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"combn2"<-
function(x, n)
{
# DATE WRITTEN: 14 April 1994 LAST REVISED: 14 April 1994
# AUTHOR: Scott D. Chasalow
#
# DESCRIPTION:
# Generate all combinations of the elements of x taken two at a time.
# If x is missing, generate all combinations of 1:n taken two
# at a time (that is, the indices of x that would give all
# combinations of the elements of x if x with length n had been given).
# Exactly one of arguments "x" and "n" should be given.
#
if(!missing(x)) {
if(!missing(n))
warning(paste("Only one of arguments x and n allowed;",
"argument n was ignored"))
n <- length(x)
}
else if(missing(n))
stop("Arguments \"x\" and \"n\" both missing")
if(length(n) > 1) {
warning(paste("Argument n has", length(n),
"elements: only the first used"))
n <- n[1]
}
if(n == 0)
return(NULL)
rmat <- array(seq(length = n), c(n, n)) # row(matrix(0,n,n))
cmat <- t(rmat) # col(matrix(0,n,n))
lower.t <- rmat > cmat # lower.tri(matrix(0,n,n))
i1 <- cmat[lower.t]
i2 <- rmat[lower.t]
if(missing(x))
cbind(i1, i2)
else cbind(x[i1], x[i2])
}
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