How to create nested lists in python?

Question

I know you can create easily nested lists in python like this:

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

But how to create a 3x3x3 matrix of zeroes?

[[[0] * 3 for i in range(0, 3)] for j in range (0,3)]

or

[[[0]*3]*3]*3

Doesn't seem right. There is no way to create it just passing a list of dimensions to a method? Ex:

CreateArray([3,3,3])

Answer 1

Do you mean like this?

[[[0]*3]*3]*3

Answer 2

Just nest the multiplication syntax:

[[[0] * 3] * 3] * 3

It's therefore simple to express this operation using folds

def zeros(dimensions):
    return reduce(lambda x, d: [x] * d, [0] + dimensions)

Or if you want to avoid reference replication, so altering one item won't affect any other you should instead use copies:

import copy
def zeros(dimensions):
    item = 0
    for dimension in dimensions:
        item = map(copy.copy, [item] * dimension)
   return item

Answer 3

NumPy addresses this problem

http://www.scipy.org/Tentative_NumPy_Tutorial#head-d3f8e5fe9b903f3c3b2a5c0dfceb60d71602cf93

>>> a = array( [2,3,4] )
>>> a
array([2, 3, 4])
>>> type(a)
<type 'numpy.ndarray'>

But if you want to use the Python native lists as a matrix the following helper methods can become handy:

import copy

def Create(dimensions, item):
    for dimension in dimensions:
        item = map(copy.copy, [item] * dimension)
    return item
def Get(matrix, position):
    for index in position:
        matrix = matrix[index]
    return matrix
def Set(matrix, position, value):
    for index in position[:-1]:
        matrix = matrix[index]
    matrix[position[-1]] = value

Answer 4

In case a matrix is actually what you are looking for, consider the numpy package.

http://docs.scipy.org/doc/numpy/reference/generated/numpy.zeros.html#numpy.zeros

This will give you a 3x3x3 array of zeros:

numpy.zeros((3,3,3)) 

You also benefit from the convenience features of a module built for scientific computing.

Answer 5

List comprehensions are just syntactic sugar for adding expressiveness to list initialization; in your case, I would not use them at all, and go for a simple nested loop.

On a completely different level: do you think the n-dimensional array of NumPy could be a better approach?
Although you can use lists to implement multi-dimensional matrices, I think they are not the best tool for that goal.

Answer 6

This seems to be what you want:

def three_dimension_list(x,y,z):
return [[[0]*x]*y]*z

Answer 7

Or use the nest function defined here, combined with repeat(0) from the itertools module:

nest(itertools.repeat(0),[3,3,3])