What is an Array?
An array is a collection that stores multiple values of the same type in a single variable. Think of it as a list of numbered boxes, each holding a value.
// Instead of this:
int iNum1 = 10;
int iNum2 = 20;
int iNum3 = 30;
int iNum4 = 40;
int iNum5 = 50;
// Use this:
int[] iNumbers = { 10, 20, 30, 40, 50 };
Array Declaration and Initialization
Method 1: Declare and Initialize with Values
// Integer array
int[] iNumbers = { 1, 2, 3, 4, 5 };
// String array
string[] sNames = { "Alice", "Bob", "Charlie" };
// Double array
double[] dPrices = { 19.99, 29.99, 39.99 };
// Character array
char[] cGrades = { 'A', 'B', 'C', 'D', 'F' };
Method 2: Declare Size, Then Assign Values
// Create array with 5 elements
int[] iNumbers = new int[5];
// Assign values
iNumbers[0] = 10;
iNumbers[1] = 20;
iNumbers[2] = 30;
iNumbers[3] = 40;
iNumbers[4] = 50;
Method 3: Declare, Then Initialize
int[] iNumbers;
iNumbers = new int[] { 1, 2, 3, 4, 5 };
// Or
string[] sNames;
sNames = new string[3];
sNames[0] = "Alice";
sNames[1] = "Bob";
sNames[2] = "Charlie";
Accessing Array Elements
Arrays use zero-based indexing (first element is at index 0).
int[] iNumbers = { 10, 20, 30, 40, 50 };
Console.WriteLine(iNumbers[0]); // 10 (first element)
Console.WriteLine(iNumbers[1]); // 20 (second element)
Console.WriteLine(iNumbers[2]); // 30 (third element)
Console.WriteLine(iNumbers[3]); // 40 (fourth element)
Console.WriteLine(iNumbers[4]); // 50 (fifth element)
// Index: 0 1 2 3 4
// Array: [10, 20, 30, 40, 50]
Modifying Array Elements
int[] iNumbers = { 10, 20, 30 };
Console.WriteLine(iNumbers[1]); // 20
iNumbers[1] = 99; // Change second element
Console.WriteLine(iNumbers[1]); // 99
Array Length
Use the Length property to get the number of elements:
int[] iNumbers = { 10, 20, 30, 40, 50 };
Console.WriteLine($"Array length: {iNumbers.Length}"); // 5
// Last element (length - 1)
Console.WriteLine($"Last element: {iNumbers[iNumbers.Length - 1]}"); // 50
Looping Through Arrays
Using For Loop (Most Common)
int[] iNumbers = { 10, 20, 30, 40, 50 };
for (int i = 0; i < iNumbers.Length; i++)
{
Console.WriteLine($"Element at index {i}: {iNumbers[i]}");
}
// Output:
// Element at index 0: 10
// Element at index 1: 20
// Element at index 2: 30
// Element at index 3: 40
// Element at index 4: 50
Using Foreach Loop
int[] iNumbers = { 10, 20, 30, 40, 50 };
foreach (int iNum in iNumbers)
{
Console.WriteLine(iNum);
}
// Output:
// 10
// 20
// 30
// 40
// 50
For vs Foreach
Use For Loop When: ✅ You need the index ✅ You want to modify array elements ✅ You want to loop in reverse
Use Foreach Loop When: ✅ You just need to read values ✅ You don't need the index ✅ Cleaner, simpler code
Array Operations
Sum of Array Elements
int[] iNumbers = { 10, 20, 30, 40, 50 };
int iSum = 0;
for (int i = 0; i < iNumbers.Length; i++)
{
iSum += iNumbers[i];
}
Console.WriteLine($"Sum: {iSum}"); // Sum: 150
Finding Maximum Value
int[] iNumbers = { 10, 45, 30, 92, 50 };
int iMax = iNumbers[0]; // Assume first is max
