matrix %in% matrix

Question

Suppose I have two matrices, each with two columns and differing numbers of row. I want to check and see which pairs of one matrix are in the other matrix. If these were one

Answer 1

Andrie's solution is perfectly fine. But if you have big matrices, you might want to try something else, based on recursion. If you work columnwise, you can cut down on the calculation time by excluding everything that doesn't match at the first position:

fastercheck <- function(x,matrix){
  nc <- ncol(matrix)
  rec.check <- function(r,i,id){
    id[id] <- matrix[id,i] %in% r[i]
    if(i

The comparison :

> set.seed(100)
> x <- matrix(runif(1e6),ncol=10)
> a <- matrix(runif(300),ncol=10)
> a[c(3,7,9,15),] <- x[c(1000,48213,867,20459),]
> system.time(res1 <- a %inm% x)
   user  system elapsed 
  31.16    0.14   31.50 
> system.time(res2 <- fastercheck(a,x))
   user  system elapsed 
   0.37    0.00    0.38 
> identical(res1, res2)
[1] TRUE
> which(res2)
[1]  3  7  9 15

EDIT:

I checked the accepted answer just for fun. Performs better than the double apply ( as you get rid of the inner loop), but recursion still rules! ;-)

> system.time(apply(a, 1, paste, collapse="$$") %in% 
 + apply(x, 1, paste, collapse="$$"))
   user  system elapsed 
   6.40    0.01    6.41