Question

In Python I want an intuitive way to create a 3 dimensional list.

I want an (n by n) list. So for n = 4 it should be:

x = [[[],[],[],[]],[[],[],[],[]],[[],[],[],[]],[[],[],[],[]]]

I’ve tried using:

y = [n*[n*[]]]    
y = [[[]]* n for i in range(n)]

Which both appear to be creating copies of a reference.
I’ve also tried naive application of the list builder with little success:

y = [[[]* n for i in range(n)]* n for i in range(n)]
y = [[[]* n for i in range(1)]* n for i in range(n)]

I’ve also tried building up the array iteratively using loops, with no success. I also tried this:

y = []
for i in range(0,n):
    y.append([[]*n for i in range(n)])

Is there an easier or more intuitive way of doing this?

Answer 1

I think your list comprehension versions were very close to working. You don’t need to do any list multiplication (which doesn’t work with empty lists anyway). Here’s a working version:

>>> y = [[[] for i in range(n)] for i in range(n)]
>>> print y
[[[], [], [], []], [[], [], [], []], [[], [], [], []], [[], [], [], []]]