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pythonarraysnumpymultidimensional-arrayindexing
Habtamu Wubie
Habtamu Wubie

Reputation: 63

Numpy array Indexing with other arrays yields broadcasting error

I have two indexing arrays.

elim=range(130,240)
tlim=range(0,610)

The array to be indexed, I, has originally shape of (299, 3800)

When I try to index it as follow

I[elim,tlim]

I got the following error message.

shape mismatch: indexing arrays could not be broadcast together with shapes (110,) (610,)

I didn't expect such errors. Could someone explain what is happening here?

Thanks!

Upvotes: 5

Views: 1327

Answers (1)

yatu
yatu

Reputation: 88236

Let's reproduce the example with a random array of the specified shape:

elim=range(0,610)
tlim=range(130,240)
a = np.random.rand(299, 3800)

a[tlim, elim]

IndexError: shape mismatch: indexing arrays could not be broadcast together with shapes (110,) (610,)

This raises an error because you're using arrays of integer indexes to index the array, and hence advanced indexing rules will apply. You should use slices for this example

a[130:240,0:610].shape
# (110, 610)

See Understanding slice notation (NumPy indexing, is just an extension of the same concept up to ndimensional arrays.

For the cases in which you have a list of indices, not necessarily expressable as slices, you have np.ix_. For more on numpy indexing, this might help

a[np.ix_(tlim, elim)].shape
# (110, 610)

Upvotes: 7

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