Changing values of a numpy array using another array as a reference without loops

Question

Say I have this array

x = np.array([0,0,0
              1,1,1
              2,2,2])

and

y = np.array([4,5,6])

I'd like x to become

[4,4,4
 5,5,5
 6,6,6]

meaning, i in x becomes y[i]. I have a feeling fancy indexing will be my help, but I'm new to numpy and struggling.

input;

[[  0   0   0 ... 255 255 255]
 [  0   0   0 ... 255 255 255]
 [  0   0   0 ... 255 255 255]
 ...
 [  0   0   0 ... 255 255 255]
 [  0   0   0 ... 255 255 255]
 [  0   0   0 ... 255 255 255]]

and my desired output, given y of length 256, y = np.array([5,6,7.......261])

[[  5    ... 261]
 [  5    ... 261]
 [  5    ... 261]
 ...
 [  5    ... 261]
 [  5    ... 261]
 [  5    ... 261]]

Please provide a small working example (input and expected output) — Riccardo Bucco
– Riccardo Bucco, Commented Oct 27, 2020 at 15:00

Daniel F · Accepted Answer · 2020-10-27 15:43:31Z

1

If you have (which I think you want):

x = np.array([[0,0,0],
              [1,1,1],
              [2,2,2]])

y = np.array([4,5,6])

then:

y[x]

Out: 
array([[4, 4, 4],
       [5, 5, 5],
       [6, 6, 6]])

That's . . . just how indexing works :)

answered Oct 27, 2020 at 15:43

Daniel F

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1 Comment

Omri. B Over a year ago

Wow do I feel dumb. this is perfect!

Riccardo Bucco · Accepted Answer · 2020-10-27 14:52:20Z

1

You can use np.repeat:

>>> np.repeat(y, x.size / y.size)
array([4, 4, 4, 5, 5, 5, 6, 6, 6])

answered Oct 27, 2020 at 14:52

Riccardo Bucco

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3 Comments

Omri. B Over a year ago

Thanks for the reply. Will this works for 2D arrays as well?

Riccardo Bucco Over a year ago

@Omri.B Please update your question with the desired details and I'll work on it

Omri. B Over a year ago

Edited. Tried to simplify it but went too far.

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