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pythonnumpynumpy-ndarraydot-product

Showing ValueError while taking dot product of matrix

I wish to multiply two numpy matrices A and B_transformed:

    A = 
     [[-1.910095  ]
      [-1.20056174]
      [-0.77669163]
      [ 0.62406999]
      [ 1.1471159 ]
      [ 2.11616247]]

    B = 
      [[ 0.70710678 -0.70710678]
       [ 0.70710678  0.70710678]]

     B_transformed = B[1,:]
                   = [0.70710678 0.70710678]

I tried:

product = np.dot(A,B_transformed)

However I get ValueError:

ValueError: shapes (6,1) and (2,) not aligned:

By matrices rule, (6,1) X (1,2) is allowed. Then why am I getting valueError?

Desired Output

product = [[-1.35064113 -1.35064113]
           [-0.84892534 -0.84892534]
           [-0.54920392 -0.54920392]
           [ 0.44128412  0.44128412]
           [ 0.81113343  0.81113343]
           [ 1.49635283  1.49635283]]
Solution

  • Because the shape of B_transformed is (2,), based on numpy broadcasting, numpy cannot find a way to broadcast B_transformed in order to perform matmul with A.

    In order to get the desired output, you need

    np.matmul(A,B_transformed[None,:])

    B_transformed[None,:] will reshape B_transformed to (1,2)