Question

I am doing Project Euler Problem 15. I used the right algorithm, but it didn’t seem to work. Here’s my code:

f = [[0] * 21] * 21

# init the list
for i in range(21):
    f[0][i] = 1
    f[i][0] = 1

for i in range(21):
    for j in range(21):
        f[i][j] = f[i-1][j] + f[i][j-1]

print f[20][20]

when I finished initializing the list, I printed it. I expected it like [[1, 1, 1...], [1, 0, 0...]...], but it turned [[1, 1, 1...], [1, 1, 1...]...] and I can’t figure why.

I used to use C-like language and I thought the list in Python is kind of like the array in C, so I used them in the same way.

Answer 1

When multiplying a list, you are not creating separate lists but rather create multiple references to the same list.

Instead, do this:

f = [[0 for _ in range(21)] for _ in range(21)] 

You can see the difference when using the id() function:

>>> f = [[0]*21]*21
>>> for nested in f[:3]:
...     print id(nested)
... 
4523317152
4523317152
4523317152
>>> f = [[0 for _ in range(21)] for _ in range(21)] 
>>> for nested in f[:3]:
...     print id(nested)
... 
4523317512
4523317440
4523317656

Separate objects have separate memory id values, while your lists have the same result for each id() call, showing they are all the same list.