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TechTip: C# for RPGers - Easily Removing Duplicates, Sorting, and Reversing Lists

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This time around, I’ll show how to perform some operations over lists that typically require some heavy coding.

Start by opening Visual Studio and locating the IntroducingLists.sln project. I’ll be using it on this TechTip to explain some more interesting methods that can save you a lot of time. Last time around, I stopped after inserting the missing old name of the IBM i. Now let’s re-add a couple of names to see how duplicates can be easily removed with a single line of code.

Back in the first TechTip about Lists, I created a list named additionalNames with a  couple of IBM i old names:

List<string> additionalNames = new List<string>() { "iSeries", "IBM i" };

Let’s re-add that list to the main List variable. After the last line of the Main method of the Program.cs file, type (or copy and paste) the following code:

// If I re-add the second list to the first, I'll get duplicate elements

serverNames.AddRange(additionalNames);

Console.WriteLine();

Console.WriteLine("The list after the insertion of the duplicates:");

Console.WriteLine(string.Join(", ", serverNames));

I’ve done this before (back in the first TechTip about lists that I mentioned earlier), so there’s not much to explain here. The interesting part is what comes next: removing the duplicates with a single line of code. I have to say that, even though it’s a single line of code, it’s actually a couple of instructions. But let’s look at the code and I’ll explain what I mean next:

serverNames = serverNames.Distinct().ToList();

Note that I’m using the Distinct() method to remove the duplicate elements of the list, much like I’d do on an SQL SELECT statement. However, the Distinct() doesn’t return a List, so I need to convert the result to a list before I can assign it back to my serverNames variable. That’s where the ToList() method comes into play. In object-oriented (OO) languages, it’s very common to execute methods on top of methods, like in this example. It may be a bit confusing, but it’s a way to prevent additional variable declaration just to perform another instruction. What I mean is that the line above can also be written like this:

IEnumerable<string> tempNames = serverNames.Distinct();

serverNames = tempNames.ToList();

It’s true that it’s not that much code, but if there’s a shorter, universally accepted way, then you should get used to itand use it. Anyway, if you output the contents of the serverNamesvariable, you’ll see that it no longer contains the duplicate elements I inserted previously. If you don’t remember how to do it, here’s the necessary code:

Console.WriteLine();

Console.WriteLine("The list after the Distinct mehod was executed:");

Console.WriteLine(string.Join(", ", serverNames));

Reversing Lists

Another common operation that requires a bit of coding is reversing a list of elements. Typically, you’d use a For loop and a secondary array or temporary variable to perform the operation. However, because it’s a very common operation, the string class has a method for that and, you’ve probably guessed it, it’s called Reverse(). Here’s how to use it:

// Now let's reverse the list

serverNames.Reverse();

Console.WriteLine();

Console.WriteLine("The list after the Reverse mehod was executed:");

Console.WriteLine(string.Join(", ", serverNames));

Note that some methods operate directly over the object (in this case, the serverNames variable) while others, like the Distinct() method I’ve shown before, return the result as something else.

This is a bit like RPG’s procedures and functions: while the procedures perform operations over “something” and don’t return a value, the functions typically leave the input “something” alone and output the result of the operations that were performed. In C#, you’ll find these two types of methods. You’ll need to read the documentation to figure out which is which and how to use them. But don’t worry; there are usually a lot of examples online.

Sorting Lists

The Sort() method is similar to the Reverse() method and it simply sorts the List object. The way it’s sorted depends on two things: the type of list and the parameters you pass to the method.

In this case, I have a list of strings, so the sorting will be performed in ascending alphabetical order. However, I can specify a different sorting order by using variations of the Sort() method that contain input parameters. This concept is called overloading, and it’s something I’ll get back to later in this TechTip series. For now, let me just show you a simple example of how to use the Sort() method:

// And sort it

serverNames.Sort();

Console.WriteLine();

Console.WriteLine("The list after the Sort mehod was executed:");

Console.WriteLine(string.Join(", ", serverNames));

Clearing a List

Finally, let’s see how to remove all the elements of a List or, in other words, clear it. Naturally, there’s a simple and straightforward method for that: Clear(). Here’s how to use it:

// Finally, let's remove all elements from the list, using Clear()

serverNames.Clear();

Console.WriteLine();

Console.WriteLine("The list now has " + serverNames.Count + " elements.");

If you run the code above, you’ll see that serverNames is now empty. Usually, there’s a simple solution, often in the form of a method or a combination of methods to solve the most common problems. You just need to learn how to navigate the Visual Studio inline and online documentation. Visual Studio contains loads of documentation and examples, but there are also tons of web pages with tutorials, examples, and forums discussing pretty much everything you can think of.

