Functions are the building blocks of C programs. They divide large programs into smaller, manageable, and reusable modules. A function performs a specific task and can be executed whenever it is called.

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1. Brief Overview of Functions

A function is a block of code designed to perform a particular operation.
Key points:

• Improves code readability
• Supports reusability
• Makes debugging easier
• Enables modular programming

Types of functions:

1. Library Functions (built-in) → printf(), scanf(), strlen()
2. User-Defined Functions → created by the programmer

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2. Defining a Function in C

A function definition specifies what a function does.

Syntax

return_type function_name(parameter_list) {
    // function body
}

Example

int add(int a, int b) {
    return a + b;
}

Components:

• return_type → type of value the function returns
• function_name → identifier
• parameters → inputs
• function body → code executed when called

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3. Accessing (Calling) a Function

After defining a function, you must call it to execute its code.

Syntax

function_name(arguments);

Example

int result = add(5, 3);

Function calling may occur:

• in main()
• inside another function
• inside itself (recursion)

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4. Passing Arguments to a Function

C supports passing values (arguments) to functions, allowing flexible processing.

There are two ways:

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a) Pass by Value (default)

A copy of the variable is passed.
Changes inside the function do not affect the original variable.

void change(int x) {
    x = 50;
}

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b) Pass by Reference (Using Pointers)

The function receives the memory address.
Changes reflect in the original variable.

void update(int *p) {
    *p = 100;
}

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5. Specifying Argument Data Types

When declaring or defining a function, the data type of each parameter must be specified.

Example

int sum(int a, float b, double c);

Parameters:

• a → int
• b → float
• c → double

Correct data types ensure:

• proper memory allocation
• correct function execution
• type safety

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6. Function Prototypes

A function prototype tells the compiler:

• function name
• return type
• number of parameters
• parameter data types

It must be written before main() if the function is defined later.

Syntax

return_type function_name(parameter_type1, parameter_type2, ...);

Example

int multiply(int, int);

Purpose:

• Enables early type checking
• Prevents errors
• Improves readability

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Program Example – Using Function Prototype

#include <stdio.h>

int multiply(int, int);  // prototype

int main() {
    int result = multiply(4, 5);
    printf("Result = %d", result);
    return 0;
}

int multiply(int a, int b) {  // definition
    return a * b;
}

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7. Recursion

Recursion is a process in which a function calls itself to solve a problem.

Every recursive function must have:

1. Base condition → stops the recursion
2. Recursive call → calls itself

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Example: Factorial using Recursion

int factorial(int n) {
    if (n == 0)
        return 1;        // base condition
    else
        return n * factorial(n - 1);  // recursive call
}

Calling:

factorial(5);

Recursion is ideal for:

• factorial
• Fibonacci
• tree traversal
• divide-and-conquer algorithms

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8. Complete Program Demonstrating All Concepts

#include <stdio.h>

// Function prototype
int sum(int, int);
int factorial(int);

int main() {
    int a = 5, b = 10;

    // Accessing a function
    printf("Sum = %d\n", sum(a, b));

    // Recursion
    printf("Factorial = %d", factorial(5));

    return 0;
}

// Defining a function
int sum(int x, int y) {
    return x + y; 
}

// Recursive function
int factorial(int n) {
    if (n == 0)
        return 1;
    return n * factorial(n - 1);
}

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Citations

🔗 View other articles about C Programming:
https://savanka.com/category/learn/c-programming/

🔗 External C Documentation:
https://www.w3schools.com/c/

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