🔹 What is a Function in C?

A function is a block of code that performs a specific task. In C, functions help to:

• Break down complex programs into smaller, manageable parts.
• Reuse code by calling the same function multiple times.
• Improve readability and maintainability.

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🔹 Types of Functions in C

1. Library Functions

• Built-in functions provided by C (e.g., printf(), scanf(), sqrt())
• Defined in header files like stdio.h, math.h, etc.

1. User-defined Functions

• Created by the programmer to perform specific tasks.

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🔹 Syntax of a User-defined Function

return_type function_name(parameter_list) {
    // body of the function
}

Example:

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

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🔹 Components of a Function

Component	Description
Return type	Type of value returned (e.g., int, void)
Function name	Identifier name of the function
Parameters	Input values (optional)
Function body	Code block performing specific operations
Return statement	Sends result back to caller (optional)

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🔹 Function Declaration / Prototype

Before using a function, you must declare it (unless defined before the main function).

int add(int, int);  // Function prototype

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🔹 Function Call

To execute a function, you need to call it using its name and pass arguments:

int result = add(5, 3);

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🔹 Complete Example

Let’s see a complete C program using a user-defined function:

#include <stdio.h>

// Function declaration
int add(int, int);

// Main function
int main() {
    int x = 10, y = 20, sum;

    sum = add(x, y);  // Function call

    printf("Sum = %d\n", sum);
    return 0;
}

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

💡 Output:

Sum = 30

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🔹 Categories of Functions Based on Arguments and Return Value

Type	Description & Example
1. No arguments, no return value	void greet();
2. Arguments, no return value	void display(int);
3. No arguments, return value	int getValue();
4. Arguments and return value	int multiply(int, int);

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🔹 Example: No Arguments, No Return Value

#include <stdio.h>

void greet() {
    printf("Hello! Welcome to C programming.\n");
}

int main() {
    greet();
    return 0;
}

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🔹 Why Use Functions?

• Modularity – Breaks large programs into smaller chunks.
• Reusability – Same function can be used multiple times.
• Easier testing – Each function can be tested independently.
• Better organization – Code is cleaner and more readable.

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If you want, I can also explain recursive functions, inline functions, or help you write functions for specific tasks like factorial, prime check, file handling, etc.