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205 lines
4.2 KiB
Markdown
205 lines
4.2 KiB
Markdown
# Arrays and Lists
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---
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# Overview
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Arrays
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Multidimensional Arrays
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Lists
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Iterating a List
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Foreach
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# Arrays
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Arrays are a collection of variables of the same type, which allocate neighboring memory locations
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A single value in this collection is called an __element__
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Arrays can be __declared __ with square brackets following the type of the elements:
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int[] userIds;
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Declaring an array does not yet allocate space from memory
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# Arrays (continued)
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__Initialize __ an array with a length of 3 using the following syntax:
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int[] userIds = new int[3];
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__Assign a value__ to an element in an array by specifying the index in square brackets:
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userIds[0] = 104;
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Indexing starts from 0, so the above line assigns a value of 104 to the first element of the array
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You can also create an array containing values with one statement:
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string[] names = new string[3] { "Johannes", "Rene", "Ville" };
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The same works without specifying the length in the brackets:
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double[] balances = new double[] { 1.3, 200.3, 9332.14 };
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# Multidimensional Arrays
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C# supports multidimensional arrays:
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char[,] letters = new char[3, 4]
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{
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{'a', 'b', 'c', 'd'},
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{'e', 'f', 'g', 'h'},
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{'i', 'j', 'k', 'l'}
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};
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Console.WriteLine(letters[1, 3]); // Outputs "h"
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# Lists
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The .NET Framework Class Library offers another way to hold multiple variables: the _List _ object
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Declaration and initialization:
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List<int> userIds = new List<int>();
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Value assignment:
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userIds[0] = 22;
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# Lists (continued)
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The key benefit of using Lists lies in its built-in functionalities
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You can see a list of available methods in VSC2019 by following a variable with a dot
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# Arrays vs Lists
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The memory allocation of an array is static/fixed, but list memory is dynamic
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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
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# Arrays vs Lists (continued)
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When to use arrays and when lists?
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Use __arrays __ if you need __high performance__
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Use __lists __ if you need support for __list operations__
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# Iterating an Array With for Loop - Example
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string[] names = new string[]
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{
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"Harry Potter",
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"Luke Skywalker",
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"Harley Quinn"
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};
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for(int i = 0; i < names.Length; ++i)
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{
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Console.WriteLine(names[i]);
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}
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# Iterating a List
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You can iterate the elements of a list with a for loop the same way as an array
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Just use List.Count instead of Array.Length
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List<int> numbers = new List<int>() { 1, 5, 3 };
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int sum = 0;
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for (int i = 0; i < numbers.Count; ++i)
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{
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sum += numbers[i];
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}
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// sum is now 9
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# foreach Statement
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There is one more statement to use to iterate arrays and lists: the foreach statement
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foreach is useful when you have to execute code for each element of an array or a list:
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string[] names = new string[]
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{
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"Harry Potter",
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"Luke Skywalker",
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"Harley Quinn"
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};
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foreach (string name in names)
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{
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Console.WriteLine(name);
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}
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# foreach Statement (continued)
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However, foreach creates a copy of each element in the object so the element cannot be mutated directly:
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Performance-wise, using foreach is also more costly because it uses more memory space
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---
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Instead, create a new list
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# Exercise 1: Expanding the Console Application
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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.
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After the user has inputted the note, it is saved to a list, and the program returns back to listening to commands.
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Add another command "list" which prints all the saved notes.
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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.
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( __Note__ : you can use int.Parse() -method to parse the user input string to an integer)
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# Assignments
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[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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