Re: [Lapackpp-devel] Matrix Multiplication to return only diagonal?

[Christian S. <sti...@tu...>]

Dear Jack,

I have searched through all of the available BLAS documentation but I 
did not find any such routine.

You could write one by yourself that will calculate the dot product 
between all rows of the left and columns of the right matrix:

// calculate only the diagonal of A times B
int msize = std::min(A.size(0), B.size(1));
LaGenVectorDouble result(msize);
assert(A.size(1) == B.size(0));
for (int i=0; i < msize; i++)
   result(i) = Blas_Dot_Prod(A(LaIndex(i), LaIndex(0,A.size(1)),
			    B(LaIndex(0,B.size(0)), LaIndex(i)));

As always, you would need to check whether this code really does the 
right thing. I'm always unsure whether I got the indices right and I 
don't confuse the "beginning with 0" or the last index.

Christian


Jacob (Jack) Gryn schrieb:

> Hi,
> 
>  
> 
> I was wondering if you know of any functions that will do a matrix 
> multiplication only to obtain the diagonal of the resulting 
> multiplication (that avoids the excess computations and temporary space 
> needed)?
> 
>  
> 
> Jack
>