C interface

All access to the matrix A is made through the user-supplied routine Aprod with the prototype:

  int Aprod( int mode, int m, int n, int nix, int ix[],
             double x[], double y[], void *ptrA );

At each call to Aprod, BCLS will set the variable "mode" to describe if a product with A or with A' is required:

    If      mode == 1,  compute  y = A *x,  with  x  untouched;
    and if  mode == 2,  compute  x = A'*y,  with  y  untouched.

The integers m and n describe the number of rows and columns in A, and also the lengths of the vectors y and x, respectively.

This is important! Only some columns of A are needed for any given matrix-vector multiply. The vector of indices ix (with length nix) describes which columns of A should contribute to the product. In other words, Aprod should return

    if mode == 1,  y = A(:, ix) * x(ix),    where  len(ix) = nix

(using Matlab notation). In fact, many of the calls to Aprod have nix = 1, so that only a single column of A is required.

On the other hand,

    if mode == 2,  x(ix) = A(:, ix)' * y,   where  len(ix) = nix.

However, BCLS will simply ignore elements of x that are not indexed by ix, so that the call to Aprod with mode == 2 is not as critical the mode == 1 call.

See the routine Aprod in bcsol.c (under Examples) for an example of how to code this routine.

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