[R] ifelse to speed up loop?

[Bert Gunter]

Your query is a little unclear to me, but I suspect
?rle
is what you want.

-- Bert

On Thu, Jan 24, 2013 at 1:20 PM, Jeffrey Fuerte <fuertejn at vcu.edu> wrote:
> Hello,
>
> I'm not sure how to explain what I'm looking for but essentially I have a
> test dataset that looks like this:
>
>  test:
>    V1
> 1   1
> 2   1
> 3   1
> 4   2
> 5   2
> 6   2
> 7   1
> 8   1
> 9   1
> 10  2
> 11  2
> 12  2
>
> And what I want to be able to do is create another column that captures a
> "grouping" variable that looks like this:
>
>  test
>    V1 group
> 1   1  1
> 2   1  1
> 3   1  1
> 4   2  2
> 5   2  2
> 6   2  2
> 7   1 3
> 8   1  3
> 9   1  3
> 10  2  4
> 11  2  4
> 12  2  4
>
> So, it's keeping track of the changes in V1, but even though V1 could be
> the same in different instances, the group is treating it as a new group.
>  I have written a loop that does what I want, but this takes too long to
> run, so I was hoping for either a faster approach or an ifelse statement.
>  Any ideas?
>
> By the loop I was using looks like this:
>
> groupings <- 1
> test$group[1] <- groupings
> for (i in 2:length(test$V1))
> {
> if (test$V1[i]==test$V1[i-1])
> {
> test$group[i] <- groupings
> } else {
> groupings <- groupings+1
> test$group[i] <- groupings
> }
> }
>
> Thanks for the help.
>
>         [[alternative HTML version deleted]]
>
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-- 

Bert Gunter
Genentech Nonclinical Biostatistics

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