Four free-format operations help you construct programs that execute conditional logic in a structured way.

Editor's Note: This article is an excerpt from Chapter 6, "Program Flow Using Free Format," of Free-Format RPG IV, 3rd Edition (MC Press, 2015) by Jim Martin.

In nearly every program you write, you need to control the flow of instructions. Free-format RPG IV offers experienced RPG programmers a new style for controlling program flow. In this article, we look at the operations available for controlling flow in free format through conditional logic: If, Else, Elseif, and Endif.

The If Group

Four free-format operations—If, Else, Elseif, and Endif—help you construct programs that execute conditional logic in a structured way. Together, these operations form a powerful arsenal for managing the flow of instructions in your program.

If

Since IBM modified the If operation for the extended Factor 2 format, this operation has become a favored method for coding conditional logic. Rather than write the following:

         A           IFGT         B

         C           ANDLT       D

it makes more sense to code

         If A > B and C < D;

To use the If operation in free-format, just take the extended Factor 2 expression and assume you have the freedom to put it anywhere in positions 8–80. For every If, you must also code a corresponding Endif. The generic End operation (used in fixed format) is not available.

The If operation performs exactly like its extended Factor 2, fixed-format counterpart. The compiler checks factors in the comparison expression according to the operators given and the dictates of precedence (order of checking). The order of checking is as follows:

1. Complete checking is performed to true or false within parentheses.
2. When no parentheses are present, ANDs are checked before ORs.

If the premise expression of the If statement resolves to true, the operations that follow the statement are performed. The program continues with any number of operations until it reaches an Else, Elseif, or Endif statement.

If the premise expression resolves to false, program control passes to the Elseif premise, if present, or to an Else statement, if coded. If the If block includes neither an Elseif nor an Else, program control passes to the next operation after the Endif.

Listing 1 shows an example of If precedence.

       // Consider the following If expression:

       If (Acctbal > 1000 or Status = 'A')

                         and Date_1 > Date_2;

       // Due to the higher precedence of parentheses,

       // the logic group inside the parentheses will be

       // resolved first. If either comparison is true,

       // the group is true. If the group is true, the next

       // part of the expression is checked. If this

       // expression is also true, the If resolves to true.

       // Now consider the same expression without the

       // parentheses:

       If Acctbal > 1000 or Status = 'A'

                       and Date_1 > Date_2;

       // This time, with no parentheses, the "and" has the

       // highest precedence, so the Status check is now

       // paired with the date comparison. If Acctbal is

       // greater than 1000, the entire If is true; if

       // false, both the Status check and the date

       // comparison must be true.

Listing 1: Precedence in If comparison expressions

Free-format's greatest advantage for If (and other program-control operations) is the option to indent subordinate program logic. Indenting gives your program a pseudo-formatted look, entirely the same as in other contemporary languages, and it makes the flow of the program's logic easier to discern. Listing 2 illustrates this method.

       If Status = 'A' and Amount > 100;

         OK_Record = *On;

         Total_Amount += Amount;

       Else;

         Count_No += 1;

       Endif;

Listing 2: If logic using indenting

Else

Most of the time, you won't need an Else operation, but Else (as well as Elseif) is available to handle the "false" leg of programming logic when you need to do so. If the premise of an If expression is false and you have coded an Else operation, program control passes to the first operation after the Else operation.

Just as with If, any number of operations can follow the Else. Indenting the operations specified after the Else gives future readers of your program a visual cue to the logic being used in the program.

Elseif

The Elseif operation is a relative newcomer to RPG IV. It replaces the combination of Else followed immediately by another If operation. By using If with Elseif, you form a control structure that lets only one group of operations be performed. If the group includes no Else, it is possible that no group will be performed. By coding an Else operation at the end of the group, you create an option for the condition "none of the above."

Endif

As I have noted, each If operation must conclude with an Endif. Free-format RPG IV provides no generic End operation like that in fixed-format. The new approach makes your code "tighter" in the sense that only specified "end" operations work, reducing the chance of accidental error or misunderstanding by a future reader.

Listing 3 shows examples of If used with Elseif, Else, and Endif.

       If Action = 'A';

         Write Record_a;

         Message = 'Record added';

       Elseif Action = 'C';

         Update Record_a;

         Message = 'Record updated';

       Elseif Action = 'D';

         Delete Record_a;

         Message = 'Record deleted';

       Else;

         Message = 'Invalid action code entered.';

       Endif;

Listing 3: Using If, Elseif, Else, and Endif operations

Read On

RPG offers other free-format operations that let you control program flow in free format through "do" functionality, loop interrupts, and selection logic. To learn more about these operations, see Chapter 6 of Free-Format RPG IV, 3rd Edition.