Today's function is very straightforward. It adds numbers.
num = sum( argument, argument, ... );
So straightforward in fact, that some of you new to SAS may be wondering why you might even need such a simple function? There's certainly nothing wrong with the old "+ sign" right? Well, the sum() function can do something that the plus sign cannot do. And that is treat missing values as if they are 0. That can be a very important distinction if there is the possibility of adding variables that may contain missing values.
Consider the following data step:
a = 1;
b = 2;
c = .; * our missing value;
r1 = a + b + c;
r2 = sum( a, b, c );
The value of r1 will be missing since the plus operator returns missing if one of it's arguments is missing. The value for r2 will be 3 since it treats the missing value as if it were a 0