First variable when use two varaibles

[Ronein]

Hello

I don't understand how first variable's values are determined when there are two first variables.

For example:

When I use two first statements then I don't understand the result o the second first statement.

Related to  first.group2 I was sure that it looks only on values of group2 and give value 1 to first value.

Values in group2 are:              one,one,one,one.one.one.one,two,two,two,two

so I was sure that I will get: 1,0,0,0,0,0,0,1,0,0,0

But I see that I got values:  1,0,0,0,1,0,0,1,0,0,0

Can anyone explain please?

Thank you so much

Data raw_tbl;
input ID group1 $  group2 $ Y;
cards;
1 a One 5
2 a One 10
3 a One 15
4 a One 20
5 b One 25
6 b One 30
7 b One 35
8 c Two 40
9 c Two 45
10 c Two .
11 c Two 50
;
run;

 data zz;
set raw_tbl;
by group1 group2;
first_group1=first.group1;
first_group2=first.group2;
Run;
 

data ww;
set raw_tbl;
by group2;
first_group2=first.group2;
Run;

[PeterClemmensen]

This is because in zz, you have two variables in your By Statement. Group1 is considered first. Group2 is considered within Group1. This means that Group2 in obs5 is the first value in the Group1 group of 'b'.

[Kurt_Bremser]

When you use more than one variable in a by statement, you build a hierarchy from left to right. A change in a variable further up (left) forces a change in all "lower" variables (to the right).

by a b c;

As soon as first.a is 1, first.b and first.c will also be set 1. If first.a is 0, but first.b is 1, then first.c will also be 1, even if lag(c) = c.