Re: [R] sampling vectors

From: Eric Pante <ericpante_at_earthlink.net>
Date: Wed 05 Oct 2005 - 01:47:09 EST

Hi Dan,

I just tried your code with long vectors, and the sampling stops to be random. Do you see any reason why this is ?

examples:

ex = c(30,13,9,8,7,7,7,6,6,5,2,2,2,1,1,1,1,1,1,1,1,1,1,1,1,1)

 > vectorSample(ex)
  [1] 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 3 1 7 6
[24] 4 2 75
 > vectorSample(ex)
  [1] 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 2 1 1 1 2 11
[24] 7 8 67
 >
 > vectorSample(ex)
  [1] 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
[24] 1 6 87
 > vectorSample(ex)
  [1] 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 2 1 1 1 1 1 2 3
[24] 1 4 85
 > vectorSample(ex)
  [1] 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 19
[24] 16 42 18
 > vectorSample(ex)
  [1] 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 2 1 3 5 4 4 17
[24] 2 53 10
 > vectorSample(ex)
  [1] 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 3 5
[24] 3 11 74
 > vectorSample(ex)
  [1] 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 3 3 1 2 3
[24] 21 23 43
 > vectorSample(ex)
  [1] 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 5 9
[24] 18 20 44
 > vectorSample(ex)
  [1] 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 2 1 3 6 14
[24] 8 6 59

On Oct 4, 2005, at 12:02 PM, Daniel Nordlund wrote:

>> -----Original Message-----
>> From: r-help-bounces@stat.math.ethz.ch
>> [mailto:r-help-bounces@stat.math.ethz.ch]
>> On Behalf Of ecatchpole
>> Sent: Monday, October 03, 2005 11:57 PM
>> To: Eric Pante
>> Cc: r-help@stat.math.ethz.ch
>> Subject: Re: [R] sampling vectors
>>
>> Eric,
>>
>> If you want samples of size 3 from 0:50, with sum==50, this seems to
>> do
>> the job (with apologies to those who really know how to program in R):
>>
>> tot <- 50
>> ii <- 0
>> aa <- list()
>> for(i in 0:tot){
>> for(j in 0:(tot-i)){
>> k <- tot-i-j
>> ii <- ii+1
>> aa[[ii]] <- list(i=i,j=j,k=k)
>> }
>> }
>>
>> aa[sample(ii, 4)] # for a sample of 4.
>>
>> If you want a sample of size n from x[1],...x[N], such that the sample
>> sum is T, then that is much trickier!
>>
>> Ted.
>>
>> On 04/10/05 07:31, Eric Pante wrote,:
>>> Hello Listers,
>>>
>>> I am trying to sample a vector to create a new one of sample length,
>>> witha sum equal to the sum of the initial vector:
>>>
>>> initial = 10, 30, 10 (sum=50)
>>> sample example = 5, 35, 10 (sum=50) or 25, 15, 10 (sum=50), etc ...
>>>
>>> My problem is to control the sum, so it stays constant.
>>> Any suggestions would be very helpful !
>>>
>>> Thank you in advance,
>>> Eric
>>>

>
> Eric,
>
> Here is another solution, which allows vectors of different lengths 
> and sums.
>
> vectorSample <- function(vec) {
>   tot<-sum(vec)
>   Len<-length(vec)
>   v < -rep(0,Len)
>   for(i in Len:2) {
>     UL <- tot - sum(v) - i + 1
>     v[i]<-sample(1:UL,1)
>     }
>   v[1] <- tot - sum(v)
>   v
>   }
>
>  vectorSample( c(10,30,10) )
>
> Hope this helps,
>
> Dan Nordlund
> Bothell, WA
>
>
>
>

Eric Pante

Graduate Student in Marine Biology
Grice Marine Laboratory
205 Fort Johnson Road, Charleston SC 29412
	"On ne force pas la curiosite, on l'eveille ..."
	Daniel Pennac

	[[alternative text/enriched version deleted]]

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