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TechTip: C# for RPGers: Making Stuff Happen, Part 2 - More Flow Control Structures

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Last time around, I explained the IF statement. Now it’s time to get to know a couple more C# flow control structures.

If you read the previous TechTip of this series, you already know that the IF statement in C# is similar to its RPG counterpart. It’s true that there are differences, but in general they’re pretty much alike. Even though I didn’t mention it in the previous TechTip, you can create IF-ELSE-IF structures in C# just like you do in RPG. As you know, abusing this facility usually leads to a stairwell-like structure (if you’re using RPG /free and proper indentation) or a Find-The-Right-EndIf game (similar to Where’s Wally, but not so fun). To avoid these pitfalls, a sensible programmer uses a Select-When structure instead of nested IF-ELSE-IF blocks. There’s also a similar control flow structure in C#, under a different name: Switch-Case.

C#’s Switch-Case Is RPG’s Select-When

In order to establish a proper comparison, let’s first revisit the Select-When structure with a simple example:

Select;

When myNumber = 1;

// execute appropriate action for the value 1

When myNumber = 2;

// execute appropriate action for the value 2

Other;

// execute the default action if all previous comparisons evaluate to false

EndSl;

This flow control structure is delimited by the Select and EndSl instructions. In the middle, you can have multiple When instructions, each with a different condition and its own block of code, which will be executed if the condition of the respective When instruction evaluates to true. Finally, you can also have an Other block, which will be executed only if none of the preceding When conditions evaluate to true. It’s also important to mention that, once a condition evaluates to true, the program execution resumes after the EndSl instruction.  

Now let’s see how the C# version looks:

            switch(myNumber)

            {

                case 1:

                    // execute appropriate action for the value 1

                    break;

                case 2:

                    // execute appropriate action for the value 2

                    break;

                default:

                    // execute the default action if all previous comparisons evaluate to false

                    break;

            }

Probably, the first thing that comes to mind is “the two examples are not the same, because the RPG example doesn’t have the myNumber variable in the Select instruction!” Well, the same way a Select instruction simply marks the beginning of the control flow structure and can’t take a variable in front of it, the switch instruction must take a variable in front of it and—just like in the IF statement—the parentheses are not optional.

Note that each block of instructions belonging to a case instruction must end with the break instruction. The fact that the switch instruction must take a variable in front of it means that all the case instructions are limited to evaluate a condition built using whichever variable is used on the switch instruction. If you think that this limits your choices, wait because it gets worse: you can’t use variables in the case instructions. In other words, you’re limited to actual static values (numbers or strings or whatever)! You can’t even use composed conditions, even though you can “fake” an OR composed condition with this neat trick:

(…)

             case 3:

             case 4:

             case 5:

                    // execute appropriate action for 3, 4 or 5

                    break;

(…)

It’s not brilliant, but it works. I learned to live with this limitation in my C# code, but I sometimes miss the flexibility RPG’s Select-When structure offers.

A Shorter (and Weirder) IF

A common workaround to the Switch-Case structure limitations is going back to nested IF statements. It’s not the perfect option, but sometimes you don’t actually have a better choice. You have, however, a shorter (and weirder) way of writing a simple IF statement. It’s often referred to as the ternary if operator because, unlike most operators you’re used to in RPG and other programming languages, this one takes three arguments. However, you’ve seen a “version” of this operator before. I’d dare say you use it frequently—just not in a programming language! No, I’m not kidding (or going mad): If you ever used Excel’s IF formula, then you’ve used a “clone” of the ternary if operator. As a quick reminder, Excel’s IF looks like this:

=IF(<condition to evaluate>;<cell value if true>;<cell value if false>)

C#’s ternary if operator is similar, but it does without the IF and the parentheses. It also replaces the first semicolon with a question mark, almost asking if the condition being evaluated is true:

<condition to evaluate>?<value if true>;<value if false>

Here’s an example that assigns 5 to myNumber if myString is equal to “Five” and 0 otherwise:

myNumber = myString.Equals("Five") ? 5 : 0;

Even though I’m using the ternary if operator to assign a value to a variable, I can also use it as part of an IF-ELSE-IF structure (or anywhere else in fact) because it simply returns one of two values, depending on the evaluation of the condition it contains. The only drawback is that you’re limited to one instruction that produces a value on each “section” of the operator. You can use methods, as long as they return something (a number, a string, and so on). You’ll find a more detailed explanation here and additional examples here.

