I want to compare two matrices or data frames and select or get an index for those rows which are the same in both. I have tried the following :
a = matrix ( 1:10, ncol = 2 ) a b = matrix ( c ( 2,3,4,7,8,9 ), ncol = 2 ) b a[a==b] a = as.data.frame ( matrix ( 1:10, ncol = 2 ) ) a b = as.data.frame ( matrix ( c ( 2,3,4,7,8,9 ), ncol = 2 ) ) b a[a==b] Any ideas please. Thanks. Simon Parker Imperial College 
I'm not sure what you want, but take a look at ?merge and %in%

In reply to this post by ppaarrkk
Dear Simon,
Try this: # Index  FALSE, TRUE sapply(1:nrow(a),function(x) all(a[x,]%in%b)) # Rows of a that are in b which(sapply(1:nrow(a),function(x) all(a[x,]%in%b))) # Reporting a[sapply(1:nrow(a),function(x) all(a[x,]%in%b)),] HTH, Jorge On Tue, Dec 9, 2008 at 9:57 AM, ppaarrkk <[hidden email]> wrote: > > I want to compare two matrices or data frames and select or get an index > for > those rows which are the same in both. I have tried the following : > > > > > > > a = matrix ( 1:10, ncol = 2 ) > a > > b = matrix ( c ( 2,3,4,7,8,9 ), ncol = 2 ) > b > > a[a==b] > > > > > > > a = as.data.frame ( matrix ( 1:10, ncol = 2 ) ) > a > > b = as.data.frame ( matrix ( c ( 2,3,4,7,8,9 ), ncol = 2 ) ) > b > > a[a==b] > > > > > > > > > Any ideas please. > > > Thanks. > > > Simon Parker > Imperial College > >  > View this message in context: > http://www.nabble.com/Selectingrowsthatarethesameinseparatedataframestp20916243p20916243.html > Sent from the R help mailing list archive at Nabble.com. > > ______________________________________________ > [hidden email] mailing list > https://stat.ethz.ch/mailman/listinfo/rhelp > PLEASE do read the posting guide > http://www.Rproject.org/postingguide.html > and provide commented, minimal, selfcontained, reproducible code. > [[alternative HTML version deleted]] ______________________________________________ [hidden email] mailing list https://stat.ethz.ch/mailman/listinfo/rhelp PLEASE do read the posting guide http://www.Rproject.org/postingguide.html and provide commented, minimal, selfcontained, reproducible code. 
In reply to this post by ppaarrkk
Try this:
1) Rows: merge(as.data.frame(a), as.data.frame(b), sort = FALSE) 2) Index: merge(cbind(as.data.frame(a), Idx = 1:nrow(a)), as.data.frame(b), sort = FALSE)$Idx On Tue, Dec 9, 2008 at 12:57 PM, ppaarrkk <[hidden email]> wrote: > > I want to compare two matrices or data frames and select or get an index > for > those rows which are the same in both. I have tried the following : > > > > > > > a = matrix ( 1:10, ncol = 2 ) > a > > b = matrix ( c ( 2,3,4,7,8,9 ), ncol = 2 ) > b > > a[a==b] > > > > > > > a = as.data.frame ( matrix ( 1:10, ncol = 2 ) ) > a > > b = as.data.frame ( matrix ( c ( 2,3,4,7,8,9 ), ncol = 2 ) ) > b > > a[a==b] > > > > > > > > > Any ideas please. > > > Thanks. > > > Simon Parker > Imperial College > >  > View this message in context: > http://www.nabble.com/Selectingrowsthatarethesameinseparatedataframestp20916243p20916243.html > Sent from the R help mailing list archive at Nabble.com. > > ______________________________________________ > [hidden email] mailing list > https://stat.ethz.ch/mailman/listinfo/rhelp > PLEASE do read the posting guide > http://www.Rproject.org/postingguide.html > and provide commented, minimal, selfcontained, reproducible code. >  Henrique Dallazuanna CuritibaParanáBrasil 25° 25' 40" S 49° 16' 22" O [[alternative HTML version deleted]] ______________________________________________ [hidden email] mailing list https://stat.ethz.ch/mailman/listinfo/rhelp PLEASE do read the posting guide http://www.Rproject.org/postingguide.html and provide commented, minimal, selfcontained, reproducible code. 
In reply to this post by Bart Joosen
Thanks for reply.
What I want is the equivalent of this : xxx = 1:10 which(xxx %in% c(2,5)) .......but where there is more than one criterion for matching. which (b %in% a) in the code I included does nothing (not surprisingly). I'm not sure that I can use merge, because I want the whole of a, but to mark those rows which are also in b. If I do merge ( a,b ), I just get b. If I do merge ( a,b, all.x =TRUE), I get a.

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