What are and how to use functions in C++

In C++, functions are reusable blocks of code that perform specific tasks and can be invoked from other places in the program.

Functions allow us to divide the code into more manageable fragments (this enables us to create more reusable code and improve the readability and maintainability of the program).

What is a function

A function in C++ is a set of instructions that performs a specific task. A function can be defined to receive certain parameters, process this data, and return a result if necessary. The basic structure of a function in C++ is as follows:

return_type function_name(parameter_type parameter1, parameter_type parameter2, ...)
{
    // Function code
    return return_value; // Optional
}

• Return type: This is the type of value that the function returns. It can be any data type, such as int, double, char, etc. If the function does not return any value, void is used.
• Function name: This is the identifier of the function. It must be unique within the scope in which it is defined.
• Parameter list: These are variables that are passed to the function so that it can operate with this data (Parameters are optional and a function may have no parameters)
• Function code: This is the block of code that executes when the function is called.
• