Get the data in rows and/or columns from a matrix in a MEX file

Hello,

I am writing a MEX routine in which I need to multiply a row and a vector from a matrix (among other things). In Matlab's language it would look like this:

tmp(i) = A(i,:) * A(:,i)

I want to use the ddot BLAS function. Is there a MEX function to get a complete row/column from a matrix?

Answer 1

[James Tursa]

The signature for ddot from this link:

http://www.netlib.org/blas/

is:

        DOUBLE PRECISION FUNCTION DDOT(N,DX,INCX,DY,INCY)
  *     .. Scalar Arguments ..
        INTEGER INCX,INCY,N
  *     ..
  *     .. Array Arguments ..
        DOUBLE PRECISION DX(*),DY(*)
  *     ..

So you can just give the starting address (DX and DY) and increment (INCX and INCY) of each, no need to extract anything. E.g., a code snippet for calling to do the basic calculation could be this:

// Assume 1st input prhs[0] is A, 2nd input prhs[1] is i
mwSignedIndex N, I, ONE;
double *pr, *DX, *DY;
double tmp;
ONE = 1;
N = mxGetN(prhs[0]); // assume input matrix is square
pr = mxGetPr(prhs[0]); // assume input matrix is double
I = mxGetScalar(prhs[1]);
DX = pr + (I-1); // Use 0-based indexing
DY = pr + (I-1)*N;
tmp = ddot(&N,DX,&N,DY,&ONE); // A(i,:)*A(:,i)

[gire]

Thanks for the code snippet.