一个array小问题

shiyiming

有一个数据集donate，有四个变量qtr1-qtr4,有下面一段程序：
data goal(drop=i g1-g4);
    set donate;
    array contrib(4) qtr1-qtr4;
    array differ(4) diff1-diff4;
    array goal(4) g1-g4 (10 15 5 10);
    do i=1 to 4:
         differ(i)=contrib(i)-goal(i);
    end;
run;

我想问的是：
1、在数据步每次循环中，是不是三个array语句都要重新执行一次？或者更进一步问：这三个array语句在数据步的编译和执行过程中分别会有哪些动作？
2、g1-g4中的初值为什么在数据步的每次循环的开始不被置为缺失值？

shiyiming

First, ARRAY statement is not an excutable statement, it will run only once for this data step.  In the other words, if you move 'set donate' statement after all the array statement, it will work the same.

Second, I believe that if you set an initial value to an array, SAS will automatically RETAIN the value.  

I recommand you test the following:

donate data set:
1 2 3 4
2 3  . 4
2 4 5 6

try to run your program twice, once with
  array differ(4) diff1-diff4;
another with
  array differ(4) diff1-diff4 (1 1 1 1);

You will see the difference and it will help you to understand how does it work.

shiyiming

[quote="xic":34345]
try to run your program twice, once with
  array differ(4) diff1-diff4;
another with
  array differ(4) diff1-diff4 (1 1 1 1);

You will see the difference and it will help you to understand how does it work.[/quote:34345]
两次结果一样的，没什么差别。不管你赋不赋初值，到后面碰到循环语句中的赋值语句，数组中的值当然会改变的。

shiyiming

xic的解答很精确！

  两次运行的结果当然不一样！对数组进行初始化赋值和不进行初始化赋值是完全不同的。
  此程序中，没有其它语句对g1-g4四个变量进行赋值，所以每一次循环，此四个变量取其初始化值（若有）或缺失值（若无初始值）。
  如果g1-g4四个变量在原数据集中存在，且无缺失值，那程序中（以上程序）对其进行初始化赋值的操作就没有什么作用，两次操作的结果会完全相同。

shiyiming

I am sorry that I forgot to mention that if you write the line to assign the value to differ as

if contrib(i) ne . then differ(i)=contrib(i)-goal(i);

you will see the difference.  I tried it in program, but did not write it in the email.  I would like to apoligize.