Add X days to each subsequent each row, by group

[pamplemouse22]

Hi everyone, 

I am stuck on a data management task and thought someone here may be able to help me. 

I am working with a dataset where each individual has ~30 rows of repeated data. The variables with information that differs are dates and a variable that indicates the first for each group. I'd like to add 7 number of days to every row, by group. I have a start date for the first row for each individual and need to add 7 days to each row until the next individual is reached (e.g., row 31) where the same process is repeated. Any suggestions? Here is what I have and what i want the data to look like. Thank you!! 

data have; 
	input id date first;
	datalines;
	1 03OCT2010 1
	1         . 0
	1         . 0
	1         . 0
	2 01JAN2011 1
	2         . 0
	2         . 0
	;
run; 

data want; 
	input id date first;
	datalines; 
	1 03OCT2010 1
	1 10OCT2010 0
	1 17OCT2010 0
	1 24OCT2010 0
	2 01JAN2011 1
 	2 08JAN2011 0
	2 15JAN2011 0
	;
run; 

[mkeintz · Accepted solution]

As long as FLAG=1 for the first record of each id, this will work:

data want;
  set have;
  retain tmpdate 0;
  tmpdate=ifn(flag=1,date,tmpdate+7);
  if flag^=1 then date=tmpdate;
  drop tmpdate;
run;

[pamplemouse22]

This works beautifully! Thank you. 

I'm trying to understand how this works. 

What does the carrot in following line do?

flag^=1

[ballardw]

---

@pamplemouse22 wrote:

This works beautifully! Thank you. 

I'm trying to understand how this works. 

What does the carrot in following line do?

flag^=1

---

^=   is "not equal" . There are some other ways depending on different keyboards/operating systems that are also acceptable.

so

Flag not equal to 1

[pamplemouse22]

oh, hah! that makes sense. thank you!