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-dimensional, I would normally just do a %in% x to get my results. match seems only to work on vectors.

> a
      [,1] [,2]
[1,]    1    2
[2,]    4    9
[3,]    1    6
[4,]    7    7
> x
     [,1] [,2]
[1,]    1    6
[2,]    2    7
[3,]    3    8
[4,]    4    9
[5,]    5   10

I would like the result to be c(FALSE,TRUE,TRUE,FALSE).

Answer 1

Recreate your data:

a <- matrix(c(1, 2, 4, 9, 1, 6, 7, 7), ncol=2, byrow=TRUE)
x <- matrix(c(1, 6, 2, 7, 3, 8, 4, 9, 5, 10), ncol=2, byrow=TRUE)

Define a function %inm% that is a matrix analogue to %in%:

`%inm%` <- function(x, matrix){
  test <- apply(matrix, 1, `==`, x)
  any(apply(test, 2, all))
}

Apply this to your data:

apply(a, 1, `%inm%`, x)
[1] FALSE  TRUE  TRUE FALSE

---

To compare a single row:

a[1, ] %inm% x
[1] FALSE

a[2, ] %inm% x
[1] TRUE

Answer 2

Another approach would be:

> paste(a[,1], a[,2], sep="$$") %in% paste(x[,1], x[,2], sep="$$")
[1] FALSE  TRUE  TRUE FALSE

A more general version of this is:

> apply(a, 1, paste, collapse="$$") %in% apply(x, 1, paste, collapse="$$")
[1] FALSE  TRUE  TRUE FALSE