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 chapter, we look at the numerous operations available for controlling flow in free format through conditional and selection logic, “do” functionality, and loop interrupts.
Editor's Note: This article is excerpted from chapter 5 of Free-Format RPG IV: Third Edition, by Jim Martin.
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:
it makes more sense to code
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:
- Complete checking is performed to true or false within
- When no parentheses are present, ANDs are checked before OR
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 6-1 shows an example of If precedence.
Listing 6-1: 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 6-2 illustrates this method.
Listing 6-2: 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 6-3 shows examples of If used with Elseif, Else, and Endif.
Listing 6-3: Using If, Elseif, Else, and Endif operations
More of Jim's Free-Format RPG IV is coming soon in an upcoming issue of MC RPG Developer. Can't wait? You can pick up Jim Martin's book, Free-Format RPG IV: Third Edition at the MC Press Bookstore Today!
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