Create 3D array using Python

Question

I would like to create a 3D array in Python (2.7) to use like this:

distance[i][j][k] 

And the sizes of the array should be the size of a variable I have. (nnn)

I tried using:

distance = [[[]*n]*n] 

but that didn't seem to work.

I can only use the default libraries, and the method of multiplying (i.e.,[[0]*n]*n) wont work because they are linked to the same pointer and I need all of the values to be individual

Answer 1

You should use a list comprehension:

>>> import pprint >>> n = 3 >>> distance = [[[0 for k in xrange(n)] for j in xrange(n)] for i in xrange(n)] >>> pprint.pprint(distance) [[[0, 0, 0], [0, 0, 0], [0, 0, 0]],  [[0, 0, 0], [0, 0, 0], [0, 0, 0]],  [[0, 0, 0], [0, 0, 0], [0, 0, 0]]] >>> distance[0][1] [0, 0, 0] >>> distance[0][1][2] 0 

You could have produced a data structure with a statement that looked like the one you tried, but it would have had side effects since the inner lists are copy-by-reference:

>>> distance=[[[0]*n]*n]*n >>> pprint.pprint(distance) [[[0, 0, 0], [0, 0, 0], [0, 0, 0]],  [[0, 0, 0], [0, 0, 0], [0, 0, 0]],  [[0, 0, 0], [0, 0, 0], [0, 0, 0]]] >>> distance[0][0][0] = 1 >>> pprint.pprint(distance) [[[1, 0, 0], [1, 0, 0], [1, 0, 0]],  [[1, 0, 0], [1, 0, 0], [1, 0, 0]],  [[1, 0, 0], [1, 0, 0], [1, 0, 0]]] 

Answer 2

numpy.arrays are designed just for this case:

 numpy.zeros((i,j,k)) 

will give you an array of dimensions ijk, filled with zeroes.

depending what you need it for, numpy may be the right library for your needs.