问题
I'm trying to get the pairwise products of a vector, say
a = c(1,2,3,4)
What I'm trying to get is
2,3,4,6,8,12 (in that order).
I've tried using outer:
outer(1:4,2:4)
and that gives me a matrix that includes the products I want but I'm not sure how to extract them from the matrix in a way that scales to vectors of higher dimensions.
Thanks!
回答1:
combn() is nice for this sort of thing:
a <- 1:4
combn(a, m = 2, FUN = prod)
# [1] 2 3 4 6 8 12
回答2:
lower.tri selects them in that order:
out <- outer(1:4,1:4)
out[lower.tri(out)]
# [1] 2 3 4 6 8 12
来源：https://stackoverflow.com/questions/18899332/r-pairwise-product