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
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