# [R] random section of samples based on group membership

Sebastian Luque spluque at gmail.com
Mon Jul 24 21:25:54 CEST 2006

```On Mon, 24 Jul 2006 11:18:10 -0400,

> Hi all, I have a matrix of 474 rows (samples) with 565 columns
> (variables).  each of the 474 samples belong to one of 120 groups, with
> the groupings as a column in the above matrix. For example, the group
> column would be:

> 1 1 1 2 2 2 .  .  .  120 120

> I want to randomly select one from each group.  Not all the groups have
> the same number of samples, some have 4, some 3 etc.  Is there a
> function to do this, or would I need to write a looping statement to
> look at each successive group?

I use the following for that (some of it hacked from help("sample")):

".resample" <- function(x, size, ...) {
if(length(x) <= 1) {
if(!missing(size) && size == 0) x[FALSE] else x
} else sample(x, size, ...)
}

"randpick" <- function(x, by, size = 1, ...)
{
nx <- seq(nrow(x))
ind <- unlist(tapply(nx, by, .resample, size, ...))
x[nx %in% ind, ]
}

So, for instance:

R> randpick(Indometh, Indometh\$Subject, 3)
Subject time conc
2        1 0.50 0.94
7        1 3.00 0.12
11       1 8.00 0.05
15       2 1.00 0.70
16       2 1.25 0.64
19       2 4.00 0.20
25       3 0.75 1.16
29       3 3.00 0.22
32       3 6.00 0.08
34       4 0.25 1.85
43       4 6.00 0.07
44       4 8.00 0.07
48       5 1.00 0.39
54       5 6.00 0.10
55       5 8.00 0.06
58       6 0.75 1.03
64       6 5.00 0.13
65       6 6.00 0.10
R> randpick(Indometh, Indometh\$Subject, 2)
Subject time conc
8        1 4.00 0.11
10       1 6.00 0.07
14       2 0.75 0.71
20       2 5.00 0.25
23       3 0.25 2.72
28       3 2.00 0.39
39       4 2.00 0.40
43       4 6.00 0.07
48       5 1.00 0.39
52       5 4.00 0.11
57       6 0.50 1.44
66       6 8.00 0.09

The 'by' argument allows to sample within any combination of factors
desired.

Cheers,

--
Seb

```