[R] within group sequential subtraction

[natalie.vanzuydam]

Hi Everyone,

I would like to do sequential subtractions within a group so that I know the
time between separate observations for a group of individuals.  

My data:

data <- structure(list(group = c("IND1", "IND1", "IND2", 
"IND2", "IND2", "IND3", "IND4", "IND5", 
"IND6", "IND6"), date_obs = structure(c(6468, 
7063, 9981, 14186, 14372, 5129, 9767, 11168, 10243, 10647), class =
"Date")), .Names = c("group", 
"date_obs"), row.names = c(NA, 10L), class = "data.frame")

So I start with:

 group   date_obs
1   IND1 1987-09-17
2   IND1 1989-05-04
3   IND2 1997-04-30
4   IND2 2008-11-03
5   IND2 2009-05-08
6   IND3 1984-01-17
7   IND4 1996-09-28
8   IND5 2000-07-30
9   IND6 1998-01-17
10  IND6 1999-02-25

what I would like:

 group   date_obs     time
1   IND1 1987-09-17 NA  
2   IND1 1989-05-04 595
3   IND2 1997-04-30 NA
4   IND2 2008-11-03 4205
5   IND2 2009-05-08 186
6   IND3 1984-01-17 NA
7   IND4 1996-09-28 NA
8   IND5 2000-07-30 NA
9   IND6 1998-01-17 NA
10  IND6 1999-02-25 404

So that if there is one entry/individual a 0/NA would be acceptable and if
there is more than one entry/individual the sequential difference would be
calculated.

I started with some code but it I cannot edit it appropriately.

x <- do.call(rbind, lapply(split(data, data$group), 
        function(dat) { 
			dat <- dat[order(dat$date_obs), ] 
			d<-diff(dat$date_obs)
			 dat <- rbind(dat,d)
			}))

I get this error: "Error in as.Date.numeric(value) : 'origin' must be
supplied" so I'm not sure if it does what I need it to do.  In addition to
this the vector lengths won't match up as the first date in the sequence
won't be subtracted from itself.

I'm not sure if anyone knows an easier way to achieve this.  

Thanks for the help,
Natalie
			



-----
Natalie Van Zuydam

PhD Student
University of Dundee
nvanzuydam at dundee.ac.uk
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