# [Rd] uniform sampling without replacement algorithm

Wed Oct 18 16:04:47 CEST 2017

```> From: "Pavel S. Ruzankin" <ruzankin at math.nsc.ru>

> Let us consider the current uniform sampling without replacement
> algorithm. It resides in function do_sample in
> https://svn.r-project.org/R/trunk/src/main/random.c
> Its complexity is obviously O(n), where the sample is selected from
> 1...n, since the algorithm has to create a vector of length n. So when
> the sample size is much lesser than n, the algorithm is not effective.

Recent R Core implementations have a faster hashing scheme, but it is
enabled by default only for very large n, since it produces a
different result from the old, simple algorithm, and so (for example)
re-running an old analysis might give different results if it were
enabled.

The development version of pqR (at https://github.com/radfordneal/pqR
with the latest development branch being "73") implements a fast
hashing scheme that produces the same result as the old method, and so
is always enabled (when it would seem advantageous).  Here is the core
part of the code used (which calls a few pqR internal routines not in
R Core versions):

/******  Copyright 2017 by Radford M. Neal, GPL 2 or later  ******/

/* Equal probability sampling; without-replacement case.

This version is written to produce the same result as earlier versions,
in which the algorithm was as follows (with x being temporary storage,
and y being the result):

for (i = 0; i < n; i++)
x[i] = i;
for (i = 0; i < k; i++) {
j = n * unif_rand();
y[i] = x[j] + 1;
x[j] = x[--n];
}

When k <= 2, special code is used, for speed.

When n is small or k is not much smaller than n, a modification of the
above algorithm is used, which avoids the need for temporary storage - the
result is allocated as of length n, and then has its length reduced to k
(usually with no copy being done).

When k is much smaller than n, and n is not small, a hashing scheme is
used, in which hash entries record which elements of x in the above
algorithm would have been modified from their original setting in which
x[i] == i.
*/

static SEXP SampleNoReplace (int k, int n)
{
SEXP r;

if (k <= 2) {

/* Special code for k = 0, 1, or 2, mimicing effect of previous code. */

if (k == 0)
return allocVector(INTSXP,0);

int i1 = 1 + (int) (n * unif_rand());
if (k == 1)
return ScalarInteger (i1);

int i2 = 1 + (int) ((n-1) * unif_rand());
if (i2 == i1) i2 = n;
r = allocVector(INTSXP,2);
INTEGER(r) = i1;
INTEGER(r) = i2;
}
else if (n < 100 || k > 0.6*n) {

/* Code similar to previous method, but with temporary storage avoided.
This reqires storing the initial sequence in decreasing rather than
increasing order, and picking elements from the tail rather than the
head, so that the space no longer used after each choice can hold the
result, at the front of the vector.  Note:  Unlike the previous
code, the indexes in the sequences are from 1 to n, not 0 to n-1. */

r = allocVector(INTSXP,n);
int *y = INTEGER(r);
int i;
for (i = 0; i < n; i++)
y[i] = n-i;
for (i = 0; i < k; i++) {
int j = n - 1 - (int) ((n-i) * unif_rand());
int t = y[j];
y[j] = y[i];
y[i] = t;
}
if (k < n)
r = reallocVector(r,k,1);
}

else {

/* Hash table implementation, producing same result as previous code.
Mimics previous code by using a hash table to record how 'x' would
have been changed.  At each iteration, it looks up x[j] in the
hash table (j from 1 up), taking its value to be j if it is not in
the table, and using this value as the next sampled value.  Also
lookups up x[n-i], which is taken to be n-i if not present, and
replaces/creates the entry for x[j] as having value x[n-i].  The
hash table is non-chaining, with linear search. */

/* Decide on the size of the hash table. */

unsigned tblsize, mintblsize;
mintblsize = 1.5 * k;
tblsize = 32;
while (tblsize < 0x80000000U && tblsize < mintblsize)
tblsize <<= 1;
unsigned tblmask = tblsize - 1;

/* Allocate hash table, as auto variable if small, else with R_alloc. */

struct tblentry { int pos, val; } *tbl;
struct tblentry local [ tblsize < 1000 ? tblsize : 1 ];
void *vmax = VMAXGET();
tbl = tblsize < 1000 ? local
: (struct tblentry *) R_alloc(tblsize,sizeof *tbl);

/* Clear all entries to zero.  Non-empty pos values start at 1. */

memset (tbl, 0, tblsize * sizeof *tbl);

/* Allocate vector to hold result. */

r = allocVector(INTSXP,k);
int *y = INTEGER(r);

/* Do the sampling as described above. */

int i;
for (i = 0; i < k; i++) {
int j = 1 + (int) ((n-i) * unif_rand());
unsigned h;
for (h = j & tblmask; ; h = (h+1) & tblmask) {
if (tbl[h].pos == 0) {
y[i] = j;
break;
}
if (tbl[h].pos == j) {
y[i] = tbl[h].val;
break;
}
}
unsigned h2;
for (h2 = (n-i) & tblmask; ; h2 = (h2+1) & tblmask) {
if (tbl[h2].pos == 0) {
tbl[h].val = n-i;
break;
}
if (tbl[h2].pos == n-i) {
tbl[h].val = tbl[h2].val;
break;
}
}
tbl[h].pos = j;  /* don't set until after search for entry n-i */
}

VMAXSET(vmax);
}

return r;
}

This will be in a new version of pqR that will have many other performance
improvements as well, which I expect to release in a few weeks.