R help - Nov 2011 - Sampling with conditions

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

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Not sure this is valid that you can have 9 random samples out of 10,
but the last one has to be fixed to meet the restraint, sum=10.

Weidong

On Mon, Nov 7, 2011 at 5:22 PM, SarahJoyes <sjoyes at uoguelph.ca>
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
>
> --
> View this message in context:
http://r.789695.n4.nabble.com/Sampling-with-conditions-tp4014036p4014036.html
> Sent from the R help mailing list archive at Nabble.com.
>
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>

> -----Original Message-----
> From: r-help-bounces at r-project.org [mailto:r-help-bounces at
r-project.org]
> On Behalf Of SarahJoyes
> Sent: Monday, November 07, 2011 2:23 PM
> To: r-help at r-project.org
> Subject: [R] Sampling with conditions
> 
> 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,

Does something like this do what you want?

n <- matrix(0,nrow=5, ncol=10)
repeat{
  c1 <- sample(0:10, 4, replace=TRUE)
  if(sum(c1) <= 10) break
}
n[,1] <- c(c1,10-sum(c1))
n


Hope this is helpful,

Dan

Daniel Nordlund
Bothell, WA USA

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

This gives each integer in (0:10) equal chances of being
in the sample. For unequal chances, vary 'prob'.

Hoping this helps,
Ted.

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E-Mail: (Ted Harding) <ted.harding at wlandres.net>
Fax-to-email: +44 (0)870 094 0861
Date: 08-Nov-11                                       Time: 00:25:54
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