[R] sample unique pairs from a matrix

[jos matejus]

Dear Ritchie and David,

Thanks very much for your advice. I had thought of this potential
solution, however it doesn't really fullfill my second criteria which
is that once a particular cell has been sampled, the row and column of
that cell can't be sampled from subsequently. In other words, the next
sample would be taken from a 5x5 matrix, and then a 4x4 matrix and so
on until I have my 6 values.

I will keep thinking!
Cheers
Jos

2009/5/28 David Winsemius <dwinsemius at comcast.net>:
>
> On May 28, 2009, at 6:33 AM, jos matejus wrote:
>
>> Dear R users,
>>
>> I have a matrix of both negative and positive values that I would like
>> to randomly sample with the following 2 conditions:
>>
>> 1. only sample positive values
>> 2. once a cell in the matrix has been sampled the row and column of
>> that cell cannot be sampled from again.
>>
>> #some dummy data
>> set.seed(101)
>> dataf <- matrix(rnorm(36,1,2), nrow=6)
>>
>> I can do this quite simply if all the values are positive by using the
>> sample function without replacement on the column and row indices.
>>
>> samrow <- sample(6,replace=F)
>> samcol <- sample(6,replace=F)
>> values <- numeric(6)
>> for(i in 1:6){
>>        values[i] <- dataf[samrow[i], samcol[i]]
>> }
>>
>> However, I am not sure how to include the logical condition to only
>> include postitive values
>> Any help would be gratefully received.
>> Jos
>
>> M <- matrix(rnorm(36),nrow=6)
>
>> M[M[,]>0]
>  [1] 1.65619781 0.56182830 0.23812890 0.81493915 1.01279243 1.29188874
> 0.64252343 0.53748655 0.31503112
> [10] 0.37245358 0.07942883 0.56834586 0.62200056 0.39478167 0.02374574
> 0.04974857 0.56219171 0.52901658
>
>
>> sample(M[M[,]>0],6,replace=F)
> [1] 0.56834586 0.07942883 0.31503112 0.62200056 0.02374574 0.64252343
>
> --
> David Winsemius, MD
> Heritage Laboratories
> West Hartford, CT
>
>