What are Lists?
Imagine we have a situation where we want to store the names of all the students that are taking a certain course. The first solution we can come up is to create an Array:
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using System; class Program { string[] studentNames = new string[50]; }
However, if at some point are more than 50 students, we won't be able to store their names. Similarly, in a situation where there are less than 50 students, the unoccupied spaces in the array will be a waste of memory - this becomes a problem in especially large arrays.
Here we need a new structure which can store a variable amount of elements. Luckily such a structure already exists, it's called a List.
Lists are very similar to Arrays, however the number of elements that are stored in a list are changeable.
Following is the syntax for declaring an empty list:
Using this syntax we can create a list for storing the Students' names:
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using System; using System.Collections.Generic; class Program { static void Main(string[] args) { List<string> students = new List<string>(); } }
It is important to note that we need to import the Generic
module to be able to use Lists.
You simply need to add this line under using System;
to import the required module.
Add() Method
We can add elements to a list by using the Add
method:
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using System; using System.Collections.Generic; class Program { static void Main(string[] args) { List<string> students = new List<string>(); students.Add("Anna"); students.Add("Laura"); students.Add("Jacob"); students.Add("Aron"); } }
The above code will add four elements to the students
list.
Just like an Array of type string
can only contain string
elements. A list of type string
can only accept string
elements.
Indexing
The first element is Anna
hence it will have the index 0
, while Laura
will have the index 1
and so on. Elements of a list can be accessed via indexing just like an array:
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using System; using System.Collections.Generic; class Program { static void Main(string[] args) { List<string> students = new List<string>(); students.Add("Anna"); students.Add("Laura"); students.Add("Jacob"); students.Add("Aron"); Console.WriteLine(students[2]); } }
Count() Method
We can retrieve the length of a list using its Count
attribute:
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using System; using System.Collections.Generic; class Program { static void Main(string[] args) { List<string> students = new List<string>(); students.Add("Anna"); students.Add("Laura"); students.Add("Jacob"); students.Add("Aron"); Console.WriteLine(students[2]); Console.WriteLine(students.Count); } }
Dynamic Length
Note that the length of a list is dynamic (changeable), so it changes as we add elements:
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using System; using System.Collections.Generic; class Program { static void Main(string[] args) { List<string> students = new List<string>(); Console.WriteLine(students.Count); students.Add("Anna"); Console.WriteLine(students.Count); students.Add("Laura"); Console.WriteLine(students.Count); students.Add("Jacob"); Console.WriteLine(students.Count); students.Add("Aron"); Console.WriteLine(students.Count); } }
Initialization
We can also initialize a list with some elements using the following syntax:
List<dataType> listName = new List<dataType>{ element1, element2, … };
For-example:
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using System; using System.Collections.Generic; class Program { static void Main(string[] args) { List<string> students = new List<string> { "Anna", "Laura", "Jacob", "Aron" }; // It is still possible to add additional elements after initialization students.Add("Markus"); Console.WriteLine(students.Count); Console.WriteLine(students[4]); } }
Looping Through Lists
Looping through a List similar to how we would loop through Arrays
- Using a for-loop:
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using System; using System.Collections.Generic; class Program { static void Main(string[] args) { List<string> students = new List<string> { "Anna", "Laura", "Jacob", "Aron" }; for (int i = 0; i < students.Count; i++) { Console.WriteLine(students[i]); } } }
- Using a foreach loop
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using System; using System.Collections.Generic; class Program { static void Main(string[] args) { List<string> students = new List<string> { "Anna", "Laura", "Jacob", "Aron" }; foreach (string studentName in students) { Console.WriteLine(studentName); } } }
Tip:
To make the declaration syntax shorter we can also use implicit declaration. To recall, an explicit declaration is when we specify the data type during variable declaration, for-example:
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float number = 7.9f;
On the other hand, in implicit declaration we can simply use the var
keyword and the compiler automatically infers the data type of the variable according to the assigned value:
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var number = 7.9f;
We can use implicit declaration when declaring lists as well:
var listName = new List<dataType>();// orvar listName = new List<dataType>{ element1, element2, … };
For-example:
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var numbers = new List<int> { 1, 2, 3, 4, 5 };
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