如何保留每个id前两行observation?

shiyiming

比如：有数据集a:
id price amount
1    5     3
1    6     4
1    7     8
1   10     11
2    3     5
2    4     7
2    5     6
2    7     3
如何得到
id price amount
1   5     3
1   6     4
2   3     5
2   4     7
就是说，要保留每个id前两行的observation该怎么做？谢谢！！！！

shiyiming

/* Assume the dataset being sorted by id. */
[code:24794]data a(drop=flag); set a; by id;
if first.id then flag=0;
flag+1;
if flag <=2 then output;
run;
[/code:24794]

shiyiming

数据集c就是要的结果。[code:fb1ef]data a;
input id price amount;
cards;
1 5 3
1 6 4
1 7 8
1 10 11
2 3 5
2 4 7
2 5 6
2 7 3
;
data b;
set a;
by id;
if first.id then i=1;
else i+1;
data c(drop=i);
set b;
if i<=2;
run;[/code:fb1ef]

shiyiming

谢谢两位！