Module 7 : Arrays

Overview
 Overview of Arrays
 Creating Arrays
 Using Arrays

Overview of Arrays
 What Is an Array?
 Array Notation in C#
 Array Rank
 Accessing Array Elements
 Checking Array Bounds
 Comparing Arrays to Collections

What Is an Array?
 An array is a sequence of elements
 All elements in an array have the same type
 Structs can have elements of different types
 Individual elements are accessed using integer indexes
Integer index 0
(zero)
Integer index 4
(four)

Array Notation in C#
 You declare an array variable by specifying:
 The element type of the array
 The rank of the array
 The name of the variable
This specifies the rank of the array
This specifies the name of the array variable
This specifies the element type of the array
type[ ] name;

Array Rank
 Rank is also known as the array dimension
 The number of indexes associated with each element
Rank 1: One-dimensional
Single index associates with
each long element
Rank 2: Two-dimensional
Two indexes associate with
each int element
long[ ] row; int[,] grid;

Accessing Array Elements
 Supply an integer index for each rank
 Indexes are zero-based
3
2
1
long[ ] row;
...
row[3];
int[,] grid;
...
grid[1,2];

Checking Array Bounds
 All array access attempts are bounds checked
 A bad index throws an IndexOutOfRangeException
 Use the Length property and the GetLength method
row grid
row.GetLength(0)==6
row.Length==6
grid.GetLength(0)==2
grid.GetLength(1)==4
grid.Length==2*4

Comparing Arrays to Collections
 An array cannot resize itself when full
 A collection class, such as ArrayList, can resize
 An array is intended to store elements of one type
 A collection is designed to store elements of different
types
 Elements of an array cannot have read-only access
 A collection can have read-only access
 In general, arrays are faster but less flexible
 Collections are slightly slower but more flexible

 Creating Array Instances
 Initializing Array Elements
 Initializing Multidimensional Array Elements
 Creating a Computed Size Array
 Copying Array Variables
Creating Arrays

Creating Array Instances
 Declaring an array variable does not create an array!
 You must use new to explicitly create the array instance
 Array elements have an implicit default value of zero
row
0 0 0 0
grid
0 0 0
0 0 0
Variable Instance
long[ ] row = new long[4];
int[,] grid = new int[2,3];

Initializing Array Elements
 The elements of an array can be explicitly initialized
 You can use a convenient shorthand
row
0 1 2 3
Equivalent
long[ ] row = new long[4] {0, 1, 2, 3};
long[ ] row = {0, 1, 2, 3};

Initializing Multidimensional Array
Elements
 You can also initialize multidimensional array elements
 All elements must be specified
grid
5 4 3
2 1 0
Implicitly a new int[2,3] array


int[,] grid = {
{5, 4, 3},
{2, 1, 0}
};
int[,] grid = {
{5, 4, 3},
{2, 1 }
};

Creating a Computed Size Array
 The array size does not need to be a compile-time
constant
 Any valid integer expression will work
 Accessing elements is equally fast in all cases
Array size specified by compile-time integer constant:
Array size specified by run-time integer value:
string s = Console.ReadLine();
int size = int.Parse(s);
long[ ] row = new long[size];

Copying Array Variables
 Copying an array variable copies the array variable only
 It does not copy the array instance
 Two array variables can refer to the same array instance
copy
row
0 0 0 0
Variable Instance
long[ ] copy = row;
...
row[0]++;
long value = copy[0];
Console.WriteLine(value);

 Array Properties
 Array Methods
 Returning Arrays from Methods
 Passing Arrays as Parameters
 Command-Line Arguments
 Demonstration: Arguments for Main
 Using Arrays with foreach
 Quiz: Spot the Bugs
Using Arrays

Array Properties
row
0 0 0 0
grid
0 0 0
0 0 0
row.Rank
row.Length
grid.Rank
grid.Length
int[,] grid = new int[2,3];
2
4
1
6

Array Methods
 Commonly used methods
 Sort – sorts the elements in an array of rank 1
 Clear – sets a range of elements to zero or null
 Clone – creates a copy of the array
 GetLength – returns the length of a given dimension
 IndexOf – returns the index of the first occurrence of a
value

Returning Arrays from Methods
 You can declare methods to return arrays
class Example {
static void Main( ) {
int[ ] array = CreateArray(42);
...
}
static int[ ] CreateArray(int size) {
int[ ] created = new int[size];
return created;
}
}

Passing Arrays as Parameters
 An array parameter is a copy of the array variable
 Not a copy of the array instance
class Example2 {
static void Main( ) {
int[ ] arg = {10, 9, 8, 7};
Method(arg);
System.Console.WriteLine(arg[0]);
}
static void Method(int[ ] parameter) {
parameter[0]++;
}
}
This method will modify
the original array
instance created in Main

Command-Line Arguments
 The runtime passes command line arguments to Main
 Main can take an array of strings as a parameter
 The name of the program is not a member of the array
class Example3 {
static void Main(string[ ] args) {
for (int i = 0; i < args.Length; i++) {
System.Console.WriteLine(args[i]);
}
}
}

Using Arrays with foreach
 The foreach statement abstracts away many details of
array handling
class Example4 {
static void Main(string[ ] args) {
foreach (string arg in args) {
System.Console.WriteLine(arg);
}
}
}

Quiz: Spot the Bugs
2
3
4
5
1
int [ ] array;
array = {0, 2, 4, 6};
int [ ] array;
System.Console.WriteLine(array[0]);
int [ ] array = new int[3];
System.Console.WriteLine(array[3]);
int [ ] array = new int[ ];
int [ ] array = new int[3]{0, 1, 2, 3};

Module 7 : Arrays

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Module 7 : Arrays