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Blitz Support : |
From: Julian Cummings (cummings_at_[hidden])
Date: 2004-01-06 13:55:35
Hi,
There are (incomplete and not well documented) Matrix classes in blitz
that perform the expected matrix-matrix multiply behavior. The *
operation for Arrays does an elementwise multiplication, which is not
what you intended. Also, you normally need to allocate the array that
receives a result before assigning it. Below is a program using the
blitz Matrix class that does what your original program was supposed to do.
Regards, Julian C.
#include <blitz/matrix.h>
using namespace blitz;
int main(int argc, char* argv[])
{
Matrix<double> m1(1,100);
Matrix<double> m2(100,24);
for (int i=0; i<100; ++i) {
m1(0,i) = 0;
for (int j=0; j<24; ++j)
m2(i,j) = i;
}
cout << m1 << endl;
cout << m2 << endl;
Matrix<double> m3(1,24);
m3 = m1 * m2;
cout << m3 << endl;
}
Google Poster wrote:
>Please forgive my ignorance - I am not a
>mathematician.
>
>I am involved in a project that requires high
>performance math. I have been replacing the old
>legacy routines with blitz++ and have been
>experiencing encouraging results thus far. I have hit
>a problem with matrix multiplication and I am sure the
>root cause is my lack of understand.
>
>I *think*.
>
>A matrix with dimensions A x B can be multiplied by a
>matrix with dimensions B x C to produce a matrix of
>dimensions A x C.
>
>For this code:
>
>// ---------------------------------------------
>#include <blitz/array.h>
>
>using namespace blitz;
>
>int main(int argc, char* argv[])
>{
> double* t = new double[100];
> double* t2 = new double[2400];
>
> Array<double,2> m1( t , shape( 1 , 100 ) ,
>neverDeleteData );
> Array<double,2> m2( t2 , shape( 100 , 24 ) ,
>neverDeleteData ,blitz::ColumnMajorArray<2>() );
>
> blitz::firstIndex iw;
>
> m1 = iw;
> m2 = iw;
>
> cout << m1 << endl;
> cout << m2 << endl;
>
> Array<double,2> m3( m1 * m2 );
>
> cout << m3 << endl;
>}
>// ----------------------------------------
>
>M3 is a matrix of 1 x 1 - I would expect 1 x 24
>matrix.
>
>Any assistance would be gratefully appreciated.
>
>--
>gp
>
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>
-- Dr. Julian C. Cummings E-mail: cummings_at_[hidden] California Institute of Technology Phone: 626-395-2543 1200 E. California Blvd., Mail Code 158-79 Fax: 626-584-5917 Pasadena, CA 91125