Sum pairwise rows with R?

Question

My input is

 df1 <- data.frame(Row=c(\"row1\", \"row2\", \"row3\", \"row4\", \"row5\"),
                   A=c(1,2,3,5.5,5), 
                   B=c(2,2,2         


        

Answer 1

1. Create all the combinations you need with combn. t is used to transpose the matrix as you expect it to be formatted.
2. Use apply to iterate over the indices created in step 1. Note that we use negative indexing so we don't try to sum the Row column.
3. Bind the two results together.

`

ind <- t(combn(nrow(df1),2))
out <- apply(ind, 1, function(x) sum(df1[x[1], -1] * df1[x[2], -1]))
cbind(ind, out)

           out
[1,] 1 2  6.00
[2,] 1 3  7.00
[3,] 1 4 12.65
 .....

Answer 2

Yes! This is a matrix multiplication! :-))

First, just to prepare the matrix:

m = as.matrix(df1[,2:4])
row.names(m) = df1$Row

and this is the operation, how easy!

m %*% t(m)

That's it!

One tip - you could define the data.frame this way and it will save you the row.names command:

df1 <- data.frame(row.names=c("row1", "row2", "row3", "row4", "row5"),A=c(1,2,3,5.5,5), B=c(2,2,2,2,0.5), C= c(1.5,0,0,2.1,3))