[R] simple loop questions

[David Winsemius]

On Apr 23, 2011, at 6:31 AM, <xueke at pdx.edu> wrote:

> Hi all,
>
> I am trying to write a loop to return the matched index for a column  
> of values. Here is an simple example of this,
>
> A   B
> 0   5
> 1   2
> 2   2
> 3   4
> 4   1
> 5   4
> 6   2
>
> In this case, A is an index column for B. Now I have this new column  
> C with just two values of 2 and 4. I want to match column C with  
> column B, and return the matched indexes. So what I desire is to  
> return:
>
> [1] 1 2 6
> [2] 3 5
>
> Since value 2 corresponds to indexes 1,2,6, and 4 corresponds to  
> indexes 3,5.

Assuming this to be in a data.frame named dat:

 > dat$A[which(dat$B==2) ]
[1] 1 2 6
 > dat$A[which(dat$B==4) ]
[1] 3 5

Results of varying lengths generally need to be returned in list form:

 > sapply(c(2,4), function(x) dat$A[which(dat$B==x) ] )
[[1]]
[1] 1 2 6

[[2]]
[1] 3 5

>
> Is there any way to write a loop to have this done? Thank you for  
> the help.
>
> Xueke
>
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David Winsemius, MD
West Hartford, CT