[R] Getting elements of a matrix by a vector of column indice s
Liaw, Andy
andy_liaw at merck.com
Thu Jul 8 16:44:56 CEST 2004
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
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> > >
> > >
> >
> >
> >
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