Numpy sum over planes of 3d array, return a scalar

Question

I'm making the transition from MATLAB to Numpy and feeling some growing pains. I have a 3D array, lets say it's 3x3x3 and I want the scalar sum of each plane. In matlab, I would use: ``` sum_vec = sum(3dArray,3); ``` TIA wbg EDIT: I was wrong about my matlab code. Matlab only vectorizes in one dim, so a loop wold be required. So numpy turns out to be more elegant...cool. ``` MATLAB for i = 1:3 sum_vec(i) = sum(sum(3dArray(:,:,i)); end ```

Accepted Answer

You can do

```
sum_vec = np.array([plane.sum() for plane in cube])
```

or simply

```
sum_vec = cube.sum(-1).sum(-1)
```

where `cube` is your 3d array. You can specify `0` or `1` instead of `-1` (or `2`) depending on the orientation of the planes. The latter version is also better because it doesn't use a Python loop, which usually helps to improve performance when using `numpy`.

Numpy sum over planes of 3d array, return a scalar

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