[R] Sampling problem

[Michael Bedward]

Hello,

Is this what you want ?

sampleX <- function(X, nGrp1, nsamples)
# X is matrix or data.frame with cols for two groups of variables
# with grp1 in cols 2:5 and grp2 in cols 6:9
#
# nGrp1 <- number of variables to sample from group 1
#
# nsamples <- number of rows in output matrix

  if (nGrp1 >= 4) stop("can't sample all group 1 variables")

  out <- matrix(0, nsamples, nGrp1+1)
  for (i in 1:nsamples) {
  	# choose grp1 vars to sample
    grp1 <- sample(4, nGrp1)

    # choose complentary grp2 var to sample
    grp2 <- sample((1:4)[-grp1], 1)

    # sample 1 value from each var
    out[i, ] <- apply(X[,c(grp1+1, grp2+5)], 2, sample, 1)
  }

  out
}

Michael


On 16 November 2010 07:59, wangwallace <talenttree at gmail.com> wrote:
>
> Hey,
>
> I am hoping someone can help me with a sampling question.
>
> I have a data frame of 8 variables (the first column is the subjects' id):
>
>    SubID    CSE1 CSE2 CSE3 CSE4 WSE1 WSE2 WSE3 WSE4
>      1          6      5       6       2      6      2        2       4
>      2          6      4       7       2      6      6        2       3
>      3          5      5       5       5      5      5        4       5
>      4          5      4       3       4      4      4        5       2
>      5          5      6       7       5      6      4        4       1
>      6          5      4       3       6      4      3        7       3
>      7          3      6       6       3      6      5        2       1
>      8          3      6       6       3      6      5        4       7
>
> the 6 variables are categorized into two groups with CSE1, CSE2, CSE3, and
> CSE4 in one group and the rest in another group.
>
>>sample(data[,2:4],2,replace=FALSE)
>
>   CSE1 CSE2
> 1      6    5
> 2      6    4
> 3      5    5
> 4      5    4
> 5      5    6
> 6      5    4
> 7      3    6
> 8      3    6
>
> Now I want to sample 1 column from another group of variables (i.e., WSE1,
> WSE2, WSE3, WSE4), but I want to restrict a vector I am going to sample from
> to only those columns that are not correspond to GROUP 1 variables I have
> sampled. That is, I want to sample a column from WSE3, WSE4  Columns
> corresponding to CSE1 and CSE2 (i.e., WSE1, WSE2) need to be dropped.
>
> How can I do this? what if I want to repeat this whole process (drawing 2
> random columns from CSE1, CSE2, CSE3, and CSE4 first, AND then another
> random column from WSE1, WSE2, WSE3, and WSE4) for 1000 times. any ideas?
>
> Many thanks in advance!!
>
> --
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>
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