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copied!<p>You could first find all 2-way combinations, and then just combine them with the 3d value while saving them every time. This takes a lot less memory:</p> <pre><code>combn.mod <- function(x,fname){ tmp <- combn(x,2,simplify=F) n <- length(x) for ( i in x[-c(n,n-1)]){ # Drop all combinations that contain value i id <- which(!unlist(lapply(tmp,function(t) i %in% t))) tmp <- tmp[id] # add i to all other combinations and write to file out <- do.call(rbind,lapply(tmp,c,i)) write(t(out),file=fname,ncolumns=3,append=T,sep=",") } } combn.mod(x,"F:/Tmp/Test.txt") </code></pre> <p>This is not as general as Joshua's answer though, it is specifically for your case. I guess it is faster -again, for this particular case-, but I didn't make the comparison. Function works on my computer using little over 50 Mb (roughly estimated) when applied to your x.</p> <p>EDIT</p> <p>On a sidenote: If this is for simulation purposes, I find it hard to believe that any scientific application needs 400+ million simulation runs. You might be asking the correct answer to the wrong question here...</p> <p>PROOF OF CONCEPT :</p> <p>I changed the write line by <code>tt[[i]]<-out</code>, added <code>tt <- list()</code> before the loop and return(tt) after it. Then:</p> <pre><code>> do.call(rbind,combn.mod(letters[1:5])) [,1] [,2] [,3] [1,] "b" "c" "a" [2,] "b" "d" "a" [3,] "b" "e" "a" [4,] "c" "d" "a" [5,] "c" "e" "a" [6,] "d" "e" "a" [7,] "c" "d" "b" [8,] "c" "e" "b" [9,] "d" "e" "b" [10,] "d" "e" "c" </code></pre>

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