@supp:
You've got a cornucopia of fine solutions, so methought another one, based on the hash object's prowess, wouldn't hurt at least from the standpoint of learning how it can be done. Note that like MEANS/SUMMARY it doesn't need the input file to be sorted, so I use it unsorted and have also added another satellite variable, VAR, to see if it gets transplanted from the input to the output correctly (it does). Despite the unsorted input, the output comes out sorted (again sharing the trait with MEANS/SUMMARY).
data have ;
input personID $ attribute1 $ qty date yymmdd10. var ;
format date yymmdd10. ;
cards ;
A m3 20 2019-02-01 1
B m3 10 2019-02-15 2
A m1 20 2019-03-01 3
B m3 20 2019-02-15 4
A m1 20 2019-02-01 5
C m2 20 2019-01-15 6
D m1 20 2019-01-15 7
A m1 10 2019-03-01 8
D m1 20 2019-02-15 9
A m2 20 2019-01-01 10
;
run;
data _null_ ;
dcl hash h (dataset:"have(obs=0)", ordered:"a") ;
h.definekey ("personID", "attribute1") ;
h.definedata (all:"y") ;
h.definedone () ;
do until (z) ;
set have (rename = (qty=_q date=_d var=_v)) end = z ;
if not (h.find() or (_d < date or _d = date and _q > qty)) then continue ;
date = _d ;
qty = _q ;
var = _v ;
h.replace() ;
end ;
h.output (dataset:"want") ;
format date yymmdd10. ;
run ;
Output:
person
ID attribute1 qty date var
------------------------------------------------
A m1 20 2019-02-01 5
A m2 20 2019-01-01 10
A m3 20 2019-02-01 1
B m3 20 2019-02-15 4
C m2 20 2019-01-15 6
D m1 20 2019-01-15 7
Frankly, I prefer the brevity of @FreelanceReinh's MEANS/SUMMARY offering, except for one subtlety: With the hash, bringing another satellite variable like VAR above is a simple business and it works properly. I've tried to do the same by adding the ID statement in SUMMARY; but the values of VAR it generated came out incorrectly in the sense that not all of them aligned with the corresponding output (date,qty) pairs. Hope @FreelanceReinh will chime in on that.
Kind regards
Paul D.
