[R] sample unique pairs from a matrix

David Winsemius dwinsemius at comcast.net
Thu May 28 17:26:55 CEST 2009

```Your last step will either be a single number (not really a sampling
operation) or a
non-positive number. So at best you really only have an number that
depends
entirely on the prior sequence of draws.

--
David.

On May 28, 2009, at 8:21 AM, jos matejus wrote:

> Dear Ritchie and David,
>
> 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
>>
>>

David Winsemius, MD
Heritage Laboratories
West Hartford, CT

```