How to access numpy array with a set of indices stored in another numpy array?

Question

I have a numpy array which stores a set of indices I need to access another numpy array.

I tried to use a for loop but it doesn't work as I expected.

The situation is like this:

>>> a
array([[1, 2],
       [3, 4]])
>>> c
array([[0, 0],
       [0, 1]])
>>> a[c[0]]
array([[1, 2],
       [1, 2]])
>>> a[0,0]         # the result I want
1

Above is a simplified version of my actual code, where the c array is much larger so I have to use a for loop to get every index.

Seems very relevant - stackoverflow.com/questions/41802319

Divakar
– Divakar

2017-01-31 06:42:21 +00:00
Commented Jan 31, 2017 at 6:42 — Divakar
– Divakar, Commented Jan 31, 2017 at 6:42

MSeifert · Accepted Answer · 2017-01-31 06:40:25Z

3

Convert it to a tuple:

>>> a[tuple(c[0])]
1

Because list and array indices trigger advanced indexing. tuples are (mostly) basic slicing.

answered Jan 31, 2017 at 6:40

MSeifert

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Kasravnd · Accepted Answer · 2017-01-31 06:42:56Z

1

Index a with columns of c by passing the first column as row's index and second one as column index:

In [23]: a[c[:,0], c[:,1]]
Out[23]: array([1, 2])

answered Jan 31, 2017 at 6:42

Kasravnd

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