My challenge was to generate random codes of variable length and size. The codes should be all unique and build of alphanumeric symbols. I wrote the following function using the build-in function sample():

alpha.rnd <- function(length, size) {
	if(size > 36^length) stop("\n size cannot be greater than 36^length")
	b <- NULL
	repeat {
		a <- NULL	
		for (i in 1:length) {
			a <- paste0(sample(c(letters, 0:9), size, replace = TRUE), a)
		}
		b <- c(a,b)
		if(sum(duplicated(b)) == 0) break
	  	size = sum(duplicated(b))
		b <- b[!duplicated(b)]		
	}
	return(b)
}

Example:

> set.seed(77)
> alpha.rnd(3,5)
[1] "5qk" "94z" "f45" "au8" "qq0"

Enjoy!