[R] matching rows in matrices

[Marc Schwartz]

On Jul 2, 2009, at 7:27 AM, dreamworx wrote:

>
> I have a matrix such as this,
>
>     [,1] [,2] [,3]
> [1,]    1    1    2
> [2,]    2    5    5
>
> and a larger matrix such as this,
>
>     a  b
> [1,] 1  5
> [2,] 2  5
> [3,] 4  9
> [4,] 5  8
> [5,] 7  8
> [6,] 7 10
> [7,] 9 10
>
> what I want to do is check the number of times the columns in the  
> first
> matrix appear as rows in the second matrix. So for instance in this  
> example
> 1,5  and 2,5 appear in both so I'd want R to return the number 2. Is  
> there a
> function to do this?


My first approach would be to use merge(), which performs a relational  
join. So, given mat1:

# Note the rownames here, to match the colnames in mat2
 > mat1
   [,1] [,2]
a    1    2
b    5    5


and mat2:

 > mat2
      a  b
[1,] 1  5
[2,] 2  5
[3,] 4  9
[4,] 5  8
[5,] 7  8
[6,] 7 10
[7,] 9 10


Using merge() we get:

 > merge(t(mat1), mat2)
   a b
1 1 5
2 2 5

This gives us the rows that match between the two matrices. Note that  
I transpose mat1 so that the structure matches that of mat2.

Then just use nrow() to get a count:

 > nrow(merge(t(mat1), mat2))
[1] 2


See ?merge for more information.

HTH,

Marc Schwartz