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.
注解：Count + 1为一个Sum Statement，等价于：
RETAIN Count 0;
Count = SUM(Count, 1);
RETAIN的作用是初始化一个变量，并且在每次进入新的DATA step时，保留该变量的值。值得注意的是，RETAIN statement在编译代码时执行，即在执行DATA step（读取数据）之前，所以RETAIN statement只执行一次。SUM function的作用是求合，并且忽略missing value。上面程序第一句的意思为初始化一个叫Count的变量，并且其初始值为0。第二行的作用是将Count的值增加1，与Count = Count + 1类似，但两者有一个重要的区别，SUM function会忽略missing value，而后一种表达式则不会。假设Count的值为missing，Sum(Count, 1)的结果为1，而Count + 1的结果则为missing，missing value加上一个值其结果还是missing。