太感谢了，我去试试！nobs未知啊！

问题是数据到底有多少行是未知的啊，这时候nobs怎么办！还是不好使啊！

Use the point option directly access the last obs. The most efficient way. The stop is necessary here. Otherwise sas will run forever. Check the doc for more details.

the nobs option will provide the number of obs at compiling time.

proc print data=sashelp.class;
run;

data needed;
   set sashelp.class nobs=nobs point=nobs;
   output;
   stop;
run;

proc print data=needed;run;