topic Re: Data step with Lag function in SAS Programming

Data step with Lag function

Aman4SAS — Tue, 25 May 2021 15:49:05 GMT

Hi All,
Data x;
Input A B$ ;
Datalines;
101 a
101 b
101 b
101 c
101 d
102 a
102 a
102 a
102 b
102 b
102 c
103 a
103 b
103 c
103 d
103 d
103 e
;
Run;
Required data like:
101 a 1
101 b 2
101 b 2
101 c 3
101 d 4
102 a 1
102 a 1
102 a 1
102 b 2
102 b 2
102 c 3
103 a 1
103 b 2
103 c 3
103 d 4
103 d 4
103 e 5
My code is:
Data new;
Set x;
If first.a then i=1;
Else if b ne lag(b) then i=i+1;
Retain i;
Run;

I am facing problem in 102 2nd row, in 2nd row i is being 2 which should not as b = lag(b)

Please help me
Thanks

Re: Data step with Lag function

Reeza — Tue, 25 May 2021 16:01:45 GMT

You shouldn't use LAG() for this at all, this is better done with BY group processing.
https://stats.idre.ucla.edu/sas/faq/how-can-i-create-an-enumeration-variable-by-groups/

proc sort data=x;
by a b;
run;

data new;
set x;
by A B;
retain i;
if first.a then i=0;
if first.b then i+1;
run;

@Aman4SAS wrote:
Hi All,
Data x;
Input A B$ ;
Datalines;
101 a
101 b
101 b
101 c
101 d
102 a
102 a
102 a
102 b
102 b
102 c
103 a
103 b
103 c
103 d
103 d
103 e
;
Run;
Required data like:
101 a 1
101 b 2
101 b 2
101 c 3
101 d 4
102 a 1
102 a 1
102 a 1
102 b 2
102 b 2
102 c 3
103 a 1
103 b 2
103 c 3
103 d 4
103 d 4
103 e 5
My code is:
Data new;
Set x;
If first.a then i=1;
Else if b ne lag(b) then i=i+1;
Retain i;
Run;

I am facing problem in 102 2nd row, in 2nd row i is being 2 which should not as b = lag(b)

Please help me
Thanks

Re: Data step with Lag function

FreelanceReinh — Tue, 25 May 2021 16:05:52 GMT

Hi @Aman4SAS,

If you really want to use the LAG function, make sure that it is executed unconditionally:

data new;
set x;
by a;
if b ne lag(b) then i+1;
if first.a then i=1;
run;

Re: Data step with Lag function

Aman4SAS — Tue, 25 May 2021 16:27:45 GMT

Thanks a lot. If there anyway where i can find my mistake n learn

Re: Data step with Lag function

Aman4SAS — Tue, 25 May 2021 16:29:02 GMT

Thanks a lot. Is there anyway where i can find my mistake n learn the problem

Re: Data step with Lag function

Reeza — Tue, 25 May 2021 16:31:12 GMT

Googling "issue with lag" would likely have brought up many posts that illustrate this issue. It's a common misunderstanding with the lag function that once you understand how it works you're fine. Don't forget it now 🙂

Re: Data step with Lag function

FreelanceReinh — Tue, 25 May 2021 17:59:51 GMT

See also the description and examples in the documentation of the LAG function. Then be creative and write your own DATA steps using the LAG function (applied to small datasets like SASHELP.CLASS or datasets you create for this purpose), moving from very simple to more advanced little programs. Always examine the output dataset or printed output they may create (and of course the log) and check if the result matches your understanding. If it doesn't, adapt your understanding. Thus you will more and more develop the ability to predict the results of new examples (even tricky ones as in If statement Short circuiting and Lag function).