[R] help with loop

[Wacek Kusnierczyk]

is your data a data frame or a matrix?  do you want to compute the
differences columnwise, i.e., for each column independently?  consider
this example:

    # generate and display dummy data
    (d = as.data.frame(replicate(3, sample(5))))

    # compute successive differences columnwise
    as.data.frame(apply(d, 2, diff))
    apply(as.matrix(d), 2, diff)

see ?apply and ?diff for details.  note that apply will return a matrix
both when given a data frame and when given a matrix.

vQ


Rafael Moral 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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