即一个变量var从第n行开始出现值，那么怎么变成新变量New_var 即第n行之前的记录仍为空值，第n后出现的空值变为字符串"缺失"。
- data test;
- infile cards missover;
- input
- id $
- var $;
- cards;
- 1
- 1 A
- 1
- 1 B
- ;
- run;
- data want;
- set test;
- retain temp;
- if not missing(var) then temp=1;
- new_var=ifc(temp=1 and missing(var),'缺失',var);
- drop temp;
- run;
