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

The 'ave' function is the first thing you should think of when the results is to be a vector with the same length as the number of rows in the dataframe:

 df$elapsed <- ave(df$time, df$id, FUN=function(x) x -min(x) )
 df
  id time elapsed
1  a    1       0
2  a    2       1
3  a    3       2
4  b    4       0
5  b    5       1