sas issue with retain to replace missing data - sas

Question

The following inherited simplified code is meant to replace missing values of a column with the values of not missing entries in a group:
DATA WORK.TOYDATA;
INPUT Category $ PRICE;
DATALINES;
Cat1 2
Cat1 .
Cat1 .
Cat2 .
Cat2 3
Cat2 .
;
DATA WORK.OUTTOYDATA;
SET WORK.TOYDATA;
BY Category ;
RETAIN _PRICE;
IF FIRST.Category THEN _PRICE=PRICE;
IF NOT MISSING(PRICE) THEN _PRICE=PRICE;
ELSE PRICE=_PRICE;
DROP _PRICE;
RUN;
Unfortunately, this will not work if the first entry in a group is missing. How could this be fixed?

Answer 1

As SAS works row by row through the dataset there is no value to replace if the first value is missing.
You could sort the data by Category and Price DESCENDING to circumvent this.
proc sort data= WORK.TOYDATA; by Category DESCENDING PRICE; run;
Or if there is only one NON-missing value by category you could use a sql join e.g.
proc sql;
create table WORK.OUTTOYDATA as
select a.Category, coalesce(a.PRICE, b.PRICE) as PRICE
from WORK.TOYDATA a
left join (select distinct Category, PRICE
from WORK.TOYDATA
where PRICE ne .
) b
on a.Category eq b.Category
;
quit;

Answer 2

As #Jetzler pointed out, the easiest way is just to sort the data. However, if you have multiple columns with missing values then you'd need to do multiple sorts, which isn't efficient.
Another option from doing a join is proc stdize which can be used to replace missing values with a simple measure (mean, median, sum etc). The default method will suffice in your example, you just need to add the reponly option which only replaces missing values and does not standardize the data.
DATA WORK.TOYDATA;
INPUT Category $ PRICE;
DATALINES;
Cat1 2
Cat1 .
Cat1 .
Cat2 .
Cat2 3
Cat2 .
;
run;
proc stdize data=TOYDATA out=want reponly;
by category;
var price;
run;