The R Project SVN R

/* Original (by permission) from
 * MASS/MASS.c by W. N. Venables and B. D. Ripley  Copyright (C) 1994-9

 * Find maximum column: designed for probabilities.
 * Uses reservoir sampling to break ties at random.
 */

#include <R_ext/Arith.h>    /* NA handling */
#include <Rmath.h>      /* fmax2 */
#include <R_ext/Random.h>   /* ..RNGstate */

#include <R_ext/Applic.h>   /* NA handling */

#define RELTOL 1e-5

void R_max_col(double *matrix, int *nr, int *nc, int *maxes)
{
    int   r, c, m, ntie, n_r = *nr;
    double a, b, tol, large;
    Rboolean isna, used_random=FALSE;

    for (r = 0; r < n_r; r++) {
    /* first check row for any NAs and find the largest entry */
    large = 0.0;
    isna = FALSE;
    for (c = 0; c < *nc; c++) {
        a = matrix[r + c * n_r];
        if (ISNAN(a)) { isna = TRUE; break; }
        large = fmax2(large, fabs(a));
    }
    if (isna) { maxes[r] = NA_INTEGER; continue; }
    tol = RELTOL * large;

    m = 0;
    ntie = 1;
    a = matrix[r];
    for (c = 1; c < *nc; c++) {
        b = matrix[r + c * n_r];
        if (b >= a + tol) {
        ntie = 1;
        a = b;
        m = c;
        } else if (b >= a - tol) {
        ntie++;    
        if (!used_random) { GetRNGstate(); used_random = TRUE; }
        if (ntie * unif_rand() < 1.) m = c;
        }
    }
    maxes[r] = m + 1;
    }
    if(used_random) PutRNGstate();
}