[R] Create a time interval from a single time variable

[Terry Therneau]

-- begin included message --
I am trying to set up a data set for a survival analysis with time-varying  
covariates. The data is already in a long format, but does not have a variable 
to signify the stopping point for the interval. The variable DaysEnrolled is the 
variable I would like to use to form this interval. This is what I have now:

...
---- end inclusion

 I would have expected a dozen solutions from the list - data manipulation 
problems usually get a large following.  It can be done in 4 lines, assuming 
that the parent data set is sorted by subject and time within subject.
 
newdata$start <- olddata$DaysEnrolled   #start time = the current variable
temp <- olddata$DaysEnrolled[-1]        # shift column up by one position
temp[diff(olddata$id) !=0] <- NA        # NA for last line of each subject
newdata$stop <- c(temp, NA)             # add the NA for the last subject

   I will leave it to others to compress this into a 1-line application of one 
of the apply functions.  (Unreadable perhaps, but definitely more elegant :-)
   
   	Terry T.