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
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