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data a;
  input company date:yymmn6. sales;
  format date yymmn6.;
  datalines;
1001 200509 20
1001 200603 30
1016 200603 451
1016 200606 521
1016 200609 358
1016 200612 650  
;
run;
proc sort data=a;
by company date;
run;
data c;
set a;
by company date;
if _n_=1 then date2=date;
else date2=lag(date);
y=intck("month",date2,date );
run;

这里面为什么date2第二行是有缺失值的？请教，谢谢！

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关键词：SAS问题 company compan format yymmn 请教 SAS

努力回帖 勤工俭学

SAS HELP:
The queue for LAGn is initialized with n missing values, where n is the length of the queue (for example, a LAG2 queue is initialized with two missing values). When LAGn is executed, the value at the top of the queue is removed and returned, the remaining values are shifted upwards, and the new value of the argument is placed at the bottom of the queue. Hence, missing values are returned for the first n executions of LAGn, after which the lagged values of the argument begin to appear.
你的代码当_N_＝2首次执行，故此时return的为缺失值。一般调用lagn队列都是在初始就赋给一个变量，再进行操作。

一般不在条件语句中使用lag函数

先赋值，再使用，多注意点，就可以了......

data a;
  input company date:yymmn6. sales;
  format date yymmn6.;
  datalines;
1001 200509 20
1001 200603 30
1016 200603 451
1016 200606 521
1016 200609 358
1016 200612 650  
;
run;
proc sort data=a;
by company date;
run;
data c;
set a;
by company date;
retain date2;
date2=lag(date);
if _n_=1 then date2=date;
y=intck("month",date2,date );
run;

楼问的问题处在_N_=1这个里面
因为ELSE之后，就是从_N_=2开始了，那么你第二行肯定就是一个缺失值了

学习学习

It is very good to provide detailed code for beginner like me. Thanks.

crackman 发表于 2010-9-2 10:43
data a;
  input company date:yymmn6. sales;
  format date yymmn6.;
  datalines;
1001 200509 20
1001 200603 30
1016 200603 451
1016 200606 521
1016 200609 358
1016 200612 650  
;
run;
proc sort data=a;
by company date;
run;
data c;
set a;
by company date;
retain date2;
date2=lag(date);
if _n_=1 then date2=date;
y=intck("month",date2,date );
run;