Here is another approach :
rowmatch.count <- function(a,b) {
f <- function(...) paste(..., sep=":")
a2 <- do.call("f", as.data.frame(a))
b2 <- do.call("f", as.data.frame(b))
c(table(c(a2,unique(b2)))[b2] - 1)
}
# example
a <- matrix(c(1,2,1,5,2,5), 2, 3)
b <- matrix(c(1,2,4,5,7,7,9,2, 5,5, 9,8,8,10,10, 5), 8, 2)
rowmatch.count(b, t(a))
Interestingly, this was suggested by either Marc or Gabor about 3 years ago
when I had the same question; you may be able to tack it down in r-help
archives.
Hope this helps,
Ravi.
----------------------------------------------------------------------------
-------
Ravi Varadhan, Ph.D.
Assistant Professor, The Center on Aging and Health
Division of Geriatric Medicine and Gerontology
Johns Hopkins University
Ph: (410) 502-2619
Fax: (410) 614-9625
Email: rvarad...@jhmi.edu
Webpage:
http://www.jhsph.edu/agingandhealth/People/Faculty_personal_pages/Varadhan.h
tml
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--------
-----Original Message-----
From: r-help-boun...@r-project.org [mailto:r-help-boun...@r-project.org] On
Behalf Of Marc Schwartz
Sent: Thursday, July 02, 2009 10:20 AM
To: dreamworx
Cc: r-help@r-project.org
Subject: Re: [R] matching rows in matrices
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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and provide commented, minimal, self-contained, reproducible code.
