xlinsist - 1 year ago 109

Python Question

I have an numpy ndarray with shape (2,3,3),for example:

`array([[[ 1, 2, 3],`

[ 4, 5, 6],

[12, 34, 90]],

[[ 4, 5, 6],

[ 2, 5, 6],

[ 7, 3, 4]]])

I am getting lost in np.sum(above ndarray ,axis=1), why that answer is:

`array([[17, 41, 99],`

[13, 13, 16]])

Thanks

Answer

Axes are defined for arrays with more than one dimension. A 2-dimensional array has two corresponding axes: the first running vertically downwards across rows (axis 0), and the second running horizontally across columns (axis 1).

Let A be the array, then in your example when axis is 1, [i,:,k] are added. Likewise, for axis 0, [:,j,k] are added and when axis is 2, [i,j,:] are added.

```
A = np.array([[[ 1, 2, 3],[ 4, 5, 6],
[12, 34, 90]],
[[ 4, 5, 6],[ 2, 5, 6],
[ 7, 3, 4]]])
np.sum(A,axis = 0)
array([[ 5, 7, 9],
[ 6, 10, 12],
[19, 37, 94]])
np.sum(A,axis = 1)
array([[17, 41, 99],
[13, 13, 16]])
np.sum(A,axis = 2)
array([[ 6, 15, 136],
[ 15, 13, 14]])
```

Source (Stackoverflow)