Easily Avoid a Common Data Structure Error
February 25, 2009 Ted Holt
|
A shop with which I’m acquainted had an undesirable experience recently. A program that, to my understanding, had been working properly, ended abnormally. The error could have easily been avoided, had the programmer followed one simple rule of thumb. Take a look at the following program fragment.
D MyData ds
D OneField 4a
D TwoField 7p 0
D RedField 3s 0
D BlueField 5u 0
D
D Number s 15p 5
/free
Number = TwoField;
If the calculation is the first executable statement in the program, what is the new value of Number? The answer is that Number never gets a new value. TwoField has an initial value of hexadecimal 40404040, which is not a valid packed decimal value. The program receives message MCH1202 and that’s the end of that. Suppose the first executable statement is this one:
Number = BlueField;
What’s the value of Number after this assignment? That’s right! Sixteen thousand, four hundred forty-eight, the decimal equivalent of hex 4040. In this case, the program runs, but has a logic error. Such errors won’t occur if you habitually initialize data structures. That is, add the INZ keyword to the first line. D MyData ds inz The best way to debug a program is not to put bugs into it in the first place. Little rules of thumb, like “always initialize data structures,” move you toward that goal.
|
