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Define a two-dimensional array in Python

David Y.

The Problem

How can I create an empty two-dimensional array in Python, e.g. for matrix operations?

The Solution

The best way to create two-dimensional (2D) arrays or matrices is by using Python’s numpy module, which provides comprehensive and performant functionality for manipulating multidimensional arrays. We can create a 2D array by passing a tuple to the zeros function. The following code will create a 3-by-3 2D array with all values set to zero:

import numpy

matrix = numpy.zeros((3,3))

# array([[0., 0., 0.],
#        [0., 0., 0.],
#        [0., 0., 0.]])

Note that numpy.matrix is deprecated and should not be used for this operation.

To create a 2D array without using numpy, we can initialize a list of lists using a list comprehension. This will be slower and deprive us of the use of numpy’s matrix operations, but may be desirable in cases where we don’t want to add the dependency. We can use code similar to the following:

matrix = [[0 for x in range(3)] for y in range(3)] # x = width, y = height
# [[0, 0, 0], [0, 0, 0], [0, 0, 0]]

In both of these structures, we can read and write individual elements with the following syntax:

matrix[0][0] = 1 # set top-left corner to 1

print(matrix[2][2]) # print value of bottom-right corner

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