[R] Getting elements of a matrix by a vector of column indices
Uwe Ligges
ligges at statistik.uni-dortmund.de
Thu Jul 8 13:21:53 CEST 2004
Wolfram Fischer wrote:
> 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"
>
Two solutions are (there might be better ones):
a)
mapply(function(x,y) t[x,y], 1:nrow(t), i)
b)
t(t)[i + (1:nrow(t) - 1) * (ncol(t))]
Note that calling the matrix "t" is not the best idea.
Uwe Ligges
> Thanks - Wolfram
>
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