csharp-basics/5-arrays-and-lists.md

marp	paginate	math	theme	title
true	true	mathjax	buutti	5. Arrays and Lists

Arrays and Lists

Overview

• Arrays
• Multidimensional arrays
• Lists
• Iterating a list
• Foreach

Arrays

• Arrays are a collection of variables of the same type
• To optimize memory usage, arrays allocate neighbouring memory addresses
• A single value in this collection is called an element
• Arrays can be declared with square brackets following the type of the elements:

  int[] userIds;
  

• Declaring an array does not yet allocate space from memory

Initializing arrays

• Initialize an array with a length of 3 using the following syntax:

  int[] userIds = new int[3];
  

• Assign a value to an element in an array by specifying the index in square brackets:

  userIds[0] = 104;
  

• Indexing starts from 0, so the above line assigns a value of 104 to the first element of the array

---

• You can also create an array and instantly populate it with values with one statement:

  string[] names = new string[3] { "Johannes", "Rene", "Ville" };
  

• The same works without specifying the length in the brackets:

  double[] balances = new double[] { 1.3, 200.3, 9332.14 };
  

Multidimensional Arrays

• C# supports multidimensional arrays
• Here's an example of a two-dimensional array.
• You can think of it as an array consisting of arrays

  char[,] letters = new char[3, 4]
  {
    {'a', 'b', 'c', 'd'},
    {'e', 'f', 'g', 'h'},
    {'i', 'j', 'k', 'l'}
  };
  
  Console.WriteLine(letters[1, 3]);       // Outputs "h"
  

Lists

• The .NET Framework Library System.Collections.Generic offers another structure to contain multiple variables: the List class
• Importing the library:

  using System.Collections.Generic;
  

• Declaration and initialization:

  List<int> userIds = new List<int>();
  

• Value assignment (exactly like arrays):

  userIds[0] = 22;
  

Why lists?

• The key benefit of using Lists lies in its built-in functionalities
• You can see a list (heh) of available methods in Visual Studio by following the list variable with a dot

Arrays vs Lists

• The memory allocation of an array is static a.k.a. fixed, but list memory is dynamic
• This allows the flexible mutation of lists, meaning you can always add or remove elements, thus changing the length of the list without issues

  List<int> intList = new List<int>() { 1, 9, 4 };
  intList.Remove(0); // List is now [9, 4]
  
  int[] intArray = new int[] { 1, 9, 4 };
  intArray.Remove(0);
  // Can't do that, array is static
  // and doesn't include such method
  

When to use arrays or lists?

• Use arrays when you...
  • ...need high performance
  • ...can predetermine the number of elements
• Use lists when you...
  • ...need to add/remove elements dynamically
  • ...need the list operations

Example: Iterating an array with a for loop

string[] names = new string[]
{
  "Harry Potter",
  "Luke Skywalker",
  "Harley Quinn"
};

for (int i = 0; i < names.Length; ++i)
{
  Console.WriteLine(names[i]);
}

Iterating a List

• You can iterate the elements of a list with a for loop the same way as an array
• Just use List.Count instead of Array.Length

  List<int> numbers = new List<int>() { 1, 5, 3 };
  int sum = 0;
  for (int i = 0; i < numbers.Count; ++i)
  {
    sum += numbers[i];
  }
  // sum is now 9
  

foreach Statement

• There is one more statement to use to iterate arrays and lists: the foreach statement
• foreach is useful when you have to execute code for each element of an array or a list, and don't need the corresponding index:

string[] names = new string[]
{
  "Harry Potter",
  "Luke Skywalker",
  "Harley Quinn"
};

foreach (string name in names)
{
  Console.WriteLine(name);
}

Note about foreach

• However, foreach creates a copy of each element in the object so the element cannot be mutated directly:
• Performance-wise, using foreach is also more costly because it uses more memory space
• If you need to change every element, consider creating a new list instead

Exercise 1: Expanding the Console Application

1. Continue working on the command line application you created in Lecture 4, Exercise 1: The Main Loop . Add a new command add which prompts the user to write a note.
2. After the user has inputted the note, save it to a list, and return back to listening to commands.
3. Add another command list which prints all the saved notes.
4. Add one more command remove which prints all the saved notes with the index of the note, and then prompts the user for a number. After entering the number the note with the corresponding index is deleted from the list.

• Note: you can use the int.Parse() method to parse the user input string to an integer

Assignments

Assignments about this topic can be found here