Calculate elapsed “times”, where the reference time depends on a factor

Question

I\'m trying to calculate elapsed times in a data frame, where the \'start\' value for the elapsed time depends on the value of a factor column in the data frame. (To simply the

Answer 1

Here is a ddply solution

ddply(df, .(id), summarize, time = time, elapsed = seq(length(id))-1)

and one using rle instead

df$elapsed <- unlist(sapply(rle(as.numeric(df$id))$lengths, seq))-1