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

[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
> > 
> > 
> 
> 
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