C# For Loops

What is a For Loop?

A for loop is used when you know exactly how many times you want to repeat code. It's more compact than a while loop for counting scenarios.

Basic Syntax

for (initialization; condition; increment/decrement)
{
    // Code to execute
}

Three Parts:

Initialization: Sets starting value (runs once)
Condition: Checked before each iteration
Increment/Decrement: Updates counter after each iteration

Simple Examples

Example 1: Count from 1 to 5

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

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

Example 2: Count from 0 to 4

for (int i = 0; i < 5; i++)
{
    Console.WriteLine(i);
}

// Output:
// 0
// 1
// 2
// 3
// 4

Example 3: Count Down

for (int i = 5; i >= 1; i--)
{
    Console.WriteLine(i);
}
Console.WriteLine("Blast off!");

// Output:
// 5
// 4
// 3
// 2
// 1
// Blast off!

Example 4: Count by 2s

for (int i = 0; i <= 10; i += 2)
{
    Console.WriteLine(i);
}

// Output: 0, 2, 4, 6, 8, 10

How For Loops Work

for (int i = 1; i <= 3; i++)
{
    Console.WriteLine("Iteration " + i);
}

Step by step: 1. int i = 1 → Initialize i to 1 2. i <= 3? → Check condition (true) 3. Execute code block → Print "Iteration 1" 4. i++ → Increment i to 2 5. i <= 3? → Check condition (true) 6. Execute code block → Print "Iteration 2" 7. i++ → Increment i to 3 8. i <= 3? → Check condition (true) 9. Execute code block → Print "Iteration 3" 10. i++ → Increment i to 4 11. i <= 3? → Check condition (false) 12. Exit loop

Practical Examples from Course

Example 1: Temperature Conversion Table (From Practical Test 1)

using System;

namespace TemperatureTable
{
    class Program
    {
        static void Main(string[] args)
        {
            string sMsg = "Celsius\tFahrenheit\t|\tFahrenheit\tCelsius\n";
            sMsg += "========================================================\n";

            for (double dCelsius = 40.0, dFahrenheit = 120.0; 
                 dFahrenheit >= 30.0; 
                 dCelsius--, dFahrenheit -= 10)
            {
                sMsg += dCelsius.ToString("0.0") + "\t" +
                       CelsiusToFahrenheit(dCelsius).ToString("0.0") + "\t\t|\t" +
                       dFahrenheit.ToString("0.0") + "\t\t" +
                       FahrenheitToCelsius(dFahrenheit).ToString("0.0") + "\n";
            }

            Console.WriteLine(sMsg);
            Console.WriteLine("Press any key to exit...");
            Console.ReadKey();
        }

        static double CelsiusToFahrenheit(double dCelsius)
        {
            return (9.0 / 5) * dCelsius + 32;
        }

        static double FahrenheitToCelsius(double dFahrenheit)
        {
            return (5.0 / 9) * (dFahrenheit - 32);
        }
    }
}

Example 2: Random Number Statistics (From Worksheet 1)

using System;

namespace RandomStats
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.Write("How many random numbers: ");
            int iN = int.Parse(Console.ReadLine());

            Random rnd = new Random();
            int iCountEven = 0, iCountOdd = 0;
            int iSumEven = 0, iSumOdd = 0;

            Console.WriteLine();

            for (int i = 1; i <= iN; i++)
            {
                int iR = rnd.Next(0, 31);  // Random 0-30
                Console.Write(iR + " ");

                if (iR % 2 == 0)
                {
                    iCountEven++;
                    iSumEven += iR;
                }
                else
                {
                    iCountOdd++;
                    iSumOdd += iR;
                }
            }

            double dEvenPercentage = iCountEven * 100.0 / iN;
            double dOddPercentage = iCountOdd * 100.0 / iN;
            double dAvgEven = (iCountEven > 0) ? iSumEven * 1.0 / iCountEven : 0;
            double dAvgOdd = (iCountOdd > 0) ? iSumOdd * 1.0 / iCountOdd : 0;

            Console.WriteLine("\n\nStatistics:");
            Console.WriteLine("Percentage of Even numbers: " + dEvenPercentage.ToString("0.00") + "%");
            Console.WriteLine("Percentage of Odd numbers: " + dOddPercentage.ToString("0.00") + "%");
            Console.WriteLine("Sum of even numbers: " + iSumEven);
            Console.WriteLine("Sum of odd numbers: " + iSumOdd);
            Console.WriteLine("Average of even numbers: " + dAvgEven.ToString("0.00"));
            Console.WriteLine("Average of odd numbers: " + dAvgOdd.ToString("0.00"));

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

Example 3: Multiplication Table

using System;

namespace MultiplicationTable
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.Write("Enter a number: ");
            int iNumber = int.Parse(Console.ReadLine());

