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

This code below works.

# Create list of the 20 records
list <- c(1:20)

# Generate all combinations including repetitions
c <- data.frame(expand.grid(rep(list(list), 4))); rm(list)
c$combo <- paste(c$Var1, c$Var2, c$Var3, c$Var4)
# Remove repetitions
c <- subset(c, c$Var1 != c$Var2 & c$Var1 != c$Var3 & c$Var1 != c$Var4 & c$Var2 != c$Var3 & c$Var2 != c$Var4 & c$Var3 != c$Var4)

# Create common group labels (ex. abc, acb, bac, bca, cab, cba would all have "abc" as their group label).
key <- data.frame(paste(c$Var1, c$Var2, c$Var3, c$Var4))
key$group  <- apply(key, 1, function(x) paste(sort(unlist(strsplit(x, " "))), collapse = " "))
c$group <- key$group; rm(key)

# Sort by common group label and id combos by group
c <- c[order(c$group),]
c$Var1 <- NULL; c$Var2 <- NULL; c$Var3 <- NULL; c$Var4 <- NULL;
c$rank <- rep(1:24)

# Pivot
c <- reshape(data=c,idvar="group", v.names = "combo", timevar = "rank", direction="wide")