How to make a variable to be '1' if it is the first instance of a new variable in a by group?

[Tom_87]

See the title and the attached file. How do I write the SAS code to create variable 3? I am not very experienced with SAS, so any help would be greatly appreciated. Thank you!

Edit- Thank you Bruno, that worked perfectly!

[BrunoMueller · Accepted solution]

Hi

Assuming the input data is sorted by var_1, then the following code should do what you want:

data want;
  set have;
  by Var_1;
  if first.var_1 = 1 then do;
    retain checkForA;
    checkForA = 1;
  end;
  if checkForA = 1 and var_2 = "A" then do;
    var_3_sas = 1;
    checkForA = 0;
  end;
  var_3_sas = coalesce(var_3_sas, 0);
run;