Re: Newbie question

[Robert Kern]

Joe Hays wrote:

I'm a newbie to python. Can anyone tell me why the following little
program complains about incorrect dimensions?

<snip>

import pylab
from pylab import *

n = 10; m = 2*n;
A = randn(m,n);
b = A*rand(n,1) + 2*rand(m,1);

</snip>

The actual error I receive is,

<snip>

b = A*rand(n,1) + 2*rand(m,1);
ValueError: shape mismatch: objects cannot be broadcast to a single
shape

</snip>

This is supposed to be doing a matrix multply of an (m,n)(n,1). This
results in an (m,1). This should then be able to be added to the
2*rand(m,1), but, its not for some reason.

Ideas?

You will want to ask numpy questions on the numpy mailing list (preferably with
a subject line a bit more informative with respect to the actual problem).

http://www.scipy.org/Mailing_Lists

In this case, the problem is that * does not do matrix multiplication; it does
element-wise multiplication. Use dot(A, randn(n,1)) instead.

--
Robert Kern

"I have come to believe that the whole world is an enigma, a harmless enigma
that is made terrible by our own mad attempt to interpret it as though it had
an underlying truth."
-- Umberto Eco