10.3. Collections

10.3.1. Data Structures

A data structure lets us hold on to lots of data in a single place. It is a programming construct to aggregate many values into one value. Many types of data structures exist in various languages. A few examples are lists, dictionaries, arrays, tuples, etc.

10.3.2. C# Collections Namespace

C# provides powerful and flexible structures to store data, known as collections. The C# collections namespace refers to the various interfaces the language provides for implementing collection types.

Here, we’ll discuss a collection called List and compare it to the Array class. We’ll then introduce a third collection type called Dictionary. These three collection types will be sufficient for our basic C# needs. For more, refer to the official C# documentation on collections.

In order to be able to access these collection types, we need to include the correct using statements.

10.3.3. using

The using statement in C# allows us to access classes, methods, and data stored in different files other than the one we are currently in.

In C#, you can use any class that is available without having to import the class - subject to two very important conditions:

  1. The C# compiler must know that the class exists.

  2. You must use the full name of the class.

Classes that are available to you may be those in the project you are currently working on, such as the MainClass in the replit.com IDE or those that come along with the .NET class library, as well as anything you might get from added dependencies.

Example

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using System;

namespace TempConv
{

   class MainClass
   {
      public static void Main(string[] args)
      {
         double fahrenheit;
         double celsius;
         string input;

         Console.WriteLine("Temperature in F:");
         input = Console.ReadLine();
         fahrenheit = double.Parse(input);

         celsius = (fahrenheit - 32) * 5 / 9;
         Console.WriteLine("The Temperature in C is: " + celsius);
         Console.ReadLine();
      }
   }
}

repl.it

The class naming system in C# is very hierarchical. The full name of the Console class used first on line 14 is really System.Console. You can think of this name as having two parts. The first part, System, is called the namespace, and the last part is the class. We’ll talk more about the class naming system a bit later. You have also seen System when you are checking the data type of varialbes.

One thing to know about the using statement is that it is not responsible for loading classes into memory. That task falls on the assembly, which is the unit of compiled code created by the C# compiler.

The using statement tells the compiler that we are going to use a shortened version of the class’s name. In this example, we are going to use the class System.Console, but we can refer to it as just Console. We could use the System.Console class without any problem and without any import statement provided that we always referred to it by its full name.

Don’t just trust us, try it yourself! Remove the using statement and change Console to System.Console in the rest of the code. The program should still compile and run.

10.3.3.1. using System.Collections.Generic

So far, the using System has allowed us to work with primitive data types, such as ints and chars, but also reference types like strings. When we start using collection types, we are going to need a namespace update that will provide the compiler with needed resources to work with these types.

While working with the replit IDE, you will need to add this manually when you want to use a collection type.

Example

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using System;
using System.Collections.Generic;    //add this line

10.3.4. Gradebook, Three Ways

We’ll explore collections in C# by looking at different versions of the same program. The program functions as a gradebook, allowing a user (a professor or teacher) to enter the class roster for a course, along with each student’s grade. It then prints the class roster along with the average grade. In each variation of this program, the grading system could be anything numeric, such as a 0.0-4.0 point scale, or a 0-100 percentage scale.

A test run of the program might yield the following:

Enter your students (or ENTER to finish):
Chris
Jesse
Sally

Grade for Chris: 3.0
Grade for Jesse: 4.0
Grade for Sally: 3.5

Class roster:
Chris (3.0)
Jesse (4.0)
Sally (3.5)

Average grade: 3.5

We’ll look at the gradebook using a List first.