Ondrej, I was confused an unclear in my original question. I subsequently posted a followup, titled "Simplified question on tensordot' where I explained myself a lot better, and got some really good help. So, thank you very much for looking into this issue, but I believe that this has been resolved.
Thanks! Rick http://mail.scipy.org/pipermail/numpy-discussion/2010-September/052584.html On Tue, Sep 7, 2010 at 1:23 PM, Ondrej Certik <ond...@certik.cz> wrote: > Hi Rick! > > On Fri, Sep 3, 2010 at 4:02 AM, Rick Muller <rpmul...@gmail.com> wrote: > > Can someone help me replace a slow expression with a faster one based on > > tensordot? I've read the documentation and I'm still confused. > > > > I have two matrices b and d. b is n x m and d is m x m. I want to replace > > the expression > > > >>>> bdb = zeros(n,'d') > >>>> for i in xrange(n): > >>>> bdb[i,:] = dot(b[i,:],dot(d,b[i,:]) > > I am first trying to reproduce this --- the above is missing one ")" > and also dot() seems to produce a number, but you are assigning it to > bdb[i,:], also you declare bdb as a 1D array. So I tried this: > > http://gist.github.com/568879/ > > > > > with something that doesn't have the for loop and thus is a bit faster. > > > > The first step is > > > >>>> bd = dot(b,d) > > > > However, following this with > > > >>>> bdb = dot(bd,b.T) > > > > doesn't work, since it yields a n x n matrix instead of an n x 1 vector. > > Reading the notes on tensordot makes me think it's the function to use, > but > > I'm having trouble grokking the axes argument. Can anyone help? > > In the above gist, I did the following: > > > bd = dot(b, d) > bdb = diag(dot(bd, b.T)) > print bdb > > > which printed the same as: > > > for i in xrange(n): > bdb[i] = dot(b[i,:], dot(d, b[i, :])) > print bdb > > > but I think that this is not what you want, is it? I think you want to > do something like: > > b * d * b.T > > but since b is a (n, m) matrix, the result is a matrix, not a vector, > right? > > Ondrej > _______________________________________________ > NumPy-Discussion mailing list > NumPy-Discussion@scipy.org > http://mail.scipy.org/mailman/listinfo/numpy-discussion > -- Rick Muller rpmul...@gmail.com 505-750-7557
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