[R] Getting elements of a matrix by a vector of column indice s

[Liaw, Andy]

This should work:

> idx <- cbind(match(rowidx, rownames(m)), match(colidx, colnames(m)))
> m[idx]
[1] "b1" "c3" "d2"

Andy

> From: Wolfram Fischer
> 
> Thanks for you answer! It works.
> 
> >	m <- outer(letters[1:5], 1:4, paste, sep="")
> 
> The following works with the help of your proposition:
> >	rowidx.n <- c( 2, 3, 4)
> >	colidx.n <- c( 1, 3, 2)
> >	idx.n <- cbind( rowidx.n, colidx.n )
> >	m[idx.n]
> [1] "b1" "c3" "d2"
> 
> In my real data there was an additional difficulty:
> I had names of rows and columns as indices:
> >	rownames(m) <- paste('R', 1:nrow(m), sep="")
> >	colnames(m) <- paste('C', 1:ncol(m), sep="" )
> 
> And the following did not work anymore:
> >	rowidx <- c( 'R2', 'R3', 'R4' )
> >	colidx <- c( 'C1', 'C3', 'C2' )
> >	idx <- cbind( rowidx, colidx )
> >	m[idx]
> <NA> <NA> <NA> <NA> <NA> <NA> 
>   NA   NA   NA   NA   NA   NA 
> 
> Do you have another suggestion? - Thanks! Wolfram
> 
> --- In reply to: ---
> >Date:    08.07.04 08:21 (-0400)
> >From:    "Liaw, Andy" <andy_liaw at merck.com>
> >Subject: RE: [R] Getting elements of a matrix by a vector of 
> column indice s
> >
> > See if the following helps:
> > 
> > > m <- outer(letters[1:5], 1:4, paste, sep="")
> > > m
> >      [,1] [,2] [,3] [,4]
> > [1,] "a1" "a2" "a3" "a4"
> > [2,] "b1" "b2" "b3" "b4"
> > [3,] "c1" "c2" "c3" "c4"
> > [4,] "d1" "d2" "d3" "d4"
> > [5,] "e1" "e2" "e3" "e4"
> > > idx <- c(2, 1, 3, 4, 2)
> > > m[cbind(1:5, idx)]
> > [1] "a2" "b1" "c3" "d4" "e2"
> > 
> > Andy
> > 
> > > From: Wolfram Fischer
> > > 
> > > I have e.g.
> > >     t <- matrix( nrow=2, ncol=3, byrow=TRUE, 
> > > c('a1','a2','a3','b1','b2','b3') )
> > > and
> > >     i <- c( 3, 2)
> > > 
> > > Is it possible to formulate a simple expression that gets
> > >     c( t[ 1, i[1] ], t[ 2, i[2] ] )
> > > (and so on for longer matrices)?
> > > 
> > > The result would be:
> > >     [1] "a3" "b2"
> > > 
> > > Thanks - Wolfram
> > > 
> > > ______________________________________________
> > > R-help at stat.math.ethz.ch mailing list
> > > https://www.stat.math.ethz.ch/mailman/listinfo/r-help
> > > PLEASE do read the posting guide! 
> > > http://www.R-project.org/posting-guide.html
> > > 
> > > 
> > 
> > 
> > 
> --------------------------------------------------------------
> ----------------
> > Notice:  This e-mail message, together with any 
> attachments, contains information of Merck & Co., Inc. (One 
> Merck Drive, Whitehouse Station, New Jersey, USA 08889), 
> and/or its affiliates (which may be known outside the United 
> States as Merck Frosst, Merck Sharp & Dohme or MSD and in 
> Japan, as Banyu) that may be confidential, proprietary 
> copyrighted and/or legally privileged. It is intended solely 
> for the use of the individual or entity named on this 
> message.  If you are not the intended recipient, and have 
> received this message in error, please notify us immediately 
> by reply e-mail and then delete it from your system.
> > 
> --------------------------------------------------------------
> ----------------
> 
>