# Index numpy array by matrix of two arrays

I have a 2D numpy array like

``````weights = np.array(
[
[1, 2, 3],
[4, 5, 6],
[7, 8, 9],
]
)
``````

And i have two 1-D numpy arrays that i want to use to index into weights

``````positions1 = np.array([2, 1, 0])
positions2 = np.array([1, 1, 0])
``````

If i want the results for stepping through the arrays together and indexing into the matrix that way i can do

``````print(weights[positions1[...], positions2[...]])
``````

And get `[8 5 1]`

However now i want to index into the matrix with all possible combinations of positions1 and positions2 so that i get a matrix like

``````[
[weights[2, 1], weights[2, 1], weights[2, 0]],
[weights[1, 1], weights[1, 1], weights[1, 0]],
[weights[0, 1], weights[0, 1], weights[0, 0]],
]
``````

So

``````[
[weights[pos1, pos2], weights[pos1, pos2], weights[pos1, pos2]],
[weights[pos1, pos2], weights[pos1, pos2], weights[pos1, pos2]],
[weights[pos1, pos2], weights[pos1, pos2], weights[pos1, pos2]],
]
``````

What would be the canonical way to do that in numpy?
I know that this is kinda like an outer product but i dont actually want to multiply the values in my array but just get a tuple of their values to index into the matrix with.

### >Solution :

You need change the shape of the first indexer:

``````weights[positions1[:,None], positions2]
``````

Or for a generalized version, as pointed out by Chrysophylaxs:

``````weights[np.ix_[positions1, positions2]]
``````

Output:

``````array([[8, 8, 7],
[5, 5, 4],
[2, 2, 1]])
``````