Here's a simple function (same idea):
f <- function(x){
lenx <- length(x)
negx <- sum(x < 0)
mat <- matrix(1, lenx, choose(lenx,negx))
for(i in seq_len(choose(lenx, negx))){
mat[combn(lenx,negx)[, i], i] <- -1
}
mat
}
x <- c(-1,-1,-1,1,1,1,1)
f(x)
[combn() is now in utils]
-Peter Ehlers
Greg Snow wrote:
Here is one quick way using the combinat package:
library(combinat)
tmpfun <- function(x) {
+ tmp <- rep(1,5)
+ tmp[x] <- -1
+ tmp
+ }
combn(5,2, tmpfun)
[,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8] [,9] [,10]
[1,] -1 -1 -1 -1 1 1 1 1 1 1
[2,] -1 1 1 1 -1 -1 -1 1 1 1
[3,] 1 -1 1 1 -1 1 1 -1 -1 1
[4,] 1 1 -1 1 1 -1 1 -1 1 -1
[5,] 1 1 1 -1 1 1 -1 1 -1 -1
Of course in this case the tmpfun function needs to be rewritten for each
vector size, so is not generalizable.
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