What is a User-Defined Function?

A user-defined function in C is a function created by the programmer to perform specific tasks, rather than relying solely on built-in (library) functions. These functions are defined to meet the unique requirements of a program, enhancing modularity and enabling code reuse.

Why Do We Need User-Defined Functions?

User-defined functions offer multiple benefits that are especially important for creating scalable, efficient programs. Some of the primary reasons to use user-defined functions include:

• Modularity: Functions allow us to divide large programs into smaller, independent units. Each function performs a specific task, making the program easier to understand and manage.
• Code Reusability: Once a function is defined, it can be used multiple times throughout the program (or in different programs), reducing code duplication and enhancing efficiency.
• Improved Debugging: Breaking down code into functions makes it easier to identify and fix errors. Each function can be tested independently, simplifying the debugging process.
• Enhanced Readability: Code with functions is typically more readable, as functions give names to specific operations, making it clear what each part of the code does.
• Abstraction: User-defined functions allow programmers to hide complex code details, focusing on the function’s purpose rather than its internal workings. This improves program structure and maintainability.

Syntax:

Here's a basic syntax of User-defined Function:

return_type function_name(parameters) {
    // Function body
    return value; // Optional, depending on return type
}

Key Points
• Return Type: Specifies the type of value the function returns (e.g., int, float, void).
• Function Name: A unique identifier for the function.
• Parameters: Values passed into the function for processing.
• Function Body: The code that defines what the function does.
• Return Statement: Used to return a value to the calling function (if applicable).

#include <stdio.h>
// Function to add two numbers
int add(int a, int b) {
    return a + b;
}
// Function to multiply two numbers
int multiply(int a, int b) {
    return a * b;
}
int main() {
    int num1 = 5, num2 = 10;
    printf("Sum: %d\n", add(num1, num2));          // Calls the add function
    printf("Product: %d\n", multiply(num1, num2));  // Calls the multiply function
    return 0;
}

In this example:
The add function calculates the sum of two numbers, num1 and num2.
The multiply function calculates the product of two numbers, num1 and num2.

Using user-defined functions improves the program’s readability, as each function name clearly indicates its purpose. Additionally, these functions can be reused in other parts of the program or in other programs.