[Numpy-discussion] speeding up y[:, i] for y.shape = (big number, small number)

[David Cournapeau]

Hi,

    I was wondering if there was any way to speed up the following code:

 y   = N.zeros((n, K))
 for i in range(K):
     y[:, i] = gauss_den(data, mu[i, :], va[i, :])

Where K is of order 1e1, n of order 1e5. Normally, gauss_den is a quite 
expensive function, but the profiler tells me that the indexing y[:,i] 
takes almost as much time as the gauss_den computation (which computes n 
exp !). To see if the profiler is "right", i replaces with the (non 
valid) following function:

 y   = N.zeros((n, K))
    for i in range(K):
        yt = gauss_den(data, mu[i, :], va[i, :])
    return y

Where more than 99% of the code is spent inside gauss_den.

I guess the problem is coming from the fact that y being C order, y[:, 
i] needs accessing data in a non 'linear' way. Is there a way to speed 
this up ? I did something like this:

  y   = N.zeros((K, n))
    for i in range(K):
        y[i] = gauss_den(data, mu[i, :], va[i, :])
    return y.T

which works, but I don't like it very much. Isn't there any other way ?

David