[Numpy-discussion] Trick for fast

[Alan G Isaac]

On 2/3/2012 5:16 AM, santhu kumar wrote:
> x = nX3 vector.
> mass = nX1 vector
> inert = zeros((3,3))
> for i in range(n):
>        ri = x[i,:].reshape(1,3)
>        inert = inert + mass[i,]*(sum(ri*ri)*eye(3) - dot(ri.T,ri))
>


This should buy you a bit.

xdot = (x*x).sum(axis=1)
for (massi,xi,xdoti) in zip(mass.flat,x,xdot):
       temp = -np.outer(xi,xi)
       temp.flat[slice(0,None,4)] += xdoti
       inert += massi*temp

Alan Isaac