Indexing in numpy array

Question

Given a lookup table:

colors = [  [0,0,0],\
            [0, 255, 0],\
            [0, 0, 255],\
            [255, 0, 0]]

And an index numpy matrix input 2x2:

a = np.array([[0,1],[1,1]])

How can I map a to a 2x2x3 matrix b, where b[i][j] = colors[a[i][j]]? I want to avoid using a for loop here.

Bi Rico · Accepted Answer · 2014-06-20 00:20:54Z

2

Have you tried:

colors[a]

Here's a full example:

import numpy as np

colors = np.array([[0,0,0],
                   [0, 255, 0],
                   [0, 0, 255],
                   [255, 0, 0]
                  ])
a = np.array([[0, 1], [1, 1]])

new = colors[a]
new.shape
# (2, 2, 3)
new
# array([[[  0,   0,   0],
#         [  0, 255,   0]],
#
#        [[  0, 255,   0],
#         [  0, 255,   0]]])

edited Jun 20, 2014 at 0:20

answered Jun 20, 2014 at 0:15

Bi Rico

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2 Comments

agstudy Over a year ago

Note that you redefine colors here using numpy array otherwise you get an error with "OP colors".

Bi Rico Over a year ago

@agstudy, very true. If colors is a list of lists you can only index it using slices or integers.

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