create random subsets in R without duplicates - r

Question

my task is to divide a dataset of 32 rows into 8 groups without having duplicated entries.
i am trying to do this with a loop and by creating a new dataset after each cycle.
the data:
year pos country elo fifa cont hcountry hcont
1 2010 FRA 1851 1044 Europe RSA Africa
2 2010 MEX 1872 895 South America RSA Africa
3 2010 URU 1819 899 South America RSA Africa
4 2010 RSA 1569 392 Africa RSA Africa
5 2010 GRE 1726 964 Europe RSA Africa
6 2010 KOR 1766 632 Asia RSA Africa
8 2010 ARG 1899 1076 South America RSA Africa
9 2010 USA 1749 957 North America RSA Africa
10 2010 SVN 1648 860 Europe RSA Africa
11 2010 ALG 1531 821 Africa RSA Africa
...
my solution so far:
for (i in 1:8){
assign(paste("group", i, sep = ""), droplevels(subset(wc2010[sample(nrow(wc2010), 4),])))
wc2010 <- subset(wc2010, !(country %in% group[i]$country))
}
problem is of course: i don't know how to use the loop-variable.... :-(
help would be deeply appreciated!
thanks
Bob

Answer 1

Here is one way to create a random partition:
random.groups <- function(n.items = 32L, n.groups = 8L)
1L + (sample.int(n.items) %% n.groups)
So then you just have to do:
wc2010$group <- random.groups(nrow(wc2010), n.groups = 8L)
Then you might also be interested in doing
groups <- split(wc2010, wc2010$group)
Edit: this was not asked by the OP, but I realize that soccer draws for big tournaments usually involves hats: before the draw, teams are grouped by regions and/or rankings. Then groups are formed by randomly picking one team from each hat, so that two teams from a same hat cannot end up in the same group.
Here is a modification to my function so it can also take hats as an input:
random.groups <- function(n.items = 32L, n.groups = 8L,
hats = rep(1L, n.items)) {
splitted.items <- split(seq.int(n.items), hats)
shuffled <- lapply(splitted.items, sample)
1L + (order(unlist(shuffled)) %% n.groups)
}
Here is an example, where say, the first 8 teams are in hat #1, the next 8 teams are in hat #2, etc.:
# set.seed(123)
random.groups(32, 8, c(rep(1, 8), rep(2, 8), rep(3, 8), rep(4, 8)))
# [1] 7 8 2 6 5 3 1 4 8 7 5 3 2 4 1 6 3 2 7 6 5 8 1 4 7 6 5 4 3 2 1 8