Dear Rafael,
Perhaps:

sum(diff(x))

where x is your vector. To apply above to your data set (by rows), you could
use

apply(mydata,1,function(x) sum(diff(x)))

See ?diff, ?sum and ?apply for more information.

HTH,

Jorge


On Thu, Mar 12, 2009 at 9:04 AM, Rafael Moral
<rafa_moral2004@yahoo.com.br>wrote:

> Dear useRs,
> I'm trying to write a loop to sum my data in the following way:
> (the second - the first) + (the third - the second) + (the fourth - the
> third) + ...
> for each column.
>
> So, I wrote something like this:
>
>   c <- list()
>   for(i in 1:ncol(mydata)) {
>   for(j in 2:nrow(mydata)) {
>   c[[i]] <- sum(yc[j,i] - yc[(j-1),i])
>   }}}
>
> As for the columns it works pretty fine, but it only returns the last
> subtraction, however, I need the sum of all subtractions.
>
> Any ideas?
>
> Regards,
> Rafael.
>
>
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>
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>
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