, sublist in enumerate(a, 1):
   b.append([0] * i  + sublist[i:])