[R] Getting elements of a matrix by a vector of column indice s
Wolfram Fischer
wolfram at fischer-zim.ch
Thu Jul 8 16:26:00 CEST 2004
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
> >
> > ______________________________________________
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> >
> >
>
>
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