modify multiple observations in a by variable

Question

data a1

col1 col2 flag
a    2    .
b    3    .
a    4    .
c    1    .

For data a1, flag is always missing. I want to update multiple rows using a2.

data a2

col1 flag
a    1

Ideal output:

col1 col2 flag
a    2    1
b    3    .
a    4    1
c    1    .

But this doesn't update all the records in by statement.

data a1;
modify a1 a2;
by col1;
run;

Question edited

Actually a1 is a very large data set on server. Hence I prefer to modify it (if possible) instead of creating a new one. Otherwise I have to drop previous a1 first and copy a new a1 from local to server, which will take much more time.

Answer 1

If you want to do this with MODIFY, you have to loop over the modify dataset in some fashion or it will only replace the first row (because the other dataset will then run out of records - normally this behaves like merge, where once it finds a match it advances to next record). Here's one option - there are others.

data a1(index=(col1));
  input col1 $ col2 flag;
datalines;
a    2    .
b    3    .
a    4    .
c    1    .
;;;;
run;

data a2(index=(col1));
  col1='a'; 
  flag=1;
run;

data a1;
    set a2(rename=flag=flag2);
    do _n_ = 1 to nobs_a1;
      modify a1 key=col1 nobs=nobs_a1;
      if _iorc_=0 then do;
        flag=flag2;
        replace;
      end;
    end;
    if _iorc_=%sysrc(_DSENOM) then _error_=0;
run;