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🔥 Functions in C Programming

Complete Beginner to Advanced Guide with Deep Explanation, Logic, Line-by-Line Comments & Output

📘 Part of the C Programming Beginner Series by Learning Growth Hub
✍️ Written by Krishna Popat

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🚀 1️⃣ Introduction to Functions in C

As programs grow larger, writing everything inside main() becomes confusing and difficult to manage.

If you write 500+ lines inside only one function, debugging becomes very hard.

👉 Functions help us:

• Divide programs into smaller parts

• Reuse code

• Improve readability

• Reduce repetition

• Write structured programs

Think of a function like:

Input → Processing → Output

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🎯 Who Should Read This?

✔ Complete beginners
✔ BCA / B.Tech / Diploma students
✔ Students preparing for exams
✔ Anyone learning C fundamentals

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📌 2️⃣ What is a Function?

A function is a block of code that performs a specific task.

Instead of writing the same logic again and again, we define it once and call it whenever needed.

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3️⃣ Structure of a Function

1️⃣ Function Declaration

return_type function_name(parameters);

2️⃣ Function Definition

return_type function_name(parameters)
{
    // body
}

3️⃣ Function Call

function_name(arguments);

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🧠 4️⃣ Basic Example

No Arguments, No Return Value

🔎 Understanding 

• Function does not take any input

• Function does not return anything

• It only performs a task (printing message)

• Control goes: main() → greet() → back to main()

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#include <stdio.h>                // Include standard input-output library

void greet()                      // Define function greet with no return value
{
    printf("Welcome to Learning Growth Hub!\n");   // Print message on screen
}

int main()                        // Main function - execution starts here
{
    greet();                      // Call greet function
    return 0;                     // End program successfully
}

🖥 Output

Welcome to Learning Growth Hub!

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🧩 5️⃣ Types of Functions in C

1️⃣ No arguments, No return value
2️⃣ With arguments, No return value
3️⃣ No arguments, With return value
4️⃣ With arguments, With return value

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🔹 Type 1: No Arguments, No Return Value

🔎 Understanding 

• Function performs a task

• No input is given

• Nothing is returned

• Used for display or printing tasks

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#include <stdio.h>                // Include input-output library

void display()                    // Define function without parameters
{
    printf("Hello Krishna!\n");   // Print greeting message
}

int main()                        // Main function
{
    display();                    // Call display function
    return 0;                     // End program
}

🖥 Output

Hello Krishna!

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🔹 Type 2: With Arguments, No Return Value

🔎 Understanding 

• Function receives values from main

• Performs calculation

• Prints result inside function

• Does not return anything

Flow:
main → add(5,3) → prints result → back to main

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#include <stdio.h>                // Include input-output library

void add(int a, int b)            // Function with two parameters
{
    int sum = a + b;              // Add a and b
    printf("Sum = %d\n", sum);    // Print sum
}

int main()                        // Main function
{
    add(5, 3);                    // Pass 5 and 3 to function
    return 0;                     // End program
}

🖥 Output

Sum = 8

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🔹 Type 3: No Arguments, With Return Value

🔎 Understanding

• Function takes input

• Returns value

• main() stores returned value

• Returned value is used later

Flow:
main → getNumber → return value → main prints it

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#include <stdio.h>                // Include input-output library

int getNumber()                   // Function returning integer
{
    int num;                      // Declare integer variable
    printf("Enter number: ");     // Ask user to enter number
    scanf("%d", &num);            // Store user input in num
    return num;                   // Return num to main
}

int main()                        // Main function
{
    int n = getNumber();          // Call function and store returned value
    printf("You entered: %d", n); // Print value
    return 0;                     // End program
}

🖥 Output

Enter number: 7
You entered: 7

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🔹 Type 4: With Arguments, With Return Value

🔎 Understanding 

• Most commonly used type

• Function receives input

• Performs calculation

• Returns result

• main() prints result

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#include <stdio.h>                // Include input-output library

int multiply(int x, int y)        // Function with parameters and return type
{
    int result = x * y;           // Multiply x and y
    return result;                // Return result
}

int main()                        // Main function
{
    int output = multiply(4, 5);  // Call function and store result
    printf("Result = %d", output); // Print result
    return 0;                     // End program
}

🖥 Output

Result = 20


After understanding normal functions, let’s explore a powerful concept called Recursion.


