[R] matching matrix columns to a vector

[jim holtman]

You can use the arr.ind option of which:

> t=matrix(1:12,3,4)
> v=c(1,2,3)
> t
     [,1] [,2] [,3] [,4]
[1,]    1    4    7   10
[2,]    2    5    8   11
[3,]    3    6    9   12
> result <- sapply(v, function(x) which(t == x, arr.ind=TRUE))
> colnames(result) <- v
> result
     1 2 3     # 'v'
[1,] 1 2 3    # row
[2,] 1 1 1    # column
>


On Mon, Nov 24, 2008 at 11:03 AM, Salas, Andria Kay <aks2515 at uncw.edu> 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!!
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-- 
Jim Holtman
Cincinnati, OH
+1 513 646 9390

What is the problem that you are trying to solve?