Array

Author : MD TAREQ HASSAN | Updated : 2021/03/22

Understanding array

array is a collection of a fixed number of values of same type & referred by a common name
an array is a data structure which can store multiple elements of same data type
specific element of an array is accessed by an index
array elements are stored in contiguous memory locations

Types:

1-d
2-d (matrix)
multi-d (nested array, jagged array)

1d-array-allocated-in-memory

2d-array-allocated-in-memory

Row-major order in 2d array

in computing, row-major order and column-major order describe methods for storing multidimensional arrays in linear memory
row-major order is a method of representing multi dimension array in sequential memory
reading memory in contiguous locations is faster than jumping around among locations. Therefore row-major is better and used by most compiler (C and C++ use row-major, Fortran and matlad uses column major)

Finding array element by row-major

offset = row * columnSize + column

Array syntax

// Creating array
int[] foo = new int[10];
var foo = new int[10];            // type inferred

int[,] foo = new int[10, 5];      // multi-dimentional array
var foo = new int[10, 5];         // type inferred

var foo = Array.Empty<int>();
var my1DArray = Array.CreateInstance(typeof(int), 10);

var array = Enumerable.Repeat(String.Empty, 10).ToArray();    // Creating Array From Enumerable with same default value
var array = itemList.ToArray();                               // from any IEnumerable


// Initialization
var foo = new int[] { 1, 2 };      // dimention is inferred
int[] foo = { 1, 2 };              // dimention is inferred
var foo = new int[2] { 1, 2 };     // enforcing dimention

var arr = new int[,] { {1, 2, 3}, {4, 5} };    // dimention is inferred
int[,] arr = { {1, 2, 3}, {4, 5} };            // dimention is inferred


// Properties
arr.Length
arr.IsReadOnly 


// Array class static methods
Array.Sort(arr);
Array.BinarySearch(<overloads>)
Array.Reverse(arr);
Array.Resize(ref arr, n);
Array.Exists<T>(arr, Predicate<T>)
Array.Find(arr, Predicate<T>);
Array.ForEach<T>(arr, Action<T>)

Links

Jagged array

// Jagged array - array of arrays
int[][] jaggedArray = new int[n][]; // 2nd diention is variable, can't be specified
var jaggedArray = new int[][]{
    new int[] {1,3,5,7,9},
    new int[] {0,2,4,6},
    new int[] {11,22}
};

Readonly array

Array.AsReadOnly(T[]) // Returns a read-only wrapper for the specified array.

var items = new int[] {1 ,2, 3, 4}
var readOnlyItems = Array.AsReadOnly(items)

Array to string

var words = new string[] { "hover", "mind" };    // String.Join(string separator, params string[] values);
var str = String.Join(" ", words);

List<int> primes = GetPrimes(count: 20);          
var primeString = String.Join(" ", primes)      // String.Join(string separator, params object[] values);

String to array

string[] words = sentence.Split(" ");
string[] words = sentence.Split(new string[]{ " ", "," });
string[] words = sentence.Split(" ", StringSplitOptions.None);
string[] words = sentence.Split(" ", StringSplitOptions.RemoveEmptyEntries);

string[] words = sentence.Split(new char[]{ ' ' });
string[] words = sentence.Split(new char[]{ ',' }, StringSplitOptions.RemoveEmptyEntries);
string[] words = sentence.Split(",;.".ToCharArray());

var word = "hovermind"
char[] chars = word.ToCharArray()

Array to list

var list = new List<T>(array);
var list = array.ToList();

var list = new List<T>();
list.AddRange(array);

List to array

string[] arr = list.ToArray();

Array to dictionary

var dict = array.ToDictionary(key => key, value => value); // key == value == each item
// to avoid duplicate key, use index as key
var dictionary = array.Select((v, i) => new {Key = i, Value = v}).ToDictionary(o => o.Key, o => o.Value);
// to avoid duplicate key, use hashset
var hashset = new HashSet<string>(array);
var dict = hashset.ToDictionary(key => key, value => value);

Dictionary to array

var array = dict.Values.ToArray();
var array = dict.Keys.ToArray();

T[] array = new T[dict.Count];
dict.Values.CopyTo(array, 0); // starting from 0 index
// using LINQ - has overhead (calling Lambda each time)
var array = dict.Select(pair => pair.Value).ToArray();    // CopyTo() is faster than ToArray()