🔁  Recursion in C 
Recursion is when a function calls itself to solve a smaller part of a problem.
Every recursive function must have:
✔ Base case → stops recursion

✔ Recursive case → calls function again


🔁 Example: Factorial Using Recursion

int factorial(int n)                 // Function definition: takes integer n and returns factorial
{
    if(n == 0 || n == 1)             // Base case: if n is 0 or 1
        return 1;                    // Stop recursion and return 1 (because 0! = 1 and 1! = 1)
    else                             // Recursive case: if n is greater than 1
        return n * factorial(n - 1); // Multiply n with factorial of (n - 1)
                                     // Function calls itself with smaller value
}


🧠 How It Works (For 5)
factorial(5)

→ 5 × factorial(4)

→ 5 × 4 × factorial(3)

→ 5 × 4 × 3 × factorial(2)

→ 5 × 4 × 3 × 2 × factorial(1)

→ 120
⚠ Important
If base case is missing → infinite recursion → stack overflow.

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🛠 6️⃣ Practice Programs

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✅ Program 1: Square of a Number

🔎 Understanding /

• Function receives a number

• Multiplies number by itself

• Returns square

• main() prints result

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#include <stdio.h>                // Include input-output library

int square(int n)                 // Function to calculate square
{
    return n * n;                 // Return n multiplied by n
}

int main()                        // Main function
{
    int num;                      // Declare variable
    printf("Enter number: ");     // Ask user for input
    scanf("%d", &num);            // Read input
    printf("Square = %d", square(num)); // Call function and print result
    return 0;                     // End program
}

🖥 Output

Enter number: 6
Square = 36

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✅ Program 2: Even or Odd

🔎 Understanding 

• Check if number divisible by 2

• If remainder is 0 → Even

• Else → Odd

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#include <stdio.h>                // Include input-output library

void checkEvenOdd(int n)          // Function to check even or odd
{
    if(n % 2 == 0)                // Check divisibility by 2
        printf("Even Number");    // Print Even if condition true
    else                          // Otherwise
        printf("Odd Number");     // Print Odd
}

int main()                        // Main function
{
    int num;                      // Declare variable
    printf("Enter number: ");     // Ask input
    scanf("%d", &num);            // Read input
    checkEvenOdd(num);            // Call function
    return 0;                     // End program
}

🖥 Output

Enter number: 9
Odd Number

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✅ Program 3: Factorial Using Function

🔎 Understanding 

• Initialize factorial = 1

• Multiply numbers from 1 to n

• Return result

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#include <stdio.h>                // Include input-output library

int factorial(int n)              // Function to calculate factorial
{
    int i;                        // Declare loop variable
    int fact = 1;                 // Initialize factorial to 1

    for(i = 1; i <= n; i++)       // Loop from 1 to n
    {
        fact = fact * i;          // Multiply fact by i
    }

    return fact;                  // Return factorial result
}

int main()                        // Main function
{
    int num;                      // Declare variable
    printf("Enter number: ");     // Ask input
    scanf("%d", &num);            // Read input
    printf("Factorial = %d", factorial(num)); // Call function and print result
    return 0;                     // End program
}

🖥 Output

Enter number: 5
Factorial = 120

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🔥 7️⃣ Advanced Program 1: Prime Number (Optimized)

🔎 Understanding 

• Numbers ≤ 1 are not prime

• Check divisibility only up to √n

• If divisible → Not prime

• If no divisor → Prime

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#include <stdio.h>                // Include input-output library

int isPrime(int n)                // Function to check prime
{
    int i;                        // Declare loop variable

    if(n <= 1)                    // Check invalid case
        return 0;                 // Not prime

    for(i = 2; i * i <= n; i++)   // Loop up to square root of n
    {
        if(n % i == 0)            // If divisible
            return 0;             // Not prime
    }

    return 1;                     // Prime number
}

int main()                        // Main function
{
    int num;                      // Declare variable
    printf("Enter number: ");     // Ask input
    scanf("%d", &num);            // Read input

    if(isPrime(num))              // Call function
        printf("Prime Number");   
    else
        printf("Not Prime Number");

    return 0;                     // End program
}

🖥 Output

Enter number: 7
Prime Number

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🔥 8️⃣ Advanced Program 2: Armstrong Number

