subtracting rows for unique

Hi,
Try:
dat1<- read.table(text="
ID Date x2 x1????????? x3 
56 25-Jun-01 10 2? 126 
56 29-Oct-01 10 2? 140 
56 18-Mar-02 10 2? 445 
56 6-Jun-03 10 2??? 224 
56 16-Jan-04? 10 2??? NA 
58 10-Jan-02 10.8 1 715 
58 26-Dec-03 10.8 1??? NA 
",sep="",header=TRUE,stringsAsFactors=FALSE) 
?unlist(with(dat1,by(as.Date(Date,format="%d-%b-%y"),ID,FUN=function(x) c(diff(x),NA))),use.names=FALSE)
#[1] 126 140 445 224? NA 715? NA
#or
unlist(lapply(split(dat1,dat1$ID),function(x) c(diff(as.Date(x$Date,format="%d-%b-%y")),NA)),use.names=FALSE)
#[1] 126 140 445 224? NA 715? NA
A.K.

Hi there,

Hope you are doing well.
I have a data set which looks like below, I want to create a new variable (x3-here) which subtracts the date for unique ID.
For example for ID 56, the first value is the difference between ?29-Oct, and 25-June.
Thanks for your help and time.




ID Date x2 x1???? ? ? ?x3 
56 25-Jun-01 10 2 126 
56 29-Oct-01 10 2 140 
56 18-Mar-02 10 2 445 
56 6-Jun-03 10 2 224 
56 16-Jan-04??NA ? ? ? ? ? ? NA 
58 10-Jan-02 10.8 1 715 
58 26-Dec-03 10.8 1 ? ? ? ? ? ? NA 

Best,Farnoosh Sheikhi?