How to transform 2D array using values as another array's indices?

Question

I have a 2D array with indices refering to another array:

indexarray = np.array([[0,0,1,1],
                       [1,2,3,0]])

The array which these indices refer to is:

valuearray = np.array([8,7,6,5])

I would like to get an array with the numbers from valuearray in the shape of indexarray, each item in this array corresponds to the value in valuearray with the index on the same location in indexarray, ie:

targetarray = np.array([[8,8,7,7],
                        [7,6,5,8]])

How can I do this without iteration?

What I do now to achieve this is:

np.apply_along_axis(func1d = lambda row: valuearray[row],axis=0,arr = indexarray)

If there is a simpler way, I am interested.

targetarray = valuearray[indexarray.flatten()].reshape(indexarray.shape) — cyborg
– cyborg, Commented Dec 27, 2021 at 15:18

cyborg · Accepted Answer · 2021-12-27 15:22:05Z

1

One way is to flatten the index array and get the values and reshape it back as follows.

targetarray = valuearray[indexarray.flatten()].reshape(indexarray.shape)

answered Dec 27, 2021 at 15:22

cyborg

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2 Comments

hpaulj Over a year ago

Does the simpler valuearray[indexarray] work?

cyborg Over a year ago

Oh yeah. It works without flattening too :)

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