Random permutation

randperm.c

#include <stdio.h>
#include <stdlib.h>
#include <time.h>

/* generate a random permutation of array `a` in place */
static void randperm(int n, double *a)
{
  int i, j;
  double x;

  for ( i = 0; i < n - 1; i++ ) {
    j = i + (int) ( (n - i) * rand() / RAND_MAX );
    /* swap a[j] and a[i] */
    x = a[j], a[j] = a[i], a[i] = x;
  }
}

static void print(int n, const double *a)
{
  int i;
  for ( i = 0; i < n; i++ )
    printf("%g ", a[i]);
  printf("\n");
}
  
int main(void)
{
  int n = 5, k;
  double a[] = {1, 4, 3, 5, 2};

  srand( time(NULL) );
  for ( k = 0; k < 20; k++ ) {
    randperm(n, a);
    print(n, a);
  }

  return 0;
}