[Numpy-discussion] fastest way to do multiplication with diagonal matrices from left or right

[Pietro Berkes]

This is a small function I use to speed up multiplication with diagonal
matrices. I don't know if it's the *fastest* way to do it, but it's
pretty fast.

def mult_diag(d, mtx, left=True):
    """Multiply a full matrix by a diagonal matrix.
    This function should always be faster than dot.

    Input:
      d -- 1D (N,) array (contains the diagonal elements)
      mtx -- 2D (N,N) array

    Output:
      mult_diag(d, mts, left=True) == dot(diag(d), mtx)
      mult_diag(d, mts, left=False) == dot(mtx, diag(d))
    """
    if left:
        return (d*mtx.T).T
    else:
        return d*mtx


On Fri, 23 Mar 2007, daniel.egloff at zkb.ch wrote:

> 
> 
> Dear list
> 
> what is the fastet way to multiply with a diagonal matrix from left or
> right and without to build a square matrix from the diagonal.
> Here it what I am looking for:
> 
> import numpy as N
> 
> def diagmult(X, Y):
>     """
>     Matrix multiplication X*Y where either X or Y is a diagonal matrix.
>     """
>     if X.ndim == 1 and Y.ndim == 2:
>         R = Y.copy()
>         for i, d in enumerate(X):
>             R[i,:] *= d
>         return R
>     elif X.ndim == 2 and Y.ndim == 1:
>         R = X.copy()
>         for i, d in enumerate(Y):
>             R[:,i] *= d
>         return R
>     elif X.ndim == 1 and Y.ndim == 1:
>         return X*Y
>     else
>         raise ValueError('diagmult dimension mismatch X.ndim = %d, Y.ndim =
> %d' % (X.ndim, Y.ndim))
> 
> Freundliche Grüsse
> Daniel Egloff
> Zürcher Kantonalbank
> Leiter(in) Financial Computing, ZEF
> 
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