            Console.WriteLine($"\nMultiplication Table for {iNumber}:");
            Console.WriteLine("============================");

            for (int i = 1; i <= 10; i++)
            {
                int iResult = iNumber * i;
                Console.WriteLine($"{iNumber} x {i} = {iResult}");
            }

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

Example 4: Sum and Average Calculator

using System;

namespace SumAverage
{
    class Program
    {
        static void Main(string[] args)
        {
            int iSum = 0;
            int iCount = 5;

            Console.WriteLine("Enter 5 numbers:");

            for (int i = 1; i <= iCount; i++)
            {
                Console.Write($"Number {i}: ");
                int iNumber = int.Parse(Console.ReadLine());
                iSum += iNumber;
            }

            double dAverage = (double)iSum / iCount;

            Console.WriteLine($"\nSum: {iSum}");
            Console.WriteLine($"Average: {dAverage:0.00}");

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

Multiple Variables in For Loop

You can declare multiple variables:

// Two variables: one increases, one decreases
for (int i = 0, j = 10; i < 5; i++, j--)
{
    Console.WriteLine($"i = {i}, j = {j}");
}

// Output:
// i = 0, j = 10
// i = 1, j = 9
// i = 2, j = 8
// i = 3, j = 7
// i = 4, j = 6

Break and Continue in For Loops

Break: Exit Loop Early

for (int i = 1; i <= 10; i++)
{
    if (i == 5)
    {
        break;  // Exit loop when i is 5
    }
    Console.WriteLine(i);
}

// Output: 1, 2, 3, 4

Continue: Skip Current Iteration

for (int i = 1; i <= 10; i++)
{
    if (i % 2 == 0)
    {
        continue;  // Skip even numbers
    }
    Console.WriteLine(i);
}

// Output: 1, 3, 5, 7, 9

Common For Loop Patterns

Pattern 1: Sum of Numbers

int iSum = 0;
for (int i = 1; i <= 10; i++)
{
    iSum += i;
}
Console.WriteLine("Sum: " + iSum);  // Sum: 55

Pattern 2: Factorial

int iNumber = 5;
int iFactorial = 1;

for (int i = 1; i <= iNumber; i++)
{
    iFactorial *= i;
}

Console.WriteLine($"Factorial of {iNumber} is {iFactorial}");  // 120

Pattern 3: Print Pattern

// Print stars
for (int i = 1; i <= 5; i++)
{
    Console.WriteLine("*****");
}

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

Pattern 4: Build String

string sResult = "";

for (int i = 1; i <= 5; i++)
{
    sResult += i + " ";
}

Console.WriteLine(sResult);  // Output: 1 2 3 4 5

For Loop vs While Loop

Same Result, Different Syntax

Using For Loop:

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

Using While Loop:

int i = 1;
while (i <= 5)
{
    Console.WriteLine(i);
    i++;
}

When to Use Which?

Use For Loop When: ✅ You know the number of iterations ✅ You have a counter variable ✅ Simple incrementing/decrementing

Use While Loop When: ✅ Number of iterations is unknown ✅ Condition-based looping ✅ Input validation

Infinite For Loop

// Missing increment - infinite loop!
for (int i = 1; i <= 5; )  // No i++
{
    Console.WriteLine(i);  // Prints 1 forever
}

// Intentional infinite loop
for (;;)
{
    Console.WriteLine("This runs forever");
    // Need break statement to exit
}

Practice Exercises

Exercise 1: Even Numbers

Print all even numbers from 1 to 20.

Exercise 2: Countdown Timer

Create a countdown from 10 to 1, printing "GO!" at the end.

Exercise 3: Power Calculator

Calculate 2^10 (2 to the power of 10) using a loop.

Exercise 4: Grade Statistics

Ask user for 10 grades, calculate and display: - Total sum - Average - Number of grades above 70

Exercise 5: Fibonacci Sequence

Print first 10 numbers of Fibonacci sequence: 0, 1, 1, 2, 3, 5, 8, 13, 21, 34...

Common Mistakes to Avoid

❌ Off-by-one errors:

// Prints 0-4 (5 numbers)
for (int i = 0; i < 5; i++)

// Prints 0-5 (6 numbers)
for (int i = 0; i <= 5; i++)

❌ Wrong increment:

for (int i = 1; i <= 10; i--)  // Infinite loop! i never reaches 10

✅ Correct:

for (int i = 1; i <= 10; i++)  // Correct increment

❌ Modifying loop variable inside:

for (int i = 0; i < 10; i++)
{
    i = i + 2;  // Confusing! Avoid this
}

✅ Better:

for (int i = 0; i < 10; i += 3)  // Clear increment
{
    Console.WriteLine(i);
}

❌ Semicolon after for:

for (int i = 0; i < 5; i++);  // WRONG! Empty loop
{
    Console.WriteLine(i);  // This runs once after loop
}