Indexing numpy array with another numpy array
Question:
Suppose I have
a = array([[1, 2],
[3, 4]])
and
b = array([1,1])
I’d like to use b in index a, that is to do a[b] and get 4 instead of [[3, 4], [3, 4]]
I can probably do
a[tuple(b)]
Is there a better way of doing it?
Thanks
Answers:
According the NumPy tutorial, the correct way to do it is:
a[tuple(b)]
Suppose you want to access a subvector of a with n index pairs stored in blike so:
b = array([[0, 0],
...
[1, 1]])
This can be done as follows:
a[b[:,0], b[:,1]]
For a single pair index vector this changes to a[b[0],b[1]], but I guess the tuple approach is easier to read and hence preferable.
The above is correct. However, if you see an error like:
IndexError: only integers, slices (`:`), ellipsis (`...`), numpy.newaxis (`None`) and integer or boolean arrays are valid indices
You may have your index array in floating type. Change it to something like this:
arr[tuple(a.astype(int))]
Suppose I have
a = array([[1, 2],
[3, 4]])
and
b = array([1,1])
I’d like to use b in index a, that is to do a[b] and get 4 instead of [[3, 4], [3, 4]]
I can probably do
a[tuple(b)]
Is there a better way of doing it?
Thanks
According the NumPy tutorial, the correct way to do it is:
a[tuple(b)]
Suppose you want to access a subvector of a with n index pairs stored in blike so:
b = array([[0, 0],
...
[1, 1]])
This can be done as follows:
a[b[:,0], b[:,1]]
For a single pair index vector this changes to a[b[0],b[1]], but I guess the tuple approach is easier to read and hence preferable.
The above is correct. However, if you see an error like:
IndexError: only integers, slices (`:`), ellipsis (`...`), numpy.newaxis (`None`) and integer or boolean arrays are valid indices
You may have your index array in floating type. Change it to something like this:
arr[tuple(a.astype(int))]