在 R 中找到成对和的向量的索引

Question

When I run this code

x <- c(2, 2, 10, 8)
combn(x, 2, sum)

I get

> combn(x, 2, sum)
[1]  4 12 10 12 10 18

How do I get the index for x for any of the combn? For this question, the 4 is the sum of 2 + 2 which is x[1] + x[2]. I need the index [1] and [2] for x.

Answer 1

This code will give you the first element of the combination in the first line, the second in the second, and in the last line you'll get the sum:

combn(x, 2, function(x) c(x, sum(x)))

     [,1] [,2] [,3] [,4] [,5] [,6]
[1,]    2    2    2    2    2   10
[2,]    2   10    8   10    8    8
[3,]    4   12   10   12   10   18

If you want the output as a data.frame :

t(combn(x, 2, function(x) c(x, sum(x))))-> l
dimnames(l)[[2]] <- c(paste0("X",1:2), "Sum")
as.data.frame(l)

  X1 X2 Sum
1  2  2   4
2  2 10  12
3  2  8  10
4  2 10  12
5  2  8  10
6 10  8  18