Myujth

When I multiply two numpy arrays of sizes (n x n)*(n x 1), I get a matrix of size (n x n). Following normal matrix multiplication rules, a (n x 1) vector is expected, but I simply cannot find any information about how this is done in Python's Numpy module.

The thing is that I don't want to implement it manually to preserve the speed of the program.

Example code is shown below:

a = np.array([[ 5, 1 ,3], [ 1, 1 ,1], [ 1, 2 ,1]])
b = np.array([1, 2, 3])

print a*b
 >>
 [[5 2 9]
 [1 2 3]
 [1 4 3]]

What i want is:

print a*b
 >>
 [16 6 8]

Simplest solution

Use numpy.dot or a.dot(b). See the documentation here.

>>> a = np.array([[ 5, 1 ,3], 
 [ 1, 1 ,1], 
 [ 1, 2 ,1]])
>>> b = np.array([1, 2, 3])
>>> print a.dot(b)
array([16, 6, 8])

This occurs because numpy arrays are not matrices, and the standard operations *, +, -, / work element-wise on arrays. Instead, you could try using numpy.matrix, and * will be treated like matrix multiplication.

Other Solutions

Also know there are other options:

• 
  

  As noted below, if using python3.5+ the @ operator works as you'd expect:

  
  
  

  >>> print(a @ b)
  array([16, 6, 8])
  

  
  


• 
  

  If you want overkill, you can use numpy.einsum. The documentation will give you a flavor for how it works, but honestly, I didn't fully understand how to use it until reading this answer and just playing around with it on my own.

  
  
  

  >>> np.einsum('ji,i->j', a, b)
  array([16, 6, 8])
  

  
  


• 
  

  As of mid 2016 (numpy 1.10.1), you can try the experimental numpy.matmul, which works like numpy.dot with two major exceptions: no scalar multiplication but it works with stacks of matrices.

  
  
  

  >>> np.matmul(a, b)
  array([16, 6, 8])
  

  
  


• 
  

  numpy.inner functions the same way as numpy.dot for matrix-vector multiplication but behaves differently for matrix-matrix and tensor multiplication (see Wikipedia regarding the differences between the inner product and dot product in general or see this SO answer regarding numpy's implementations).

  
  
  

  >>> np.inner(a, b)
  array([16, 6, 8])
  
  # Beware using for matrix-matrix multiplication though!
  >>> b = a.T
  >>> np.dot(a, b)
  array([[35, 9, 10],
   [ 9, 3, 4],
   [10, 4, 6]])
  >>> np.inner(a, b) 
  array([[29, 12, 19],
   [ 7, 4, 5],
   [ 8, 5, 6]])
  

  
  

Rarer options for edge cases

• 
  

  If you have tensors (arrays of dimension greater than or equal to one), you can use numpy.tensordot with the optional argument axes=1:

  
  
  

  >>> np.tensordot(a, b, axes=1)
  array([16, 6, 8])
  

  
  



• Don't use numpy.vdot if you have a matrix of complex numbers, as the matrix will be flattened to a 1D array, then it will try to find the complex conjugate dot product between your flattened matrix and vector (which will fail due to a size mismatch n*m vs n).