Dynamically create the "KEEP" List in SAS - sas

Question

I am trying to create a keep list dynamically. Say:
%MACRO TEST(A=,B=,OUT_VAR=,KEEP_VAR=);
&OUT_VAR=MAX(&A,&B);
%IF &KEEP_VAR = 'Y' %THEN VAR_LIST=%SYSFUNC(CATS(VAR_LIST,&OUT_VAR));
%PUT VAR_LIST;
%MEND;
DATA ABC (keep = VAR_LIST);
LENGTH VAR_LIST $100.;
RETAIN VAR_LIST '';
%TEST(A=1,B=3,OUT_VAR=FIRS,KEEP_VAR='Y');
%TEST(A=2,B=4,OUT_VAR=SEC,KEEP_VAR='Y');
%TEST(A=3,B=5,OUT_VAR=THIR,KEEP_VAR='N');
RUN;
I have a datastep in which i am creating several variables calculated by the macro code.
I want to create a dynamic list of these output variables and then use it in the keep statement.
The above code does not seem to work, can someone suggest what am I missing here.

Answer 1

A running data step is source code that has already been compiled. You are trying to change the source while it is running -- that is not going to happen. This is a symptom of mixing scopes and contexts between macro code and generated code.
However, the TEST macro is generating source and thus can either
maintain state information for generating a KEEP source code statement after the macros are invoked.
generate an additional KEEP statement per TEST invocation
In general, macro only parameter values are already 'strings', and do not need to be quoted.
%MACRO TEST(A=,B=,OUT_VAR=,KEEP_VAR=);
&OUT_VAR=MAX(&A,&B);/* generates data step source code*/
%IF &KEEP_VAR = 'Y' %THEN
%LET VAR_LIST=&VARLIST &OUT_VAR;
%MEND;
* must occur before sequence of TEST invocations;
%global VARLIST; * globals generally want to be avoided, next version avoids it;
%let VARLIST =;
DATA ABC;
%TEST(A=1,B=3,OUT_VAR=FIRS,KEEP_VAR='Y');
%TEST(A=2,B=4,OUT_VAR=SEC,KEEP_VAR='Y');
%TEST(A=3,B=5,OUT_VAR=THIR,KEEP_VAR='N');
KEEP &VARLIST;
RUN;
%symdel VARLIST;
I hope this is a learning exercise -- The above pattern of coding has a macro generate code that would suffice as two normal non-macro data step statements do. Too much macroification can be obfuscating.
Additionally, the output ABC might not be what you expect. There will be one row with A=3, B=5, FIRS=4, SEC=6 and no THIR=8. So you have also some mixed up row and column data concepts.
Here is a cleaner alternative in which TEST generates the KEEP
%MACRO TEST(A=,B=,OUT_VAR=,KEEP_VAR=);
&OUT_VAR=MAX(&A,&B);/* generates data step source code*/
%IF %upcase(&KEEP_VAR) = Y %THEN %STR(KEEP &OUT_VAR;);
%MEND;
DATA ABC;
%TEST(A=1,B=3,OUT_VAR=FIRS,KEEP_VAR=Y);
%TEST(A=2,B=4,OUT_VAR=SEC,KEEP_VAR=Y);
%TEST(A=3,B=5,OUT_VAR=THIR,KEEP_VAR=N);
RUN;
The macro code still has to rely on the caller to specify a proper KEEP_VAR parameter Y or not Y.
An alternative is to push the Y/N KEEP concept into two parameters, use of one parameter over the other discriminates the KEEP.
%MACRO TEST(A=,B=,KEEP_VAR=,TEMP_VAR=);
%IF %length (&KEEP_VAR) %then %do;
&KEEP_VAR
%end;
%else %do;
&TEMP_VAR
%end;
MAX(&A,&B);
%IF %length (&KEEP_VAR) %then %do;
KEEP &KEEP_VAR;
%end;
%MEND;
DATA ABC;
%TEST(A=1,B=3,KEEP_VAR=FIRS);
%TEST(A=2,B=4,KEEP_VAR=SEC);
%TEST(A=3,B=5,TEMP_VAR=THIR);
RUN;
For me, a widely used macro should have clarity of invocation. The black box of the macro implementation can strive to be as clean and efficient but does not have to be.

Answer 2

It is not clear what you are trying to do, but you can use multiple KEEP statements if want.
data want ;
x=1;
keep x;
y=2;
z=3;
keep z;
run;
Although for your application it looks like you might want to generate a DROP statement instead.
%MACRO TEST(A=,B=,OUT_VAR=,KEEP_VAR=Y);
&OUT_VAR=MAX(&A,&B);
%IF (&KEEP_VAR ^= Y) %THEN %DO;
drop &out_var ;
%end;
%MEND;
DATA ABC ;
%TEST(A=1,B=3,OUT_VAR=FIRST)
%TEST(A=2,B=4,OUT_VAR=SECOND)
%TEST(A=3,B=5,OUT_VAR=THIRD,KEEP_VAR=N)
RUN;