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# Arrays and Lists ---
marp: true
![](imgs/5%20Arrays%20and%20Lists_0.png) paginate: true
math: mathjax
theme: buutti
title: 5. Arrays and Lists
--- ---
# Overview # Arrays and Lists
Arrays
Multidimensional Arrays
Lists
Iterating a List
Foreach
# Arrays
Arrays are a collection of variables of the same type, which allocate neighboring memory locations
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
# Arrays (continued)
__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 containing 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:
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 Class Library offers another way to hold multiple variables: the _List _ object
Declaration and initialization:
List<int> userIds = new List<int>();
Value assignment:
userIds[0] = 22;
# Lists (continued)
The key benefit of using Lists lies in its built-in functionalities
![](imgs/5%20Arrays%20and%20Lists_1.png)
You can see a list of available methods in VSC2019 by following a variable with a dot
# Arrays vs Lists
The memory allocation of an array is static/fixed, but list memory is dynamic <!-- headingDivider: 5 -->
<!-- class: invert -->
This allows the flexible mutation of lists, meaning you can always add or remove and hence change change the length of the list without issues ## Overview
![](imgs/5%20Arrays%20and%20Lists_2.png) * Arrays
* Multidimensional arrays
* Lists
* Iterating a list
* Foreach
# Arrays vs Lists (continued) ## Arrays
When to use arrays and when lists? * 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:
```csharp
int[] userIds;
```
* Declaring an array does not yet allocate space from memory
Use __arrays __ if you need __high performance__ ### Initializing arrays
Use __lists __ if you need support for __list operations__ * *__Initialize__* an array with a length of 3 using the following syntax:
```csharp
int[] userIds = new int[3];
```
* *__Assign a value__* to an element in an array by specifying the index in square brackets:
```csharp
userIds[0] = 104;
```
* Indexing starts from 0, so the above line assigns a value of `104` to the ***first*** element of the array
# Iterating an Array With for Loop - Example ---
* You can also create an array and instantly populate it with values with one statement:
```csharp
string[] names = new string[3] { "Johannes", "Rene", "Ville" };
```
* The same works without specifying the length in the brackets:
```csharp
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
```csharp
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`](https://learn.microsoft.com/en-us/dotnet/api/system.collections.generic?view=net-9.0) offers another structure to contain multiple variables: the [List](https://learn.microsoft.com/en-us/dotnet/api/system.collections.generic.list-1?view=net-9.0) class
* Importing the library:
```csharp
using System.Collections.Generic;
```
* Declaration and initialization:
```csharp
List<int> userIds = new List<int>();
```
* Value assignment (exactly like arrays):
```csharp
userIds[0] = 22;
```
### Why lists?
* The key benefit of using Lists lies in its built-in functionalities
![](imgs/5%20Arrays%20and%20Lists_1.png)
* 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
```csharp
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
<div class='columns21' markdown='1'>
<div markdown='1'>
```csharp
string[] names = new string[] string[] names = new string[]
{ {
"Harry Potter",
"Harry Potter", "Luke Skywalker",
"Harley Quinn"
"Luke Skywalker",
"Harley Quinn"
}; };
for(int i = 0; i < names.Length; ++i) for (int i = 0; i < names.Length; ++i)
{ {
Console.WriteLine(names[i]);
Console.WriteLine(names[i]);
} }
```
![](imgs/5%20Arrays%20and%20Lists_3.png) </div>
<div markdown='1'>
# 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) ![](imgs/5%20Arrays%20and%20Lists_3.png)
{ </div>
</div>
sum += numbers[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`
```csharp
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
```
// sum is now 9 ## `foreach` Statement
# 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:
There is one more statement to use to iterate arrays and lists: the foreach statement <div class='columns21' markdown='1'>
<div markdown='1'>
foreach is useful when you have to execute code for each element of an array or a list:
```csharp
string[] names = new string[] string[] names = new string[]
{ {
"Harry Potter",
"Harry Potter", "Luke Skywalker",
"Harley Quinn"
"Luke Skywalker",
"Harley Quinn"
}; };
foreach (string name in names) foreach (string name in names)
{ {
Console.WriteLine(name);
Console.WriteLine(name);
} }
```
![](imgs/5%20Arrays%20and%20Lists_4.png) </div>
<div markdown='1'>
# foreach Statement (continued)
However, foreach creates a copy of each element in the object so the element cannot be mutated directly:
![](imgs/5%20Arrays%20and%20Lists_5.png) ![](imgs/5%20Arrays%20and%20Lists_4.png)
Performance-wise, using foreach is also more costly because it uses more memory space
---
Instead, create a new list
# Exercise 1: Expanding the Console Application </div>
</div>
Continue working on the command line application you created in "The Main Loop" exercise. Add a new command "add" which prompts the user to write a note. ### Note about `foreach`
After the user has inputted the note, it is saved to a list, and the program returns back to listening to commands. * However, `foreach` creates a copy of each element in the object so the element cannot be mutated directly:
![](imgs/5%20Arrays%20and%20Lists_5.png)
* 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
Add another command "list" which prints all the saved notes. ## Exercise 1: Expanding the Console Application
<!--_class: "exercise invert" -->
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 corresponding note is deleted from the list. 1) Continue working on the command line application you created in [Lecture 4, Exercise 1: The Main Loop ](4-loops#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 int.Parse() -method to parse the user input string to an integer) * *__Note__*: you can use the `int.Parse()` method to parse the user input string to an integer
# Assignments ## Assignments
[Assignments about this topic can be found here](https://gitea.buutti.com/education/academy-assignments/src/branch/master/C%23%20Basics/5.%20Arrays%20&%20Lists) [Assignments about this topic can be found here](https://gitea.buutti.com/education/academy-assignments/src/branch/master/C%23%20Basics/5.%20Arrays%20&%20Lists)

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