Alternative to LAG function - sas

Question

My dataset is structured like this:
ID, Order Date, Delivery Date, Flag
1, 10/03/12, 15/03/12,
1, 17/03/12, 20/03/12, 1
I want to be able to calculate the date difference between the first occurring delivery date and subsequent order dates. Eventual aim is group these records with an identifier.
Have tried the monotonic() with monotonic()+1 for a self join - but the problem with this is that each ID can have multiple different numbers of rows needing to be grouped together. Am using SAS Enterprise Guide 7 - unfortunately LAG is not available.
An example of what I'm looking to achieve is:
ID, Order Date, Deliv Date, Order Date_1, Deliv Date_1, DateDIFF(Deliv Date - Order Date_1)
1, 10/03/12, 15/03/12, 17/03/12, 20/03/12, 2
Any ideas?

Answer 1

You need to proc sort the data in descending order by Delivery_Date then retain the values of the previous record while grouping by ID in order to calculate the difference.
Code:
data have;
infile datalines dlm=',' dsd;
informat id 11. Order_Date ddmmyy8. Delivery_Date ddmmyy8. flag 11.;
format Order_Date ddmmyy8. Delivery_Date ddmmyy8.;
input ID Order_Date Delivery_Date Flag;
datalines;
1, 10/03/12, 15/03/12,.
1, 17/03/12, 20/03/12, 1
2, 10/03/12, 10/03/12,.
2, 17/03/12, 20/03/12, 1
2, 10/03/12, 27/03/12,1
2, 17/03/12, 23/03/12, 1
run;
proc sort data=have;
by id descending Delivery_Date ;
run;
data want;
set have;
by id;
retain nxt_date;
if first.id=1 then do;
nxt_date = Delivery_Date;
diff=0;
end;
else do;
prv_date=nxt_date;
diff=nxt_date-Delivery_Date;
nxt_date = Delivery_Date;
end;
format prv_date ddmmyy8.
drop nxt_date;
run;
Output:
id=1 Order_Date=17/03/12 Delivery_Date=20/03/12 flag=1 diff=0 prv_date=.
id=1 Order_Date=10/03/12 Delivery_Date=15/03/12 flag=. diff=5 prv_date=20/03/12
id=2 Order_Date=10/03/12 Delivery_Date=27/03/12 flag=1 diff=0 prv_date=.
id=2 Order_Date=17/03/12 Delivery_Date=23/03/12 flag=1 diff=4 prv_date=27/03/12
id=2 Order_Date=17/03/12 Delivery_Date=20/03/12 flag=1 diff=3 prv_date=23/03/12
id=2 Order_Date=10/03/12 Delivery_Date=10/03/12 flag=. diff=10 prv_date=20/03/12