Arrays

One-Dimensional Array Declaration and Initialization

Arrays in C# are reference types stored on the heap. Declare an array using the type followed by square brackets, with zero-based indexing.

Example

int[] nums1 = new int[5];
int[] nums2 = new int[5] { 10, 20, 30, 40, 50 };
int[] nums3 = { 1, 2, 3, 4, 5 };
string[] names = { "Alice", "Bob", "Carol" };

Console.WriteLine(nums1[0]);
Console.WriteLine(nums2[2]);
Console.WriteLine(nums3.Length);
Console.WriteLine(names[1]);

0
30
5
Bob

Warning: new int[5] creates an array whose elements are automatically initialized to default values (0 for int, null for string).

Index Access and Bounds Checking

C# arrays are accessed by index, and the runtime performs bounds checking. An out-of-range index throws IndexOutOfRangeException, unlike undefined behavior in C.

Example

int[] nums = { 10, 20, 30, 40, 50 };

nums[0] = 99;
Console.WriteLine(nums[0]);
Console.WriteLine(nums[nums.Length - 1]);

try
{
    int val = nums[5];
}
catch (IndexOutOfRangeException)
{
    Console.WriteLine("Index out of range!");
}

99
50
Index out of range!

Tip: Use the Length property to get the array length, rather than sizeof as in C.

Common Array Class Methods

The Array class provides static methods for sorting, reversing, searching, copying, and more.

Example

int[] nums = { 5, 3, 8, 1, 9, 2 };

Array.Sort(nums);
Console.WriteLine(string.Join(", ", nums));

Array.Reverse(nums);
Console.WriteLine(string.Join(", ", nums));

int idx = Array.IndexOf(nums, 8);
Console.WriteLine(idx);

int[] copy = new int[3];
Array.Copy(nums, copy, 3);
Console.WriteLine(string.Join(", ", copy));

Array.Clear(nums, 0, 2);
Console.WriteLine(string.Join(", ", nums));

Array.Resize(ref nums, 8);
Console.WriteLine(nums.Length);

1, 2, 3, 5, 8, 9
9, 8, 5, 3, 2, 1
1
9, 8, 5
0, 0, 5, 3, 2, 1
8

Tip: Clear sets elements in the specified range to their default values rather than removing them. Resize can change the array length.

Multidimensional Arrays (Rectangular Arrays)

Multidimensional arrays, also called rectangular arrays, have the same length for every row. Use commas to separate dimensions in the declaration, e.g. int[,] for a 2D array.

Example

int[,] matrix = new int[3, 4];
matrix[0, 0] = 1;
matrix[1, 2] = 5;
matrix[2, 3] = 9;

int[,] grid = {
    { 1, 2, 3 },
    { 4, 5, 6 },
    { 7, 8, 9 }
};

Console.WriteLine(grid[0, 1]);
Console.WriteLine(grid[2, 0]);
Console.WriteLine(grid.Length);
Console.WriteLine(grid.Rank);

2
7
9
2

Tip: Rank returns the number of dimensions, while Length returns the total number of elements (the product of all dimensions).

Jagged Arrays

A jagged array is an array of arrays, where each row can have a different length. Declare it with multiple sets of square brackets.

Example

int[][] jagged = new int[3][];
jagged[0] = new int[] { 1, 2 };
jagged[1] = new int[] { 3, 4, 5, 6 };
jagged[2] = new int[] { 7, 8, 9 };

Console.WriteLine(jagged[0].Length);
Console.WriteLine(jagged[1][2]);
Console.WriteLine(jagged[2][0]);

int[][] scores = {
    new int[] { 90, 85 },
    new int[] { 78, 92, 88 },
    new int[] { 95 }
};

Console.WriteLine(scores[1][1]);

2
5
7
92

Warning: Jagged arrays differ from rectangular arrays: int[,] is a contiguous 2D array in memory, while int[][] is a 1D array whose elements are independent arrays.

Iterating Arrays with foreach

The foreach statement simplifies array traversal by eliminating manual index management. It is suitable for read-only iteration.

Example

string[] fruits = { "Apple", "Banana", "Orange", "Grape" };

foreach (string fruit in fruits)
{
    Console.WriteLine(fruit);
}

int[,] matrix = {
    { 1, 2, 3 },
    { 4, 5, 6 }
};

foreach (int val in matrix)
{
    Console.Write(val + " ");
}
Console.WriteLine();

Apple
Banana
Orange
Grape
1 2 3 4 5 6

Tip: You cannot modify array elements inside a foreach loop. Use a for loop if you need to change values.

Arrays as Method Parameters

Arrays are reference types, so passing an array to a method passes the reference. Modifications to array elements inside the method affect the original array.

Example

void PrintArray(int[] arr)
{
    foreach (int n in arr)
    {
        Console.Write(n + " ");
    }
    Console.WriteLine();
}

void DoubleValues(int[] arr)
{
    for (int i = 0; i < arr.Length; i++)
    {
        arr[i] *= 2;
    }
}

int[] nums = { 1, 2, 3, 4, 5 };
PrintArray(nums);
DoubleValues(nums);
PrintArray(nums);

int Sum(int[] arr)
{
    int total = 0;
    foreach (int n in arr)
    {
        total += n;
    }
    return total;
}

Console.WriteLine(Sum(nums));

1 2 3 4 5
2 4 6 8 10
30

Warning: Because arrays are passed by reference, changes made by DoubleValues are directly reflected in the caller's array.

❓ FAQ

📖 Summary

• C# arrays are reference types declared with Type[], with zero-based indexing
• Length returns the element count; runtime bounds checking throws IndexOutOfRangeException on out-of-range access
• The Array class provides static methods such as Sort, Reverse, IndexOf, Copy, Clear, and Resize
• Multidimensional arrays int[,] have equal row lengths; jagged arrays int[][] allow varying row lengths
• foreach is ideal for read-only iteration, including multidimensional arrays
• Arrays are passed by reference to methods, so modifications inside the method affect the original array

📝 Exercises

1. Declare an array of 10 integers, assign values 1 through 10 using a loop, then print all elements in reverse order
2. Use Array.Sort to sort a string array alphabetically, then print the result
3. Create a 3x3 2D integer array, compute and print the sum of the diagonal elements
4. Create a jagged array storing grades for 3 students (each with a different number of subjects), then compute each student's average score
5. Write a method int FindMax(int[] arr) that accepts an array and returns the maximum value

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