READP and Reading Backward

Learning Objective

By the end of this lesson, you will understand what READP does at a basic level, and how reading backward through a keyed file differs from the forward reading covered so far.

Simple Explanation

Every read operation covered so far, READ and READE, moves forward through a file in key order. Sometimes a program needs to move the other direction instead, such as looking at the record just before the current one. This is what READP (Read Prior) is for.

dcl-f ORDHIST disk keyed;

dcl-s custNbr packed(6:0);

custNbr = 1042;
setll custNbr ORDHIST;
readp ORDHIST;

Here, setll custNbr ORDHIST; positions the file at customer 1042's first order, and readp ORDHIST; then reads backward from that position, retrieving the record immediately before it in key order rather than the one after. Just like READ, %EOF is checked immediately afterward, since reading backward past the beginning of the file sets %EOF to *on, the same way reading forward past the end does.

Why It Matters

At a basic level, knowing that READP exists, and that it moves backward through key order rather than forward, rounds out the picture of how records can be stepped through in an RPGLE program. This lesson keeps to that basic level; combining READP with more advanced positioning operations is a topic for later, more advanced lessons.

Practical Example

Imagine a program working with ORDHIST that needs to find the last order placed by whichever customer's key comes immediately before customer 1042's, perhaps while scanning backward through customer groups. After setll custNbr ORDHIST; positions the file immediately before customer 1042's first order, readp ORDHIST; retrieves the record immediately before that position: the last order belonging to whichever customer's key sorts just before 1042, without needing to read forward through the entire file from the beginning to find it.

This is a simplified, illustrative example rather than a specific real program, but it reflects the basic, everyday shape of reading backward through a keyed file.

Common Confusions

"Is READP just READ running in reverse?" At a basic level, yes: where READ moves forward through key order, READP moves backward. Both are checked with %EOF immediately afterward, since moving past either end of the file, forward or backward, sets %EOF to *on.

"Can READP be used together with READE the way SETLL is?" That kind of combined, more advanced positioning is beyond this introductory lesson. At this level, it is enough to know that READP exists and moves backward through key order; combining it with other operations is a topic for later, more advanced coverage.

"Does READP require a keyed file, like CHAIN and SETLL do?" Yes. READP depends on key order to determine what "backward" means, so it requires the same keyed declaration on dcl-f as CHAIN, SETLL, and READE.

Quick Recap

• READP fileName; reads the record immediately before the current position, moving backward through key order.
• %EOF becomes *on if READP moves past the beginning of the file, the same way it does when READ moves past the end.
• READP requires a keyed file, since it depends on key order to know which direction is "backward."
• This lesson covers READP only at a basic level; combining it with other positioning operations is left for more advanced coverage later.

Try Asking the AI Tutor

Use the AI Tutor to ask follow-up questions about this lesson. For example, try asking:

• "What would happen if I called READP as the very first operation on a file, before any SETLL or CHAIN?"
• "Is there a version of CHAIN that works backward the way READP does for READ?"