Multiply a data frame row-by-row

Question

Input file:

df1 <- data.frame(row.names=c(\"w\",\"x\",\"y\",\"z\"), 
                  A=c(0,0,0,0),
                  B=c(0,1,0,0), 
                  C=c(1,         


        

Answer 1

If you want to multiply rows, I recommend converting to a matrix:

> m = as.matrix(df1)

> m["x", ] * m["y", ]
A B C D 
0 0 0 1 

The specific result you want you could get with plyr,

library(plyr)

ldply(1:(nrow(m)-1), function(i)
    ldply((i+1):nrow(m), function(j) {
        a = row.names(m)[[i]]
        b = row.names(m)[[j]]

        do.call(data.frame,
            c(list(a=a, b=b), m[i,] * m[j,])
        )
    })
)

Sorry part of that looks a little magical -- data.frames aren't really meant to be "row like". The lines

do.call(data.frame,
    c(list(a=a, b=b), m[i,] * m[j,])
)

pass in the 6 columns: a and b for the names, concatenated (with c) to the multiplied row.