Data Structures in RPGLE

Learning Objective

By the end of this lesson, you will understand what an RPGLE data structure is, how to declare one using dcl-ds, and why declaring one as qualified is generally good practice.

Simple Explanation

In the Variables and Data Types in RPGLE lesson, you declared standalone variables one at a time using dcl-s. A data structure, declared using dcl-ds, groups several related variables, called subfields, together under one named container.

dcl-ds customer qualified;
  custName char(30);
  custBalance packed(9:2);
end-ds;

customer.custName = 'Jordan Lee';
customer.custBalance = 250.00;

Declaring a data structure as qualified means each subfield must be referred to using the data structure's name, a period, and the subfield name, such as customer.custName. This is generally good practice: it makes it immediately clear which data structure a subfield belongs to, and avoids naming conflicts if two different data structures happen to have subfields with similar names.

Without qualified, a data structure's subfields behave more like ordinary standalone variables referred to by their own name alone, which can be more convenient for very small programs, but loses that same protection against naming conflicts as a program grows.

Why It Matters

Data structures let you group logically related values together, such as several pieces of information about the same customer, rather than managing a long, flat list of unrelated standalone variables. This mirrors an idea you already know from the Db2 for i lessons: much like a record format groups related fields together, a data structure groups related subfields together within a program.

Practical Example

Imagine a program working with several pieces of information about one customer: name, balance, and account status. Declaring a single customer data structure with custName, custBalance, and custStatus subfields keeps these clearly related values grouped together and easy to pass around as a unit, rather than managing three separate, seemingly unrelated standalone variables.

Using qualified, referring to customer.custName also makes it obvious, just from reading the code, exactly which piece of data a given line is working with. This is a simplified, illustrative example rather than a specific real program, but it reflects a genuinely common, practical reason RPGLE developers use data structures.

Common Confusions

"Is a data structure the same thing as a DDS record format?" They are related ideas, both group fields together, but a data structure is declared inside an RPGLE program to organize variables in memory, while a record format, covered in the DDS Field Definitions lesson, describes the structure of data actually stored in a physical file.

"Do I always need to use qualified?" No, but it is generally recommended, especially as a program grows. Without qualified, subfield names must be unique across the entire program, which can become harder to manage; qualified avoids that concern entirely.

"Can a data structure have subfields of different data types?" Yes. A data structure's subfields can use any of the data types covered in the Variables and Data Types in RPGLE lesson, mixed together as needed, such as combining char, packed, and int subfields within the same data structure.

Quick Recap

• A data structure, declared with dcl-ds, groups related variables, called subfields, together under one named container.
• Declaring a data structure as qualified means subfields are referred to as structureName.subfieldName, which is generally good practice.
• Data structures help organize logically related values together, similar in spirit to how a record format groups related fields.
• Subfields can use any of the standard RPGLE data types, mixed together as needed.

Try Asking the AI Tutor

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

• "What happens if I don't use qualified and two data structures have a subfield with the same name?"
• "Can a data structure be used as a parameter passed to a procedure?"