不知道通过审核了没有，先自己顶下……

there exists 2 data loops,  for the data set having less obs contains only 2 obs.
In loop 1, the system read in x=1 and x=3 sequentially;
In loop 2, the system read in x=2 and x=4 sequentially.

Actually there are 3 data iterations:

_N_=1:  read the first record from short(x=1), then output;
             read the first record from long(x=3), then output;

_N_=2:  read the second record from short(x=2), then output;
             read the second record from long(x=4), then output;

_N_=3: try to read the next record from short but none available, the DATA Step stops.

嗯嗯，学习了

多谢楼上两位大神解答