Numpy: Subtract 2 numpy arrays row wise

Question

I have 2 numpy arrays a and b as below:

a = np.random.randint(0,10,(3,2))
Out[124]: 
array([[0, 2],
       [6, 8],
       [0, 4]])
b = np.random.randint(0,10         


        

Answer 1

Reading from the doc on broadcasting, it says:

When operating on two arrays, NumPy compares their shapes element-wise. It starts with the trailing dimensions, and works its way forward. Two dimensions are compatible when

they are equal, or
one of them is 1

Back to your case, you want result to be of shape (3, 2, 2), following these rules, you have to play around with your dimensions. Here's now the code to do it:

In [1]: a_ = np.expand_dims(a, axis=0)

In [2]: b_ = np.expand_dims(b, axis=1)

In [3]: c = a_ - b_

In [4]: c
Out[4]: 
array([[[-5, -7],
        [ 1, -1],
        [-5, -5]],

       [[-2, -2],
        [ 4,  4],
        [-2,  0]]])

In [5]: result = c.swapaxes(1, 0)

In [6]: result
Out[6]: 
array([[[-5, -7],
        [-2, -2]],

       [[ 1, -1],
        [ 4,  4]],

       [[-5, -5],
        [-2,  0]]])

In [7]: result.shape
Out[7]: (3, 2, 2)