Calculating Inter-purchase Time in R

Question

I have the following data frame:

id<-c(1,1,1,3,3,3)
date<-c(\"23-01-07\",\"27-01-07\",\"30-01-07\",\"11-12-07\",\"12-12-07\",\"01-01-08\")
df<-data.         


        

Answer 1

You can use plyr:

library(plyr)
ddply(df, "id", transform, inter.time = c(0, diff(date2)))

or ave:

transform(df, inter.time = ave(as.numeric(date2), id,
                               FUN = function(x)c(0, diff(x))))

Both give

#   id     date      date2 inter.time
# 1  1 23-01-07 2007-01-23          0
# 2  1 27-01-07 2007-01-27          4
# 3  1 30-01-07 2007-01-30          3
# 4  3 11-12-07 2007-12-11          0
# 5  3 12-12-07 2007-12-12          1
# 6  3 01-01-08 2008-01-01         20

P.S.: you might want to replace these zeroes with NA.