CODEWITHSUNDEEP
pythonarraysnumpypermutationshuffle
Thomas Wagenaar
Thomas Wagenaar

Reputation: 6759

"To location array" to index array

I have some data array

a = [6,4,3,1,5]

and a "to location array", that gives indices for how these elements should be reshuffled,

b = [4,2,0,1,3]

What this means: the 1st element of a should go to index 4, the 2nd element of a should go to index 2, and so on. So when applying b to a, I should get:

c = [3,1,4,5,6]

However, I am struggling to implement this operation. Is it a common operation? If so, could someone give me an example? I want to generate some array d from b so that

c = a[d]

In the above example,

d = [2,3,1,4,0]

What I tried: d = b[b], but this is not always correct.

Upvotes: 0

Views: 49

Answers (2)

Thomas Wagenaar
Thomas Wagenaar

Reputation: 6759

One method I just found is calculating d is as follows:

d = np.zeros(5);
d[b] = np.arange(5)

But it seems very non-intuitive to me.

Upvotes: 1

Ivan
Ivan

Reputation: 40698

If you don't need d, you can directly get c with np.put_along_axis:

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

>>> c = np.zeros_like(b)
>>> np.put_along_axis(c, b, a, 0)

>>> c
array([3, 1, 4, 5, 6])

Note the operation is inplace.

Upvotes: 1

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