# [R] Sampling with conditions

(Ted Harding) ted.harding at wlandres.net
Tue Nov 8 09:13:39 CET 2011

```[Yet another correction -- this one is important.
I start from scratch this time]

On 07-Nov-11 22:22:54, SarahJoyes wrote:
> Hey everyone,
> I am at best, an amateur user of R, but I am stuck on how
> to set-up the following situation.
> I am trying to select a random sample of numbers from 0 to 10
> and insert them into the first column of a matrix (which will
> used later in a loop).
> However, I need to have those numbers add up to 10. How can
> I set those conditions?
> So far I have:
> n<-matrix(0,nr=5,ncol=10)
> for(i in 1:10){n[i,1]<-sample(0:10,1)}
> How do I set-up the "BUT sum(n[i,1])=10"?
> Thanks
> SarahJ

Sarah, your example is confusing because you have set up a
matrix 'n' with 5 rows and 10 columns. But your loop cycles
through 10 rows!

However, assuming that your basic requirement is to sample
10 integers which add up to 10, consider rmultinom():

rmultinom(n=1,size=10,prob=rep(1,10)/10)
#      [,1]
# [1,]    1
# [2,]    0
# [3,]    2
# [4,]    3
# [5,]    1
# [6,]    1
# [7,]    0
# [8,]    0
# [9,]    1
#[10,]    1
rmultinom(n=1,size=10,prob=rep(1,10)/10)
#      [,1]
# [1,]    2
# [2,]    0
# [3,]    1
# [4,]    1
# [5,]    2
# [6,]    2
# [7,]    1
# [8,]    0
# [9,]    1
#[10,]    0

This gives a uniform distribution over the positions in
the sample vector for the sampled integers, so that all
permutations are equally likely. For a non-uniform
distribution, vary 'prob'.

Ted.

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Date: 08-Nov-11                                       Time: 08:13:36
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