16. Arrays Lists Stacks Queues

Arrays, Lists, Stacks, Queues
Processing Sequences of Elements
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Table of Contents
1. Declaring and Creating Arrays
2. Accessing Array Elements
3. Reading and Printing Arrays at the Console
4. Iterating over Arrays Using for and foreach
5. Resizable Arrays: List<T>
6. Other Structures: Stack<T> and Queue<T>
7. LINQ Extension Methods for Collections
2

What are Arrays?
 An array is a sequence of elements
 All elements are of the same type
 The order of the elements is fixed
 Has fixed size (Array.Length)
0 1 2 3 4Array of 5
elements
Element index
(position)
Element of an array
… … … … …
4

Declaring Arrays
 Declaration defines the type of the elements
 Square brackets [] mean "array"
 Examples:
 Declaring array of integers:
 Declaring array of strings:
int[] intArr;
string[] stringArr;
5

Creating Arrays
 Use the operator new
 Specify the array length (fixed number of elements)
 Example: creating (allocating) array of 5 integers:
myIntArray = new int[5];
myIntArray
managed heap
(dynamic memory)
0 1 2 3 4
… … … … …
6

Creating and Initializing Arrays
 Creating and initializing can be done in a single statement:
 The new operator is not required when using curly brackets
initialization
myIntArray = {1, 2, 3, 4, 5};
myIntArray
managed heap
(dynamic memory)
0 1 2 3 4
1 2 3 4 5
7

8
 Creating an array to hold the names of the days of the week
Creating Array – Example
string[] daysOfWeek =
{
"Monday",
"Tuesday",
"Wednesday",
"Thursday",
"Friday",
"Saturday",
"Sunday"
};

Accessing Array Elements
Read and Modify Elements by Index

How to Access Array Element?
 Array elements are accessed
 Using the square brackets operator [] (indexer)
 Array indexer takes element’s index as parameter
 The first element has index 0
 The last element has index Length-1
 Elements can be retrieved and changed by the [] operator
10
string[] arr = new string[2];
string arr[1] = "Maria";
arr[0] = arr[1];

Accessing Array Elements – Examples
11
string[] towns = { "Sofia", "Varna", "Bourgas" };
Console.WriteLine(towns); // System.String[]
Console.WriteLine(towns.Length); // 3
Console.WriteLine(towns[0]); // Sofia
Console.WriteLine(string.Join(", ", towns)); // Sofia, Varna,
Bourgas
towns[0] = "Pleven";
towns[2] = null;
Console.WriteLine(string.Join(", ", towns)); // Pleven, Varna,
Console.WriteLine(towns[3]); // IndexOutOfRangeException
towns.Length = 4; // Length is read-only

Accessing Elements By Index
Live Demo

Arrays: Input and Output
Reading and Printing Arrays on the Console

14
 First, read from the console the length of the array
 Next, create the array of given size n and read its elements:
Reading Arrays From the Console
int n = int.Parse(Console.ReadLine());
int[] arr = new int[n];
for (int i = 0; i < n; i++)
{
arr[i] = int.Parse(Console.ReadLine());
}

15
 We can also read all array values separated by a space
 Or even write the above at a single line:
Reading Array Values at a Single Line
string values = Console.ReadLine();
string[] items = values.Split(' ');
int[] arr = new int[items.Length];
for (int i = 0; i < items.Length; i++)
{
arr[i] = int.Parse(items[i]);
}
int[] arr = Console.ReadLine().Split(' ')
.Select(int.Parse).ToArray();

Printing Arrays on the Console
 Process all elements of the array
 Print each element to the console
 Separate elements with white space or a new line
string[] array = {"one", "two", "three"};
// Process all elements of the array
for (int index = 0; index < array.Length; index++)
{
// Print each element on a separate line
Console.WriteLine(element[{0}] = {1}",
index, array[index]);
}
16

Printing with String.Join(…) / ForEach(…)
 Use string.Join(separator, collection) to print an array:
 Or use the functional-style for-each:
int[] arr = { 1, 2, 3 };
Console.WriteLine(string.Join(", ", arr)); // 1, 2, 3
string[] strings = { "one", "two", "three" };
Console.WriteLine(string.Join("-", strings)); // one-two-three
17
int[] arr = { 1, 2, 3 };
arr.ToList().ForEach(a => Console.WriteLine(a));

Processing Array Elements
Using for and foreach

20
 Use for loop to process an array when
 Need to keep track of the index
 Processing is not strictly sequential from the first to the last
 In the loop body use the element at the loop index
(array[index]):
Processing Arrays: for Statement
{
squares[index] = array[index] * array[index];
}

21
 Printing array of integers in reversed order:
 Initialize array elements with their index:
Processing Arrays Using for Loop – Examples
int[] arr = new int[] {1, 2, 3, 4, 5};
Console.WriteLine("Reversed: ");
for (int i = array.Length - 1; i >= 0; i--)
Console.Write(array[i] + " ");
// Result: 5 4 3 2 1
{
array[index] = index;
}

