Lesson 5: Arrays

An array is a data structure that contains a number of variables, which are accessed through computed indices. The variables contained in an array, also called the elements of the array, are all of the same type, and this type is called the element type of the array.

An array has a rank that determines the number of indices associated with each array element. The rank of an array is also referred to as the dimensions of the array. An array with a rank of one is called a single-dimensional array. An array with a rank greater than one is called a multi-dimensional array. Specific sized multidimensional arrays are often referred to as two-dimensional arrays, three-dimensional arrays, and so on.

Each dimension of an array has an associated length that is an integral number greater than or equal to zero. The dimension lengths are not part of the type of the array, but rather are established when an instance of the array type is created at run-time. The length of a dimension determines the valid range of indices for that dimension: For a dimension of length N, indices can range from 0 to N–1 inclusive. The total number of elements in an array is the product of the lengths of each dimension in the array. If one or more of the dimensions of an array have a length of zero, the array is said to be empty.

The element type of an array can be any type, including an array type.

Array creation

Array instances are created by array-creation-expressions or by field or local variable declarations that include an array-initializer.

When an array instance is created, the rank and length of each dimension are established and then remain constant for the entire lifetime of the instance. In other words, it is not possible to change the rank of an existing array instance, nor is it possible to resize its dimensions.

An array instance is always of an array type. The System.Array type is an abstract type that cannot be instantiated.

Elements of arrays created by array-creation-expressions are always initialized to their default value.

Single-Dimensional Arrays

You can declare a single-dimensional array of five integers as shown in the following example:

int[] array = new int[5];

This array contains the elements from array[0] to array[4]. The new operator is used to create the array and initialize the array elements to their default values. In this example, all the array elements are initialized to zero.

An array that stores string elements can be declared in the same way. For example:

string[] stringArray = new string[6];

Array Initialization

It is possible to initialize an array upon declaration, in which case, the rank specifier is not needed because it is already supplied by the number of elements in the initialization list. For example:

int[] array1 = new int[] { 1, 3, 5, 7, 9 };

A string array can be initialized in the same way. The following is a declaration of a string array where each array element is initialized by a name of a day:

string[] weekDays = { "Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat" };

When you initialize an array upon declaration, you can use the following shortcuts:

int[] array2 = { 1, 3, 5, 7, 9 };

string[] weekDays2 = { "Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat" };

It is possible to declare an array variable without initialization, but you must use the new operator when you assign an array to this variable. For example:

int[] array3;

array3 = new int[] { 1, 3, 5, 7, 9 }; // OK

//array3 = {1, 3, 5, 7, 9}; // Error