[R] Code of sample() in C

[Berwin A Turlach]

G'day Ranjan,

On Sun, 5 Apr 2009 17:00:56 -0500
Ranjan Maitra <maitra at iastate.edu> wrote:

> [...[
> I haven't tried this function out much myself, so YMMV.

There is an obvious problem in this code since len is not decreased but
n is when an index is sampled.  The last line in the last for loop
should be
	x[j] = x[--len];
Otherwise this algorithm will produce samples in which an index can
be repeated.  And the probabilities with which an index is sampled would
be non-trivial to determine; they would definitely not be uniform.

HTH.

Cheers,

	Berwin
 
> #include <stdlib.h>
> #ifndef USING_RLIB
> #define MATHLIB_STANDALONE 1 /*It is essential to have this before
> the call to the Rmath's header file because this decides
>                                the definitions to be set. */
> #endif
> #include <Rmath.h>
> 
> /* Note that use of this function involves a prior call to the Rmath
> library to get the seeds in place. It is assumed that this is done in
> the calling function. */
> 
> /* Equal probability sampling; without-replacement case */
> /* Adapted from the R function called SampleNoReplace */
> 
> /**
>  * Stores k out of n indices sampled at random without replacement
>  * in y.
>  */
> int srswor(int n, int k, int *y)
> { 
>   if (k > n) {
>     return 1;
>   }
>   else {
>     const double len = (double) n;
>     int i;
>     int* x = malloc(n * sizeof(int));
>     if (!x) {
>       return 2;
>     }
> 		
>     for (i = 0; i < n; ++i)	x[i] = i;
>     
>     for (i = 0; i < k; ++i) {
>       const int j = (int)(len * runif(0.0, 1.0));
>       y[i] = x[j];
>       x[j] = x[--n];
>     }
>     free(x);
>   }
>   return 0;
> }

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Berwin A Turlach                            Tel.: +65 6516 4416 (secr)
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