Can I use a list to address a numpy array?

Question

I'd like to assign a value to an element of a numpy array addressed by a list. Is this possible? It seems like the sort of thing you ought to be able to do.

I tried:

q = np.zeros((2,2,2))
index = [0,0,0]
print(index)


q[index]=4.3
print(q)

Which didn't give an error, which is promising, but q is now:

[[[ 4.3  4.3]
  [ 4.3  4.3]]

 [[ 0.   0. ]
  [ 0.   0. ]]]

As opposed to:

[[[ 4.3  0. ]
  [ 0.   0.]]

 [[ 0.   0. ]
  [ 0.   0. ]]]

As I hoped it would be.

Thanks in advance for your help.

Jon Clements · Accepted Answer · 2015-01-19 13:51:41Z

3

You can't use a list to index a single element - it has to be a tuple:

import numpy as np

q = np.zeros((2,2,2))
index = [0,0,0]
print(index)


q[tuple(index)]=4.3
print(q)
[0, 0, 0]
[[[ 4.3  0. ]
  [ 0.   0. ]]

 [[ 0.   0. ]
  [ 0.   0. ]]]

edited Jan 19, 2015 at 13:51

answered Jan 19, 2015 at 13:46

Jon Clements

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3 Comments

fjarri Over a year ago

Technically, you can use a list. q[[0,0,0]] = 4.3 is equivalent to q[0] = 4.3; q[0] = 4.3; q[0] = 4.3.

Jon Clements Over a year ago

@fjarri that's indeed true... matter of taste I s'pose

fjarri Over a year ago

I wouldn't call it a matter of taste, it's a different operation that gives a different result. Just saying that you can use a list.

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