[R] A combinatorial task. How to get rid of loops

Serguei Kaniovski Serguei.Kaniovski at wifo.ac.at
Thu Aug 2 10:40:58 CEST 2007

Dear List,

I am looking for a faster way of accomplishing this:

# n is an integer larger than two
n <- 3

# matrix of 2^n binary outcomes
bmat <- as.matrix(expand.grid( rep( list(1:0), n))[, n:1])

# I would like to know which rows of "bmat" have "1" in the "i" and "j" 
coordinates, so for n=3 in coordinates 1-2, 1- 3, and 2-3
# I would like then to construct "choose( n, 2)" binary vectors of lengths 
2^n to indicate those rows with a "1"
# The loop below accomplishes this task. Is there a faster way of doing 
the same?


        # matrix of all pairwise combinations 
        cmat<-combn( n, 2)
        temp<-matrix(NA, nrow(bmat), ncol(cmat))

        for (i in 1:nrow(bmat))
                for (j in 1:ncol(cmat))
                        temp[ i, j]<-as.numeric( bmat[i,][cmat[1,j]] & 
bmat[i,][cmat[2,j]] == 1)

Thank you very much for your help!
Serguei Kaniovski

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