Data Structure

Introduction

Data structures are a powerful tool that can make your CL programs more efficient and easier to read and maintain. By using data structures, you can group related data together and reduce the amount of code that you need to write.

Data structures in CL on the AS400 can be created using defined variables. A defined variable is a variable that is based on a portion of another variable. This allows you to group related data together and to reference it more easily. To create a defined variable, you use the DCL VAR command with the STG(*DEFINED) and DEFVAR parameters.

The term STG(*DEFINED) means that this variable is another name for part of another variable. The DEFVAR parameter tells which variable this one is based on and the position at which this variable overlays the other.

Here some of the usages of using data structures in CL:

Improved efficiency:Data structures can help to improve the efficiency of your CL programs by reducing the amount of code that you need to write and by making it easier to access and manipulate data.

Increased readability and maintainability: Data structures can help to make your CL programs more readable and maintainable by grouping related data together and by giving your data meaningful names.

Reduced errors: Data structures can help to reduce errors in your CL programs by providing a way to validate data before it is used.

Restrictions-:

• CLLE data structures are limited to 64 KB in size.
• Arrays and pointers are not permitted in CLLE data structures.
• Members that are other data structures are not permitted in CLLE data structures.
• Nested structures are not permitted in CLLE data structures.
• Variable-length members cannot be found in CLLE data structures.

Compatibility -:

Field Data Types: Ensure that the data types of fields within a data structure are compatible with the data you intend to store in them. IBM i supports various data types, including character, numeric, date, and time types.

Compatibility with Embedded SQL:
If you use embedded SQL in your CLLE programs, the data structures you define should align with the structure of the database tables you are interacting with. Field names in your data structures should match the column names in SQL statements for proper binding.

Naming Conventions:
Follow consistent naming conventions for your data structures and fields. This helps maintain code readability and makes it easier to work with other developers’ code.

Performance Optimization:
Depending on your specific application and performance requirements, you may need to optimize your data structures for efficient access and processing.

Examples

The following example shows how to create a defined variable for an employee record:

DCL VAR(&EMPREC) TYPE(*CHAR) LEN(77)

 DCL VAR(&EMPNUM) TYPE(*CHAR) LEN(6) STG(*DEFINED) DEFVAR(&EMPREC 1)

 DCL VAR(&EMPNAME) TYPE(*CHAR) LEN(20) STG(*DEFINED) DEFVAR(&EMPREC 7)

This creates a defined variable called &EMPREC that is 77 characters long. The &EMPNUM and &EMPNAME variables are defined as being based on the first 6 and 20 characters of &EMPREC, respectively.
Once you have defined your defined variables, you can use them like any other variable in your CL program. For example, the following code shows how to print the employee’s name and number to the console:

 DSPMSG MSG(&EMPNAME)

 DSPMSG MSG(&EMPNUM)

Defined variables can be used to create complex data structures, such as arrays and linked lists. They can also be used to pass data between CL programs and other programs, such as RPG programs.

Here is an example of how to use a defined variable to pass data to an RPG program:

 DCL VAR(&EMPREC) TYPE(*CHAR) LEN(77)

 DCL VAR(&EMPNUM) TYPE(*CHAR) LEN(6) STG(*DEFINED) DEFVAR(&CUSTREC 1)

 DCL VAR(&EMPNAME) TYPE(*CHAR) LEN(20) STG(*DEFINED) DEFVAR(&CUSTREC 7)

Assign values to the defined variables.

CALLP RPGPGM

 PARM(&EMPREC)

Code example:

In the above code:

DCL declares a data structure named DATASTRUCT that is 50 characters long and containing multiple fields &FLD1, &FLD2, &FLD3, &FLD4 of different data types and are defined as being based on the first 10 and 31 characters of &DATASTRUCT, respectively.

STG(*DEFINED) – Storage: the value for this variable is specified in the variable defined in the DEFVAR parameter.
DEFVAR(&DATA_STRCT ?) – Defined on variable specifies the variable that contains this subfield, and its starting position.