for (int i = 1; i < iNumbers.Length; i++)
{
if (iNumbers[i] > iMax)
{
iMax = iNumbers[i];
}
}
Console.WriteLine($"Maximum: {iMax}"); // Maximum: 92
Finding Minimum Value
int[] iNumbers = { 10, 45, 5, 92, 50 };
int iMin = iNumbers[0]; // Assume first is min
for (int i = 1; i < iNumbers.Length; i++)
{
if (iNumbers[i] < iMin)
{
iMin = iNumbers[i];
}
}
Console.WriteLine($"Minimum: {iMin}"); // Minimum: 5
Calculating Average
int[] iNumbers = { 10, 20, 30, 40, 50 };
int iSum = 0;
for (int i = 0; i < iNumbers.Length; i++)
{
iSum += iNumbers[i];
}
double dAverage = (double)iSum / iNumbers.Length;
Console.WriteLine($"Average: {dAverage}"); // Average: 30
Counting Elements
int[] iNumbers = { 10, 25, 30, 15, 40, 22 };
int iCountAbove20 = 0;
for (int i = 0; i < iNumbers.Length; i++)
{
if (iNumbers[i] > 20)
{
iCountAbove20++;
}
}
Console.WriteLine($"Numbers above 20: {iCountAbove20}"); // 4
Practical Examples
Example 1: Student Grades
using System;
namespace StudentGrades
{
class Program
{
static void Main(string[] args)
{
int[] iGrades = new int[5];
// Get grades from user
Console.WriteLine("Enter 5 grades:");
for (int i = 0; i < iGrades.Length; i++)
{
Console.Write($"Grade {i + 1}: ");
iGrades[i] = int.Parse(Console.ReadLine());
}
// Calculate statistics
int iSum = 0;
int iMax = iGrades[0];
int iMin = iGrades[0];
for (int i = 0; i < iGrades.Length; i++)
{
iSum += iGrades[i];
if (iGrades[i] > iMax) iMax = iGrades[i];
if (iGrades[i] < iMin) iMin = iGrades[i];
}
double dAverage = (double)iSum / iGrades.Length;
// Display results
Console.WriteLine("\n--- Grade Statistics ---");
Console.WriteLine($"Total: {iSum}");
Console.WriteLine($"Average: {dAverage:0.00}");
Console.WriteLine($"Highest: {iMax}");
Console.WriteLine($"Lowest: {iMin}");
Console.WriteLine("\nPress any key to exit...");
Console.ReadKey();
}
}
}
Example 2: Search in Array
using System;
namespace ArraySearch
{
class Program
{
static void Main(string[] args)
{
int[] iNumbers = { 15, 23, 8, 42, 16, 7, 35 };
Console.Write("Enter number to search: ");
int iSearch = int.Parse(Console.ReadLine());
bool bFound = false;
int iPosition = -1;
for (int i = 0; i < iNumbers.Length; i++)
{
if (iNumbers[i] == iSearch)
{
bFound = true;
iPosition = i;
break;
}
}
if (bFound)
{
Console.WriteLine($"Found {iSearch} at index {iPosition}");
}
else
{
Console.WriteLine($"{iSearch} not found in array");
}
Console.WriteLine("\nPress any key to exit...");
Console.ReadKey();
}
}
}
Example 3: Reverse Array
using System;
namespace ReverseArray
{
class Program
{
static void Main(string[] args)
{
int[] iNumbers = { 1, 2, 3, 4, 5 };
Console.WriteLine("Original array:");
foreach (int iNum in iNumbers)
{
Console.Write(iNum + " ");
}
// Reverse the array
for (int i = 0; i < iNumbers.Length / 2; i++)
{
int iTemp = iNumbers[i];
iNumbers[i] = iNumbers[iNumbers.Length - 1 - i];
iNumbers[iNumbers.Length - 1 - i] = iTemp;
}
Console.WriteLine("\n\nReversed array:");
foreach (int iNum in iNumbers)
{
Console.Write(iNum + " ");
}
