Is there a R function that applies a function to each pair of columns?

Question

I often need to apply a function to each pair of columns in a dataframe/matrix and return the results in a matrix. Now I always write a loop to do this. For instance, to mak

Answer 1

You can use mapply, but as the other answers state its unlikely to be much faster as most of the time is being used up by cor.test.

matrix(mapply(function(x,y) cor.test(df[,x],df[,y])$p.value,rep(1:3,3),sort(rep(1:3,3))),nrow=3,ncol=3)

You could reduce the amount of work mapply does by using the symmetry assumption and noting the zero diagonal, eg

v <- mapply(function(x,y) cor.test(df[,x],df[,y])$p.value,rep(1:2,2:1),rev(rep(3:2,2:1)))
m <- matrix(0,nrow=3,ncol=3)
m[lower.tri(m)] <- v
m[upper.tri(m)] <- v