[R] bootstrap subject resampling: resampled subject codes surface as list/vector indices

[Aleksander Główka]

I'm implementing a custom bootstrap resampling procedure in R. This 
procedure resamples clusters of data points obtained by different 
subjects in an experiment. Since the bootstrap samples need to have the 
same size as the original dataset, `target.set.size`, I select speakers 
compute their data point contributions to make sure I have a set of the 
right size.

     set.seed(1)
     target.sample.size = 1742
     count.lookup = rbind(levels(data$subj), as.numeric(table(data$subj)))

To this end, I create a dynamic list of resampled subjects, 
`sample.subjects`, that keep on being selected and appended to the list 
as long as their summed data point contributions do not exceed 
`target.set.size`. To conveniently retrieve the number of data points 
that a given subject contributes I constructed a reference matrix, 
`count.lookup`, where the first row contains subject codes and the 
second row contains their respective data point counts.

     > count.lookup

     [,1]  [,2]  [,3]  [,4]  [,5]
     [1,] "5"   "6"   "13"  "18"  "20"
     [2,] "337" "202" "311" "740" "152"

This is how the resampling works:

     for (iter in 1:1000){

       #select first subject
       #empty list overwrites sample subjects from previous iteration
       sample.subjects = list()
       sample.subjects[1] = sample(unique(data$subj), 1, replace=TRUE, 
prob=NULL)

       #determine subject position in data point count lookup
       first.subj.pos = which(count.lookup[1,]==sample.subjects, 
arr.ind=TRUE)

       #add contribution of first subject to data point count
       sample.size = as.numeric(count.lookup[2,first.subj.pos])

       #select subject clusters until you exceed target sample size
       while(sample.size < target.sample.size){

         #add another subject
         current.subject = sample(unique(data$subj), 1, replace=TRUE, 
prob=NULL)
         sample.subjects[length(sample.subjects)+1] = current.subject

         #determine subject's position in data point lookup
         curr.subj.pos = which(count.lookup[1,]==current.subject, 
arr.ind=TRUE)

         #add subject contribution to the data point count
         sample.size = sample.size + 
as.numeric(count.lookup[2,curr.subj.pos])
       }

       #initialize intermediate data frame; intermediate because it will 
be shortened to fit target size
       inter.set = data.frame(matrix(, nrow = 0, ncol = ncol(data)))

       #build the bootstrap sample from the selected subjects
       for(j in 1:length(sample.subjects)){

         inter.set = rbind(inter.set, data[data$subj == 
sample.subjects[j],])

       }

       #procustean bed of target sample size
       final.set = inter.set[1:target.sample.size,]

       write.csv(final.set, paste("bootstrap_sample_", iter,".csv", 
sep=""), row.names=FALSE)
       cat("Bootstrap Iteration", iter, "completed\n")

       #clean up sample.size for next bootstrap iteration
       sample.size = 0

     }

My problem is that when I sample the second subject onward and add it to 
`sample.subjects` (regardless of whether it is a list of a vector), what 
actually gets added to `sample.subjects` seems to be the index of that 
subject in `count.lookup`! When I select the first subject code and 
create a list consisting of just that subject code as the only element, 
everything is fine.

     > sample.subjects[1] = sample(unique(tt1$subj), 1, replace=TRUE, 
prob=NULL)
     > sample.subjects
     [[1]]
     [1] 5

I know this is the actual subject number because when I check the number 
of data points that this subject contributes in `count.lookup`, it is 
the number that corresponds to subject 5.

     > sample.size = as.numeric(tt1.lookup[2,first.subj.pos])
     > sample.size

However, when I append further sampled subject codes to the list, for 
some reason they surface as their index number in count.lookup.

     > sample.subjects
     [[1]]
     [1] 5

     [[2]]
     [1] 5

     [[3]]
     [1] 1

     [[4]]
     [1] 2

     [[5]]
     [1] 5

     [[6]]
     [1] 2

     [[7]]
     [1] 2

     [[8]]
     [1] 3

     [[9]]
     [1] 3

The third element, for example, is 1. This coincides with none of the 
subject codes in count.lookup.

It seems the problem lies in how I append to `sample.subjects`. I tried 
both vectors and list as data structures in which to store sampled 
subject codes. For each data type, I tried two ways of appending: the 
one I present above, and one that is more idiomatic in R:

sampled.subjects = [current.subject, sampled.subjects] (for lists)

and

sampled.subjects = c(current.subject, sampled.subjects) (for vectors)

Are these appending strategies flawed here or is there some stupid error 
I'm making somewhere else that is making the indices to surface instead 
of subject codes?

I'd appreciate all your help!