3) Functions in C

Functions in C are blocks of code that can be defined and called to perform specific tasks. They promote modularity, reusability, and easier maintenance of code. A function encapsulates a series of statements and can accept input (parameters) and return output.

Function Declaration and Definition

A function is declared and defined with the following structure:

return_type function_name(parameters) {
    // Function body (code)
    // ...
    return return_value;  // Optional return statement
}

• return_type: The data type of the value the function returns (use void for functions that don’t return a value).
• function_name: The name of the function.
• parameters: Comma-separated list of input parameters the function accepts.
• return_value: The value that the function returns (not needed for void functions).

Example of a Simple Function

#include <stdio.h>

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

int main() {
    int result = add(5, 3);  // Calling the function
    printf("Result: %d\n", result);

    return 0;
}

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

Explanation

• In this example, the function add calculates the sum of two integers.
• The function is declared at the beginning of the program with its return type and parameter list.
• The function is then defined after the main function.
• When called in the main function, it returns the sum of the provided arguments.

Function Parameters

Functions can accept parameters that act as placeholders for values passed to the function when it’s called.

#include <stdio.h>

void greet(char name[]) {
    printf("Hello, %s!\n", name);
}

int main() {
    greet("Alice");
    greet("Bob");

    return 0;
}

Explanation:

• The function greet accepts a single parameter, name, which is an array of characters.
• When called in the main function, it prints a personalized greeting based on the provided name.

Function with No Return Value (void):

A function with void as the return type doesn’t return a value. It’s commonly used for tasks that perform actions without producing a result.

#include <stdio.h>

void printMessage() {
    printf("Hello from the function!\n");
}

int main() {
    printMessage();  // Calling the function

    return 0;
}

Explanation:

• The printMessage function prints a message when called in the main function.
• Since it has a return type of void, it doesn’t need a return statement.

Function Parameters:

Variables defined inside a function are local to that function and have scope only within it.

#include <stdio.h>

void demonstrateScope() {
    int localVar = 10;
    printf("Local variable: %d\n", localVar);
}

int main() {
    // localVar is not accessible here
    demonstrateScope();

    return 0;
}

Explanation:

• The variable localVar is defined within the demonstrateScope function and is only accessible within that function.
• It’s not accessible in the main function or any other function.

Function Prototypes:

A function prototype declares the function’s existence before it’s defined, allowing you to call the function before its actual definition.

#include <stdio.h>

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

int main() {
    int result = add(5, 3);
    printf("Result: %d\n", result);

    return 0;
}

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

Explanation:

• The function prototype for add is declared before the main function.
• This allows you to call the add function in the main function before its definition.