zz: A question about how to use _N_

The original is from mysas(probably? Forgot it). Look at the answer and pay attention how to use _N_.
 

Question：

data a;

input id $ x;

cards;

a 0

a 1

a 2

a 1

b 3

c 0

c 3

d 2

d 2

e 1

;

run;

* using the data above, how to get a variable x_grp, which is made of distinct value of x in each id group. The data should be like:

id x x_grp

a 0 0_1_2

a 1 0_1_2

a 2 0_1_2

a 1 0_1_2

b 3 3

c 0 0_3

c 3 0_3

d 2 2

d 2 2

e 1 1

Answers：

Method 1：

data b;

    length x_grp $50;

    do _n_=1 by 1 until(last.id);

        set a;

        by id notsorted;

        if indexw(trim(x_grp),cats(x),'_')=0 then x_grp=catx('_',x_grp,x);

    end;

    do _n_=1 to _n_;

        set a;

        output;

    end;

run;

Method 2：

data a2;

set a;

retain grp;

by id notsorted;

if first.id then grp=x;

else if find(grp,compress(x))=0 then grp=catx('_',grp,x);

if last.id then output;

run;

data b;

merge a a2;

by id;

run;