Create Combinations in R by Groups

Question

I want to create a list for my classroom of every possible group of 4 students. If I have 20 students, how I can I create this, by group, in R where my rows are each combination

Answer 1

So you could get all the combinations with the expand.grid function just adding the vector of data four times. Then the result will have combinations like c(1,1,1,1) so i remove each row that have any duplicated value and the last part is just making the combinations. It is 2 loops and it is quite slow but it will get what you want. It could be speed up with the Rcpp package. The code is:

ids = 1:20
d2 = expand.grid(ids,ids,ids,ids)
## Remove rows with duplicated values
pos_use = apply(apply(d2,1,duplicated),2,function(x) all(x == F))
d2_temp = t(apply(d2[pos_use,],1,sort))
list_temp = list()
pos_quitar = NULL
for(i in 1:nrow(d2_temp)){
  pos_quitar = c(pos_quitar,i)
  ini_comb = d2_temp[i,]
  d2_temp_use  = d2_temp[-pos_quitar,]
  temp_comb = ini_comb
  for(j in 2:5){
    pos_quitar_new = which(apply(d2_temp_use,1,function(x) !any(temp_comb%in%x)))[1]
    temp_comb = c(temp_comb,d2_temp_use[pos_quitar_new,])
  }
  pos_quitar = c(pos_quitar,pos_quitar_new)
  list_temp[[i]] = temp_comb
}

list_temp