C# Nested Loops

What are Nested Loops?

A nested loop is a loop inside another loop. The inner loop completes all its iterations for each iteration of the outer loop.

Basic Concept

for (outer loop)
{
    for (inner loop)
    {
        // Code executes for each combination
    }
}

Simple Example

for (int i = 1; i <= 3; i++)
{
    for (int j = 1; j <= 2; j++)
    {
        Console.WriteLine($"i = {i}, j = {j}");
    }
}

// Output:
// i = 1, j = 1
// i = 1, j = 2
// i = 2, j = 1
// i = 2, j = 2
// i = 3, j = 1
// i = 3, j = 2

How Nested Loops Work

For each iteration of the outer loop, the inner loop runs completely.

for (int row = 1; row <= 2; row++)      // Outer: runs 2 times
{
    for (int col = 1; col <= 3; col++)  // Inner: runs 3 times per outer
    {
        Console.Write("*");
    }
    Console.WriteLine();  // New line after inner loop
}

// Output:
// ***
// ***

Breakdown: - row=1: col runs 1,2,3 → prints *** - row=2: col runs 1,2,3 → prints ***

Pattern Printing Examples from Course

Example 1: Increasing Letters (From Practical Test 4)

using System;

namespace Pattern1
{
    class Program
    {
        static void Main(string[] args)
        {
            char ch = 'A';

            for (int i = 1; i <= 5; i++)
            {
                for (int j = 1; j <= i; j++)
                {
                    Console.Write(ch + " ");
                }
                Console.WriteLine();
                ch++;
            }

            Console.WriteLine("\nPress any key to exit...");
            Console.ReadKey();
        }
    }
}

// Output:
// A 
// B B 
// C C C 
// D D D D 
// E E E E E

Example 2: Alternating Pattern (From Practical Test 4)

using System;

namespace Pattern2
{
    class Program
    {
        static void Main(string[] args)
        {
            string sJs = "Js";
            string sCss = "CSS";

            for (int i = 0; i < 5; i++)
            {
                for (int j = 0; j <= i; j++)
                {
                    if (j % 2 == 0)
                    {
                        Console.Write(sJs + " ");
                    }
                    else
                    {
                        Console.Write(sCss + " ");
                    }
                }
                Console.WriteLine();
            }

            Console.WriteLine("\nPress any key to exit...");
            Console.ReadKey();
        }
    }
}

// Output:
// Js 
// Js CSS 
// Js CSS Js 
// Js CSS Js CSS 
// Js CSS Js CSS Js

Example 3: Decreasing Numbers (From Practical Test 4)

using System;

namespace Pattern3
{
    class Program
    {
        static void Main(string[] args)
        {
            for (int i = 5; i >= 1; i--)
            {
                for (int j = 0; j < i; j++)
                {
                    Console.Write(i + " ");
                }
                Console.WriteLine();
            }

            Console.WriteLine("\nPress any key to exit...");
            Console.ReadKey();
        }
    }
}

// Output:
// 5 5 5 5 5 
// 4 4 4 4 
// 3 3 3 
// 2 2 
// 1

More Pattern Examples from Worksheets

Pattern 1: Increasing Numbers (From Worksheet 4)

int iA = 1;

for (int i = 0; i <= 4; i++)
{
    for (int j = 0; j < i; j++)
    {
        Console.Write(iA + " ");
        iA++;
    }
    Console.WriteLine();
}

// Output:
// (nothing - i=0)
// 1 
// 2 3 
// 4 5 6 
// 7 8 9 10

Pattern 2: C and # Alternating (From Worksheet 4)

for (int i = 0; i < 5; i++)
{
    for (int j = 0; j <= i; j++)
    {
        if (j % 2 == 0)
        {
            Console.Write("C ");
        }
        else
        {
            Console.Write("# ");
        }
    }
    Console.WriteLine();
}

// Output:
// C 
// C # 
// C # C 
// C # C # 
// C # C # C

Pattern 3: Decreasing Letters (From Worksheet 4)

char ch = 'A';

for (int i = 5; i >= 1; i--)
{
    for (int j = 0; j < i; j++)
    {
        Console.Write(ch + " ");
    }
    Console.WriteLine();
    ch++;
}

// Output:
// A A A A A 
// B B B B 
// C C C 
// D D 
// E

Common Nested Loop Patterns

Pattern: Rectangle

for (int i = 0; i < 3; i++)
{
    for (int j = 0; j < 5; j++)
    {
        Console.Write("* ");
    }
    Console.WriteLine();
}

// Output:
// * * * * * 
// * * * * * 
// * * * * *

Pattern: Right Triangle

for (int i = 1; i <= 5; i++)
{
    for (int j = 1; j <= i; j++)
    {
        Console.Write("* ");
    }
    Console.WriteLine();
}

