data raw;
input date yymmdd8. week;
format date yymmdd10.;
datalines;

20100628 1
20100629 1
20100630 1
20100701 1
20100702 1
20100705 2
20100706 2
20100707 2
20100708 2
20100709 2
20100712 3
20100713 3
20100714 3
;

data out;
_temp=0;
do _n_=1 by 1 until(last.week);
set raw ;
by week;
_temp+1;
end;

run;

请教各位大侠：其中do _n_=1 by 1 until(last.week);这句怎么理解，data步的运行机制是什么？谢谢！