[fu gang]

tab and collapse caneasily finish the task, sumat is a very good command, just like tabstat,When add save option, then a useful matrix can be generated and called.
thank you very much, I learned a very good command
I got a idea to reduce the double loops to one and then do the loop
generate a new variable c to to represent a and b

gen c = group(a b) 
or    xgroup a b,gen(c)

levelsof c,gen(i)

then a macro i can be used in loopse

[fu gang]

i am sorry, i wrote the wrong code

 egen c = group(a b)

  
 levelsof c,local(i)