Out of Time!

That’s all the time I have, so let’s wrap up with the complete code of the “IntroducingLists” project:

using System;

using System.Collections.Generic;

using System.Linq;

using System.Text;

using System.Threading.Tasks;

namespace IntroducingLists

{

    class Program

    {

        static void Main(string[] args)

        {

            // Defining an empty list of strings

            // note that I'm using var and letting the compiler figure out the type of variable

            var serverNames = new List<string>();

            // Defining and initializing a list of strings

            List<string> additionalNames = new List<string>() { "iSeries", "IBM i" };

            // Adding elements to the List, one by one

            serverNames.Add("S/38");

            serverNames.Add("AS/400");

            Console.WriteLine("After the inital additions, the list has " + serverNames.Count + " elements.");

            // Adding multiple elements at once, or "stitching together" two lists

            serverNames.AddRange(additionalNames);

            // "S/38" isn't actually an IBM i server name, so let's remove it, using its name

            serverNames.Remove("S/38");

            // Let's insert it again and see where it went

            serverNames.Add("S/38");

            Console.WriteLine("S/38 now exists on position " + serverNames.IndexOf("S/38"));

            // Now I can remove it indicating its position

            serverNames.RemoveAt(3); // or serverNames.RemoveAt(serverNames.IndexOf("S/38"));

            // Make sure the "S/38" element has been removed, using the Contains method

            Console.WriteLine();

            Console.WriteLine("It's " + serverNames.Contains("S/38") + " that the list contains S/38");

            // Output the contents of the list, using the foreach loop

            Console.WriteLine();

            Console.WriteLine("The complete list (output using foreach):");

            foreach (string name in serverNames)

            {

                Console.Write(name + ", ");

            }

            Console.WriteLine();

            // "System i" belongs between "iSeries" and "IBM i", so let's insert it there

            serverNames.Insert(2, "System i");

            // Output the contents of the list, using the string.Join method

            Console.WriteLine("The complete list, after the insertion (output using string.Join):");

            Console.WriteLine(string.Join(", ", serverNames));

            // If I re-add the second list to the first, I'll get duplicate elements

            serverNames.AddRange(additionalNames);

            Console.WriteLine();

            Console.WriteLine("The list after the insertion of the duplicates:");

            Console.WriteLine(string.Join(", ", serverNames));

            // Here's a way to remove them

            // note that the final .ToList() method call is necessary because Distinct() doesn't return a List

            serverNames = serverNames.Distinct().ToList();

            Console.WriteLine();

            Console.WriteLine("The list after the Distinct mehod was executed:");

            Console.WriteLine(string.Join(", ", serverNames));

           

            // Retrieving an element from the list

            // Note that this doesn't remove the element from the list

            string anotherString = serverNames.ElementAt(3);

            Console.WriteLine();

            Console.WriteLine("The anotherString variable now contains " + anotherString);

            // Now let's reverse the list

            serverNames.Reverse();

            Console.WriteLine();

            Console.WriteLine("The list after the Reverse mehod was executed:");

            Console.WriteLine(string.Join(", ", serverNames));

            // And sort it

            serverNames.Sort();

            Console.WriteLine();

            Console.WriteLine("The list after the Sort mehod was executed:");

            Console.WriteLine(string.Join(", ", serverNames));

            // Finally, let's remove all elements from the list, using Clear()

            serverNames.Clear();

            Console.WriteLine();

            Console.WriteLine("The list now has " + serverNames.Count + " elements.");

            Console.ReadKey(); 

        }

    }

}

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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