在 RPGLE 程序中，为什么 EXCEPT 优于 UPDATE 来更新 PF 中的字段？

Question

Eval Num = 1;

Why do we prefer:

C  EXCEPT     DDMAST
--------------------------- 
O  RDDMAST   E    DDMAST
O                 NUM

Over:

UPDATE     DDMAST

Answer 1

We don't.

It is really a shop preference. I was going to say EXCPT was easier, but I can't really even justify that in my mind. A lot of folks used EXCPT to output to the printer because it was easier than creating a print file, and sometimes that made it's way to database files as well because it was the same process, but I always preferred WRITE and UPDATE and DELETE over EXCPT, and eventually even stopped using EXCPT for reports as well since you could do more things with a printer file than you could with the program described output specs. These days I don't even use record level IO for database files, preferring SQL.

Answer 2

In 25yrs, I've never seen EXCEPT used for a Physical File. EXCEPT is used for "exception records" defined in the o-specs. I suppose it might make sense to WRITE exceptions to a table rather than a spool file. But I've never seen it.

Honestly, looking at the documentation, it's not even clear that it can be used instead of UPDATE.

A bit of googling turned up a post that indicated EXCEPT could be used to update selected fields in a PF. There is a performance benefit to doing that.

However the modern (though 21yrs old) way to update individual fields only is with the %FIELDS() BIF.

update ddmast %fields(num);

Lastly, you can't (directly) define o-specs in a fully **free RPG IV program.

Using EXCEPT instead of UPDATE would not be allowed in my shop.