How to return list of items from an array given list on indices in numpy

Question

I have an array arr and a list of indices that I want to get indices. I want to get subset of array corresponding to items in indices and the complement of that.

For example

for

arr = np.asarray([2, 4, 1, 5, 6])
indices = np.asarray([2, 4])

I would get

[1, 6] and [2, 4, 5]

Thanks

Ok, I give up. Where does the [2, 4, 5] come from in your result? — Mark Setchell
– Mark Setchell, Commented May 9, 2019 at 17:25
it is a complement . you return items given by indices and delete these items from an original array (or it can be any there way of giving complement — YohanRoth
– YohanRoth, Commented May 9, 2019 at 17:41
is your arr a numpy array? If so, please edit your code accordingly — Tarifazo
– Tarifazo, Commented May 9, 2019 at 17:43

Tarifazo · Accepted Answer · 2019-05-09 18:30:59Z

2

Using np.isin or np.in1d (using masks):

arr = np.asarray([2, 4, 1, 5, 6])
indices = np.asarray([2, 4])
m = np.in1d(np.arange(len(arr)), indices)
arr1, arr2 = arr[m], arr[~m]
arr1, arr2
>>array([1, 6]), array([2, 4, 5])

Alternatively, using np.setdiff1d for the complementary part (can be faster for larger arrays and indices):

arr1 = arr[indices]
arr2 = arr[np.setdiff1d(np.arange(len(arr)), indices, True)]

edited May 9, 2019 at 18:30

answered May 9, 2019 at 17:59

Tarifazo

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