lag Addition

[abdulla]

code	date	Rtr	exp	new
20	2011	0.115225	0.114746
20	2012	0.101424	0.11639	0.231615
20	2013	0.114746	0.055248	0.156672
23	2011	0.11639	0.094931
23	2012	0.055248	0.173508	0.289898
23	2013	0.094931	0.190566	0.245813
55	2011	0.029143	0.073573
55	2012	0.060995	0.085915	0.115057
55	2013	0.069918	0.075433	0.136429

I want to add rtr in 2011 with exp in 2012. So my result will be equal to as shown in New column. It is adding the lag value of rtr with the current value of exp. The result should be grouped by code. Please help. 

[PeterClemmensen · Accepted solution]

You can do like this

data have;
input code date Rtr exp;
datalines;
20 2011 0.115225 0.114746
20 2012 0.101424 0.11639
20 2013 0.114746 0.055248
23 2011 0.11639 0.094931
23 2012 0.055248 0.173508
23 2013 0.094931 0.190566
55 2011 0.029143 0.073573
55 2012 0.060995 0.085915
55 2013 0.069918 0.075433
;

data want;
   set have;
   by code;
   lag_Rtr=lag1(Rtr);
   new=sum(lag_Rtr, exp);
   if first.code then new=.;
run;