Question

let’s say I have a matrix like this:

     [,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8] [,9]
[1,]    1    3    5    7    9   11   13   15   17
[2,]    2    4    6    8   10   12   14   16   18

I’m looking for an easy way to substract the 3rd column from the 1st and 2nd, then the 6th from the 4th and 5th, and so on.

Can I do this without a for-loop?

Thanks in advance,
zenzen.

Answer 1

This answer isn’t beautiful. It should really be made into a function for clarity, but:

m <- matrix(1:18,nrow=5,ncol=9, byrow=TRUE)
colsA <- (1:ncol(m))[1:ncol(m)%%3!=0]
colsB <- (1:ncol(m))[1:ncol(m)%%3==0]
m[,colsA] <- m[,colsA] - m[,rep(colsB,each=2)]

no for loop! And the result is:

     [,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8] [,9]
[1,]   -2   -1    3   -2   -1    6   -2   -1    9
[2,]   -2   -1   12   -2   -1   15   -2   -1   18

edit: here it is as a function

nth <- function(x,n) {
  colsA <- (1:ncol(x))[1:ncol(x)%%n!=0]
  colsB <- rep((1:ncol(x))[1:ncol(x)%%n==0], each=n-1)
  x[,colsA] <- x[,colsA] - x[,colsB]
  x
}