sas的编译阶段

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32.Consider the following data step:

data WORK.NEW;

    set WORK.OLD;

    Count+1;

run;

The variable Count is created using a sum statement. Which statement regarding this variable is true?

     A. It is assigned a value 0 when the data step begins execution.

     B. It is assigned a value of missing when the data step begins execution.

     C. It is assigned a value 0 at compile time.

     D. It is assigned a value of missing at compile time.

Answer: C

答案说在编译阶段就给赋值为0了。可是编译阶段不是都没有值吗？

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关键词：sas的 Data step EXECUTION statement following execution following regarding assigned created

没人给说说吗？

First one needs to understand the retain statement. It is classified as non-executable statement. This is very important!

count +1;

is equivalent to

count=count+1;
retain count 0;

data t1;
   set sashelp.class;
   count=count+1;
   retain count 0;
run;

proc print;run;

If you assign a missing to count in retain statement, you will get all counts as missing.


The other way to understand it is to wrap them into a do block as below,

if 0 then do;
set sashelp.class;
   count+1;
end;

As we know that every statements will not get executed at running time because ' if 0  ' is never true. In other words these statements provide information at compiling time only.  The put statement let you see what is the value of count at compiling time .  


data _null_;
  if 0 then do;
   set sashelp.class;
   count+1;
  end;
  put count;
run;

bobguy 发表于 2012-5-26 22:40
First one needs to understand the retain statement. It is classified as non-executable statement. Th ...

Thank you very much.
就是英文能不能不这么给力啊！

bobguy 发表于 2012-5-26 22:40
First one needs to understand the retain statement. It is classified as non-executable statement. Th ...

非常感谢这个大侠给的解释。解开了我的一个盲点。
自己也运行了一下程序。
确实是这样的。

bobguy 发表于 2012-5-26 22:40
First one needs to understand the retain statement. It is classified as non-executable statement. Th ...

想问你个事情，我也看了base programing
但是上面没有讲关于retain语句的是非执行语句等有关问题。
我想知道你是看的哪些方面的书，能推荐一下吗？

tailezhu2007 发表于 2012-5-27 01:01
想问你个事情，我也看了base programing
但是上面没有讲关于retain语句的是非执行语句等有关问题。
我想 ...

Unlike a executable statement, a non-executable statement only provides compiling instructions. The positions of a non-executable statement in a data step is not important. That is difference between a executable statement and a non-executable statement.

I believe that SAS base books (manual) are the best.  Beyond SAS manuals, "Professional SAS Programming Secrets" by Rick Aster is the only other book I read before. This is a really a good book.

bobguy 发表于 2012-5-27 05:51
Unlike a executable statement, a non-executable statement only provides compiling instructions. Th ...

大侠，能不能把Professional SAS Programming Secrets这本书传一下啊，自己找了好久没找到，chengyongdu@126.com，非常感谢