🔎 Understanding 

• Store original number

• Separate digits

• Cube each digit

• Add cubes

• Compare with original

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#include <stdio.h>                // Include input-output library

int isArmstrong(int n)            // Function to check Armstrong number
{
    int original = n;             // Store original value
    int remainder;                // Store digit
    int sum = 0;                  // Initialize sum

    while(n > 0)                  // Loop until number becomes 0
    {
        remainder = n % 10;       // Get last digit
        sum += remainder * remainder * remainder; // Add cube of digit
        n = n / 10;               // Remove last digit
    }

    if(sum == original)           // Compare with original
        return 1;                 // Armstrong number
    else
        return 0;                 // Not Armstrong
}

int main()                        // Main function
{
    int num;                      // Declare variable
    printf("Enter number: ");     // Ask input
    scanf("%d", &num);            // Read input

    if(isArmstrong(num))          // Call function
        printf("Armstrong Number");
    else
        printf("Not Armstrong Number");

    return 0;                     // End program
}

🖥 Output

Enter number: 153
Armstrong Number



🔥 9️⃣ Advanced Program: Fibonacci Using Recursion
🔎 Understanding
✔ Fibonacci series:

0 1 1 2 3 5 8 13 21 ...
✔ Formula:

F(n) = F(n-1) + F(n-2)
✔ Base cases:

F(0) = 0

F(1) = 1
✔ Function calls itself twice
⚠ Note: This recursive Fibonacci method is not efficient for large values because it has exponential time complexity O(2ⁿ).
#include <stdio.h>                // Include standard input-output library

int fibonacci(int n)              // Recursive function to find nth Fibonacci number
{
    if(n == 0)                    // Base case 1
        return 0;                 // Return 0 if n is 0

    else if(n == 1)               // Base case 2
        return 1;                 // Return 1 if n is 1

    else                          // Recursive case
        return fibonacci(n - 1) + fibonacci(n - 2);  
                                   // Function calls itself twice
}

int main()                        // Main function - execution starts here
{
    int num;                      // Declare variable

    printf("Enter position: ");   // Ask user for input
    scanf("%d", &num);            // Read input

    printf("Fibonacci number = %d", fibonacci(num)); 
                                   // Call recursive function

    return 0;                     // End program successfully
}
🖥 Output
Enter position: 6
Fibonacci number = 8

📊 Visual Comparison: Function Declaration vs Definition
Now that we understand both concepts separately, let’s look at a quick side-by-side comparison to clearly see the differences.
The table below summarizes the purpose, structure, memory behavior, and usage of both function declaration and function definition in C programming.

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📝 Understanding the Comparison

From the comparison above, we can clearly see:

• A function declaration only informs the compiler that the function exists.

• A function definition provides the complete implementation of that function.

• Declarations end with a semicolon (;), while definitions use curly braces { }.

• Only the function definition contains executable code.

In simple words:

Declaration = What the function looks like
Definition = How the function works

This difference becomes especially important when working with header files and large programs.

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“Why Functions Are Important in Real Life?”

Add this small section before conclusion:

🌍 Real-Life Use of Functions

Functions are used in:

• Banking systems (calculate interest)

• E-commerce apps (calculate total price)

• Games (score calculation)

• Calculator apps

• Login systems

• Large software projects

This shows practical relevance (very important for students).

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🎯 Final Conclusion

Now you have learned:

✔ Structure of function
✔ All 4 types
✔ Practice programs
✔ Advanced programs
✔ Optimized logic
✔ Full line-by-line comments
✔ Understanding before every program

Functions are the backbone of structured programming in C.

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📌 Keep Learning. Keep Coding. Keep Growing.

✨ Written by Krishna Popat
🌱 Founder, Learning Growth Hub