# Re: [R] matching matrix columns to a vector

From: Marc Schwartz <marc_schwartz_at_comcast.net>
Date: Mon, 24 Nov 2008 12:04:29 -0600

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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