how to replace an array containing indices with values from another array?

Question

I have an array b containing indices of an array a. I want to insert values of another array c in the array b with same indices.

import numpy as np
a1=np.array([[1, 3, 5, 2, 3],[7, 6, 5, 2, 4],[2, 0, 5, 6, 4]])
a=a1.argsort()[:,:2]

## this will create an array with indices of 2 smallest values of a1

  a
[[0 3]
 [3 4]
 [1 0]]
b=np.array([[1],[2],[3],[4],[5],[6]])

now I want to replace value 0 in a with 1 in b ; 3 with 4 and so on

i tried using:

[a[index]]=b[index]

but its obviously not the right way as array a handles these indices as values

please help

Could you clarify a little more? You mention an array c, but don't show it in your example. Could you show what you want the output of the operation to be in your example? — djhoese
– djhoese, Commented Jun 21, 2014 at 20:59

pezcode · Accepted Answer · 2014-06-21 21:37:04Z

2

If I understood you correctly, you can just use the flattened version of a to index into b:

result = b.ravel()[a.ravel()]

[1, 4, 4, 5, 2, 1]

If you need it in the same dimensions as a you can reshape it:

result = result.reshape(a.shape)

[[1, 4]
 [4, 5]
 [2, 1]]

answered Jun 21, 2014 at 21:37

pezcode

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