Console.WriteLine("\n\nPress any key to exit...");
Console.ReadKey();
}
}
}
// Output:
// Original array: 1 2 3 4 5
// Reversed array: 5 4 3 2 1
Multi-Dimensional Arrays
2D Arrays (Matrix)
// Declare and initialize
int[,] iMatrix = new int[3, 3]
{
{1, 2, 3},
{4, 5, 6},
{7, 8, 9}
};
// Access elements
Console.WriteLine(iMatrix[0, 0]); // 1
Console.WriteLine(iMatrix[1, 2]); // 6
Console.WriteLine(iMatrix[2, 1]); // 8
// Loop through 2D array
for (int i = 0; i < 3; i++)
{
for (int j = 0; j < 3; j++)
{
Console.Write(iMatrix[i, j] + " ");
}
Console.WriteLine();
}
// Output:
// 1 2 3
// 4 5 6
// 7 8 9
Getting 2D Array Dimensions
int[,] iMatrix = new int[3, 4];
int iRows = iMatrix.GetLength(0); // 3
int iColumns = iMatrix.GetLength(1); // 4
Console.WriteLine($"Rows: {iRows}");
Console.WriteLine($"Columns: {iColumns}");
Array Methods
Array.Sort()
int[] iNumbers = { 45, 12, 78, 23, 56 };
Array.Sort(iNumbers);
foreach (int iNum in iNumbers)
{
Console.Write(iNum + " ");
}
// Output: 12 23 45 56 78
Array.Reverse()
int[] iNumbers = { 1, 2, 3, 4, 5 };
Array.Reverse(iNumbers);
foreach (int iNum in iNumbers)
{
Console.Write(iNum + " ");
}
// Output: 5 4 3 2 1
Array.IndexOf()
int[] iNumbers = { 10, 20, 30, 40, 50 };
int iIndex = Array.IndexOf(iNumbers, 30);
Console.WriteLine($"30 found at index: {iIndex}"); // 2
int iIndex2 = Array.IndexOf(iNumbers, 99);
Console.WriteLine($"99 found at index: {iIndex2}"); // -1 (not found)
Array.Clear()
int[] iNumbers = { 10, 20, 30, 40, 50 };
Array.Clear(iNumbers, 0, iNumbers.Length);
foreach (int iNum in iNumbers)
{
Console.Write(iNum + " ");
}
// Output: 0 0 0 0 0
Practice Exercises
Exercise 1: Sum of Even Numbers
Create an array of 10 numbers, calculate and display the sum of even numbers only.
Exercise 2: Find Second Largest
Find the second largest number in an array.
Exercise 3: Remove Duplicates
Count how many unique numbers are in an array.
Exercise 4: Shift Elements
Shift all elements one position to the right (last element goes to first).
Exercise 5: Matrix Addition
Add two 3x3 matrices together.
Common Mistakes to Avoid
❌ Index Out of Range:
int[] iNumbers = { 10, 20, 30 };
Console.WriteLine(iNumbers[3]); // ERROR! Index 3 doesn't exist
✅ Correct:
int[] iNumbers = { 10, 20, 30 };
Console.WriteLine(iNumbers[2]); // OK (last element)
❌ Wrong Loop Condition:
for (int i = 0; i <= iNumbers.Length; i++) // WRONG! Causes error
✅ Correct:
for (int i = 0; i < iNumbers.Length; i++) // Correct
❌ Forgetting Array Size:
int[] iNumbers = new int[]; // WRONG! Must specify size
✅ Correct:
int[] iNumbers = new int[5]; // OK
// Or
int[] iNumbers = { 1, 2, 3, 4, 5 }; // OK
Key Takeaways
✅ Arrays store multiple values of the same type
✅ Arrays use zero-based indexing (first element is 0)
✅ Use .Length to get number of elements
✅ Use for loop when you need index
✅ Use foreach when you just need values
✅ Always check array bounds to avoid errors
✅ Can use Array class methods for sorting, reversing, etc.
Next Topic: C# Number Systems (Binary, Hexadecimal)