In future TechTips, you’ll see more examples of IF, Switch-Case, and ternary if operator examples, but before that I’ll discuss looping structures. You already know the foreach, but there are more and they’re similar to what you know from RPG. Until then, here’s the complete code for these last two TechTips, nicely formatted for your copy-paste convenience:

        static void Main(string[] args)

        {

            int myNumber = 24;

            string myString = "This is a string!";

            // Comparing integers

            if (myNumber == 12)

            {

                // do something if myNumber is equal to 12,

                // in other words, do something

                // if the condition above evaluates to true

            }

            else

            {

                // do something else if myNumber is not equal to 12

                // in other words, do something

                // if the condition above evaluates to false

            }

            // Comparing strings

            if(myString.Equals("Another string"))

            {

                // do something

            }

            else

            {

                // do something else

            }

            // Using comparison operators

            if((myNumber >= 22) && (!myString.Equals("Some string")))

            {

                // do something if myNumber is greater or equal than 22

                // AND

                // myString is NOT EQUAL to "some string"

            }

            // Introducing Switch-Case

            switch(myNumber)

            {

                case 1:

                    // execute appropriate action for the value 1

                    break;

                case 2:

                    // execute appropriate action for the value 2

                    break;

                case 3:

                case 4:

                case 5:

                    // execute appropriate action for 3, 4 or 5

                    break;

                default:

                    // execute the default action if all previous comparisons evaluate to false

                    break;

            }

            // Introducing the ternary if operator

            // an example that assigns 5 to myNumber if myString is equal to “Five” and 0 otherwise

            myNumber = myString.Equals("Five") ? 5 : 0;

        }

Rafael Victoria-Pereira

Rafael Victória-Pereira has more than 20 years of IBM i experience as a programmer, analyst, and manager. Over that period, he has been an active voice in the IBM i community, encouraging and helping programmers transition to ILE and free-format RPG. Rafael has written more than 100 technical articles about topics ranging from interfaces (the topic for his first book, Flexible Input, Dazzling Output with IBM i) to modern RPG and SQL in his popular RPG Academy and SQL 101 series on mcpressonline.com and in his books Evolve Your RPG Coding and SQL for IBM i: A Database Modernization Guide. Rafael writes in an easy-to-read, practical style that is highly popular with his audience of IBM technology professionals.

Rafael is the Deputy IT Director - Infrastructures and Services at the Luis Simões Group in Portugal. His areas of expertise include programming in the IBM i native languages (RPG, CL, and DB2 SQL) and in "modern" programming languages, such as Java, C#, and Python, as well as project management and consultancy.


MC Press books written by Rafael Victória-Pereira available now on the MC Press Bookstore.

Evolve Your RPG Coding: Move from OPM to ILE...and Beyond Evolve Your RPG Coding: Move from OPM to ILE...and Beyond
Transition to modern RPG programming with this step-by-step guide through ILE and free-format RPG, SQL, and modernization techniques.
List Price $79.95

Now On Sale

Flexible Input, Dazzling Output with IBM i Flexible Input, Dazzling Output with IBM i
Uncover easier, more flexible ways to get data into your system, plus some methods for exporting and presenting the vital business data it contains.
List Price $79.95

Now On Sale

SQL for IBM i: A Database Modernization Guide SQL for IBM i: A Database Modernization Guide
Learn how to use SQL’s capabilities to modernize and enhance your IBM i database.
List Price $79.95

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