3 days lag

[Emma8]

I have data with daily date data. I would like to identify 3 days interval (Day_3), see below:

Date  Day_3

01012020   1

01022020   1

01032020   1

01042020    2

01052020    2

01062020    2

etc.

Thank you.

[PeterClemmensen]

If your data is daily, then it is as simple as

data have;
input Date :mmddyy8.;
format date mmddyy10.;
datalines;
01012020
01022020
01032020
01042020
01052020
01062020
;

data want;
   set have;
   if mod(_N_, 3) = 1 then day_3 + 1;
run;

[Emma8]

Sorry, my actual date does not consequentially increase and also duplicate dates, so when skipped then still would like to count for the lag, for example, see below (between a start and end date--below start is 01012020 and end is 01182020):
datalines Day_3
01012020 1
01062020 2
01062020 2
01062020 2
01102020 4
01102020 4
01182020 6
;

[PeterClemmensen]

I don't follow this logic. Why is Day_3 equal o 4 and 6 in the last few obs?

[Kurt_Bremser]

Because it's in the 4th or 6th 3-day group from the beginning.

---

@PeterClemmensen wrote:

I don't follow this logic. Why is Day_3 equal o 4 and 6 in the last few obs?

---

[Emma8]

By the actual calendar date

[Emma8]

Although the actual date from 2 January 2020 to 6 January 2020 etc. skipped in the data, I want to keep track from the beginning date to the end date

[Kurt_Bremser · Accepted solution]

Expanding from my previous post:

data have;
input group $ date :mmddyy8.;
format date yymmdd10.;
datalines;
A 01012020
A 01062020
A 01062020
A 01062020
A 01102020
A 01102020
A 01182020
;

data want;
set have;
by group;
retain first;
if first.group
then first = date;
day_3 = int((date-first)/3)+1;
drop first;
run;

If you don't have groups, do the init at _N_ = 1 instead of first.group.

[Emma8]

It should count the actual calendar date not by _n_

[Kurt_Bremser]

---

@Emma8 wrote:
It should count the actual calendar date not by _n_

---

Sorry for that, edited my post.

Sometimes the behavior of SAS Studio and Safari with regards to copy/pasting drives me crazy.

[Kurt_Bremser]

And if you need to do this for some kind of groups, see here:

data want;
set have;
by group;
if first.group
then do;
  day_3 = 1;
  count = 0;
end;
else do;
  count + 1;
  if mod(count,3) = 0 then day_3 + 1;
end;
drop count;
run;