on 11/24/2008 10:03 AM Salas, Andria Kay wrote:
> I need help with (hopefully) just one more thing. I have been
> fussing with this for quite some time and have decided just to give
> up and ask! I want to match a column in a matrix to a vector. I
> found a "which" command that I thought would be helpful as it does
> the following:
>
>> g=c(1,5,3,2,7) which(g==5)
> [1] 2
>
> As the above gave which placement in the g vector corresponded to 5
> (the second place), I need this command to give me which column in a
> matrix matches to a vector.
>
> This is just a toy example of what I am trying to do:
>> t=matrix(1:12,3,4) v=c(1,2,3) which(t[,j]==v)
>
> This does not work, and with my "real" matrices and vectors, I was
> getting outputs that did not make sense. These examples are more to
> give an idea of what I am aiming to accomplish.
>
> Thank you for all the help!!
Presuming that you are comparing integers or characters:
mat <- matrix(1:12, 3, 4)
Vec <- 1:3
> mat
[,1] [,2] [,3] [,4]
[1,] 1 4 7 10
[2,] 2 5 8 11
[3,] 3 6 9 12
> Vec
[1] 1 2 3
> which(apply(mat, 2, identical, Vec))
[1] 1
Another possible option:
> which(apply(mat, 2, function(x) all(x == Vec)))
[1] 1
See ?apply, ?identical and ?all
Note that a variation on this would be required for floating point
numbers using all.equal().
You might also want to add error checking to be sure that:
length(Vec) == nrow(mat)
lest you risk re-cycling of values.
HTH,
Marc Schwartz
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