// Output:
// * 
// * * 
// * * * 
// * * * * 
// * * * * *

Pattern: Inverted Triangle

for (int i = 5; i >= 1; i--)
{
    for (int j = 1; j <= i; j++)
    {
        Console.Write("* ");
    }
    Console.WriteLine();
}

// Output:
// * * * * * 
// * * * * 
// * * * 
// * * 
// *

Pattern: Number Pyramid

for (int i = 1; i <= 5; i++)
{
    for (int j = 1; j <= i; j++)
    {
        Console.Write(j + " ");
    }
    Console.WriteLine();
}

// Output:
// 1 
// 1 2 
// 1 2 3 
// 1 2 3 4 
// 1 2 3 4 5

Pattern: Multiplication Table

Console.WriteLine("Multiplication Table (1-10)");
Console.WriteLine("============================");

for (int i = 1; i <= 10; i++)
{
    for (int j = 1; j <= 10; j++)
    {
        Console.Write($"{i * j,4}");  // 4-character width
    }
    Console.WriteLine();
}

Practical Applications

Application 1: Grid/Matrix Processing

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

Console.WriteLine("Matrix:");
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

Application 2: Combinations

string[] sColors = { "Red", "Green", "Blue" };
string[] sSizes = { "Small", "Medium", "Large" };

Console.WriteLine("All Combinations:");

for (int i = 0; i < sColors.Length; i++)
{
    for (int j = 0; j < sSizes.Length; j++)
    {
        Console.WriteLine($"{sSizes[j]} {sColors[i]}");
    }
}

// Output:
// Small Red
// Medium Red
// Large Red
// Small Green
// Medium Green
// Large Green
// Small Blue
// Medium Blue
// Large Blue

Application 3: Calendar Grid

Console.WriteLine("Calendar Week View");
Console.WriteLine("Sun Mon Tue Wed Thu Fri Sat");

int iDay = 1;

for (int week = 0; week < 5; week++)
{
    for (int day = 0; day < 7; day++)
    {
        if (iDay <= 31)
        {
            Console.Write($"{iDay,3} ");
            iDay++;
        }
    }
    Console.WriteLine();
}

Three-Level Nested Loops

You can nest loops more than two levels deep (though it gets complex):

for (int i = 1; i <= 2; i++)
{
    for (int j = 1; j <= 2; j++)
    {
        for (int k = 1; k <= 2; k++)
        {
            Console.WriteLine($"i={i}, j={j}, k={k}");
        }
    }
}

// Output:
// i=1, j=1, k=1
// i=1, j=1, k=2
// i=1, j=2, k=1
// i=1, j=2, k=2
// i=2, j=1, k=1
// i=2, j=1, k=2
// i=2, j=2, k=1
// i=2, j=2, k=2

Break and Continue in Nested Loops

Break: Exits Only Inner Loop

for (int i = 1; i <= 3; i++)
{
    for (int j = 1; j <= 3; j++)
    {
        if (j == 2)
        {
            break;  // Only exits inner loop
        }
        Console.Write($"({i},{j}) ");
    }
    Console.WriteLine();
}

// Output:
// (1,1) 
// (2,1) 
// (3,1)

Continue: Skips Inner Loop Iteration

for (int i = 1; i <= 3; i++)
{
    for (int j = 1; j <= 3; j++)
    {
        if (j == 2)
        {
            continue;  // Skip j=2
        }
        Console.Write($"({i},{j}) ");
    }
    Console.WriteLine();
}

// Output:
// (1,1) (1,3) 
// (2,1) (2,3) 
// (3,1) (3,3)

Practice Exercises

Exercise 1: Square Pattern

Create a 7x7 square of asterisks.

Exercise 2: Number Square

Create a pattern:

Exercise 3: Pyramid

Create a centered pyramid:

    *
   ***
  *****
 *******
*********

Exercise 4: Checkerboard

Create a checkerboard pattern with X and O:

X O X O X
O X O X O
X O X O X
O X O X O
X O X O X

Exercise 5: Diamond

Create a diamond shape using asterisks.

Performance Considerations

Nested loops can be slow with large datasets:

// This is O(n²) - slow for large n!
for (int i = 0; i < 1000; i++)
{
    for (int j = 0; j < 1000; j++)
    {
        // 1,000,000 iterations!
    }
}

Tips: - Avoid unnecessary nested loops - Break early when possible - Consider if there's a better algorithm

Common Mistakes to Avoid

❌ Using same variable name:

for (int i = 0; i < 5; i++)
{
    for (int i = 0; i < 3; i++)  // WRONG! Same variable name
    {
        Console.Write(i);
    }
}

✅ Correct:

for (int i = 0; i < 5; i++)
{
    for (int j = 0; j < 3; j++)  // Different variable
    {
        Console.Write(j);
    }
}

❌ Forgetting Console.WriteLine():

for (int i = 0; i < 3; i++)
{
    for (int j = 0; j < 3; j++)
    {
        Console.Write("* ");
    }
    // Missing WriteLine! All on one line
}
// Output: * * * * * * * * *