Processing Arrays: foreach
 How the foreach loop works?
 type – the type of the element
 value – local name of a variable
 collection – the collection/array to iterate
 Used when no indexing is needed
 All elements are accessed sequentially
 Elements can not be modified (read only)
foreach (type value in collection)
22

23
 Print all elements of a string[] array:
Processing Arrays Using foreach – Example
string[] capitals =
{
"Sofia",
"Washington",
"London",
"Paris"
};
foreach (string capital in capitals)
{
Console.WriteLine(capital);
}

Static Methods in the Array Class
 Array.Clear(arr, index, length) – deletes all elements
 Array.ConvertAll(arr, convertFunction)
 Converts an array of one type to an array of another type
 Array.IndexOf(arr, item)
 Finds an item in array (returns the first occurrence index or -1)
 Array.Reverse() – reverses the elements order
 Array.Sort() – sorts an array in increasing order
25

Array Static Methods
Live Demo

29
 List<T> – array that can resize dynamically
 Can add / remove / insert elements
 Also provides indexed access with [] (like arrays)
 T is the type for the list elements
 E.g. List<int> will hold integers, List<object> will hold objects
 Basic properties
 Count – returns the current size of the list
 Capacity – returns the current capacity of the list
Lists (Resizable Arrays)

30
 This code:
 Is like this:
 The main difference
 The number of elements in List<T> is variable
List Example
List<int> intList = new List<int>();
for (int i = 0; i < 5; i++)
intList.Add(i);
int[] intArray = new int[5];
for (int i = 0; i < 5; i++)
intArray[i] = i;

32
 List<T> internally keeps its items in an array – T[]
 It has bigger Capacity (buffer) than the elements inside (Count)
 Typically "Add" is fast  just adds an element is an empty cell
 Resizing is slow, but happens rarely: log2(n) times
How The List<T> Works?
3 -2 5 11 9 -1 7 33 8
List<int>:
Count = 9
Capacity = 15
Capacity
used buffer
(Count)
unused
buffer

List<T> Methods
Brief Overview

35
 List<T>.Add(item) – adds an item to the end
 List<T>.AddRange(items) – adds many items at the end
 List<T>.Clear() – clears the list
 List<T>.IndexOf(item) – search for item
 Return the first occurrence index or -1
 List<T>.Insert(item, index) – inserts an item at position
 List<T>.InsertRange(items, index)
 Inserts many items at specified position
List<T> Methods (1)

36
 List<T>.Remove()
 Removes the first occurrence of a specific item
 List<T>.RemoveAt()
 Removes the element at the specified index
 List<T>.Reverse()
 List<T>.Sort()
 List<T>.ToArray()
List<T> Methods (2)

37
 Stack is last-in-first-out (LIFO) collection of elements
Stack – LIFO Data Structure

38
 Stack<T> holds a stack of elements
 Count – the number of elements in the Stack<T>
 Peek() – check the value of the last element
 Pop() – return the last element and remove it from the stack
 Push() – add an element to the Stack<T>
 ToArray() – converts list to array
 Contains() – determines whether an element is in the stack
Stack<T>

40
 Queue is first-in-first-out (FIFO) collection of elements
Queue – FIFO Data Structure

41
 Queue<T> holds a queue of elements:
 Enqueue() – add an element at the end of the queue
 Dequeue() – remove the first element and remove it
 Count – return the number of elements in the queue
 Peek() – check the value of the first element
 ToArray() – converts the queue to array
 Contains() – checks whether an element is in the queue
Queue<T>

LINQ Extension Methods
for Collections
Brief Overview

44
 What are extension methods?
 Attach functionality to existing types
 How to use LINQ extension methods?
 Add "using System.Linq;" at the start of your C# file
 Call them as you call a regular instance method
Using System.LINQ with collections
var array = {1, 2, 3, 4, 5};
Console.WriteLine(array.Sum()); // 15
Console.WriteLine(array.Max()); // 5

45
 Distinct() – returns the distinct elements from a sequence
 First() and FirstOrDefault()
 Intersect() and Union()
 Min(), Max(), Sum() and Average()
 Skip() – bypasses a specified number of elements in a
sequence and then returns the remaining elements
 Take() – returns a specified number of contiguous elements
from the start of a sequence

Live Demo

Summary
 Arrays are a fixed-length sequences of elements of the same type
 Array elements are accessible by index (read / modify)
 Iterate over array elements with for and foreach loops
 List<T> holds resizable arrays
 Good when we don't know the number of elements initially
 Stack<T> and Queue<T> provides LIFO and FIFO lists
 LINQ extension methods attach additional functionality for
collection processing
47

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Arrays, Lists, Stacks, Queues
https://softuni.bg/courses/advanced-csharp

License
 This course (slides, examples, demos, videos, homework, etc.)
is licensed under the "Creative Commons Attribution-
NonCommercial-ShareAlike 4.0 International" license
 Attribution: this work may contain portions from
 "Fundamentals of Computer Programming with C#" book by Svetlin Nakov & Co. under CC-BY-SA license
 "C# Part I" course by Telerik Academy under CC-BY-NC-SA license
 "C# Part II" course by Telerik Academy under CC-BY-NC-SA license
49

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