[R] ifelse to speed up loop?

[Jeff]

Thank you for the tips. I should mention that the V1 is numeric in this case but I am trying to automate the overall procedure so it may not be numeric always. Would that matter with these functions?

Sent from my iPhone

On Jan 24, 2013, at 6:34 PM, arun <smartpink111 at yahoo.com> wrote:

> Hi,
> You could do this:
> test<-read.table(text="
>   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
> ",sep="",header=T)
> 
>  test$Group<-cumsum(abs(c(0,diff(test[,1]))))+1
> test
> #   V1 Group
> #1   1     1
> #2   1     1
> #3   1     1
> #4   2     2
> #5   2     2
> #6   2     2
> #7   1     3
> #   1     3
> #9   1     3
> #10  2     4
> #11  2     4
> #12  2     4
> A.K.
> 
> 
> 
> ----- Original Message -----
> From: Jeffrey Fuerte <fuertejn at vcu.edu>
> To: r-help at r-project.org
> Cc: 
> Sent: Thursday, January 24, 2013 4:20 PM
> Subject: [R] ifelse to speed up loop?
> 
> Hello,
> 
> I'm not sure how to explain what I'm looking for but essentially I have a
> test dataset that looks like this:
> 
> test:
>    V1
> 1   1
> 2   1
> 3   1
> 4   2
> 5   2
> 6   2
> 7   1
> 8   1
> 9   1
> 10  2
> 11  2
> 12  2
> 
> And what I want to be able to do is create another column that captures a
> "grouping" variable that looks like this:
> 
> test
>    V1 group
> 1   1  1
> 2   1  1
> 3   1  1
> 4   2  2
> 5   2  2
> 6   2  2
> 7   1 3
> 8   1  3
> 9   1  3
> 10  2  4
> 11  2  4
> 12  2  4
> 
> So, it's keeping track of the changes in V1, but even though V1 could be
> the same in different instances, the group is treating it as a new group.
> I have written a loop that does what I want, but this takes too long to
> run, so I was hoping for either a faster approach or an ifelse statement.
> Any ideas?
> 
> By the loop I was using looks like this:
> 
> groupings <- 1
> test$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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