How to Number Category Variables by Category

[HannaZhang]

hi，all

I have a category variable "class" ,I want to create a new variable "NO" which number ariable "class" ,the result as follows：

Thanks a lot！

[PeterClemmensen]

data have;
input Class Grades;
datalines;
1 367
1 363
1 302
2 349
2 332
2 311
2 311
3 357
3 343
;

data want;
    set have;
    by class;
    if first.Class then No=0;
    No+1;
run;

[HannaZhang]

Actually there are some missing with the variable "class" ,so I tried like follows,but the log

notes that variable first.class uninitialized.Any  solution? thanks

data want;
    set have;
    by class;
    if  Class^=. and  first.Class then No=0;
    No+1;
run;

[PeterClemmensen · Accepted solution]

This would only appear in the log if you did not have your By Statement, which initializes the first. and last. variables. 

[HannaZhang]

THANKS!