SAS中不后缀"then..."的IF指令，意义和产出？

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SAS中不后缀"then..."的IF指令，其具体意义和产出是啥呢？什么情况下要这么写呢？请大神指点！谢谢~
以下是指令代码：

以下是该指令的输出，似乎if last.dept; 这句没有发挥特殊作用，那么如果不写这句呢？输出结果会有不同吗？请指点：

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关键词：统计 工具 求助

if last.byvar, 只保留byvar（你的code中为dept）的最后一个观测：即只保留根据dept sort后的最后一个观测。参考如下code

1. proc sort data=Sashelp.Heart(keep=Smoking_Status Weight)
   
2.           out=Heart;
   
3.    by Smoking_Status;
   
4. run;
   
5. 
   
6. data Count;
   
7.    set Heart;                 /* data are sorted by Smoking_Status */
   
8.    BY Smoking_Status;         /* automatically creates indicator vars */
   
9.    if FIRST.Smoking_Status then
   
10.       Count = 0;              /* initialize Count at beginning of each BY group */
    
11.    Count + 1;                 /* increment Count for each record */
    
12.    if LAST.Smoking_Status;    /* output only the last record of each BY group */
    
13. run;
    
14. 
    
15. proc print data=Count noobs;
    
16.    format Count comma10.;
    
17.    var Smoking_Status Count;
    
18. run;

复制代码

The subsetting IF statementcauses the DATA step to continue processing only those raw data recordsor those observations from a SAS data set that meet the conditionof the expression that is specified in the IF statement. That is,if the expression is true for the observation or record (its valueis neither 0 nor missing), SAS continues to execute statements inthe DATA step and includes the current observation in the data set.The resulting SAS data set or data sets contain a subset of the originalexternal file or SAS data set.

If the expression isfalse (its value is 0 or missing), no further statements are processedfor that observation or record, the current observation is not writtento the data set, and the remaining program statements in the DATAstep are not executed. SAS immediately returns to the beginning ofthe DATA step because the subsetting IF statement does not requireadditional statements to stop processing observations.