I have the following vector in R
x<-c(5,7,8,20,11,30)
I want to get all the possible subtractions xi-xj, 1<=i<j<=6. I used outer function but I got a matrix with zero diagonal. I do not want to have xi-xj with i=j. Additionally, I want to have a vector, not a matrix.
You can subset the output of outer using lower.tri or upper.tri.
y <- outer(x, x, '-')
y[lower.tri(y)]
# [1] 2 3 15 6 25 1 13 4 23 12 3 22 -9 10 19
Alternatively you can generate the indices using seq and rep.
k <- seq(length(x) - 1, 1)
i <- rep(seq_along(k), k)
j <- sequence(k) + i
x[j] - x[i]
# [1] 2 3 15 6 25 1 13 4 23 12 3 22 -9 10 19
Yes, outer generates a matrix with all possible combinations in both the directions (a - b and b - a) which is unnecessary in this case.
Perhaps, you can use combn :
combn(x, 2, diff)
#[1] 2 3 15 6 25 1 13 4 23 12 3 22 -9 10 19
Also as @Rui Barradas points out that outer might not be the right solution since it fails for the condition 1<=i<j<=6.
Higher order functions; less succinct/readable than above but quicker for some reason.
Filter(function(y) {y != 0},
do.call("c", (Map(function(i){y <- x - x[i]; y[i:length(x)]}, seq_along(x)))))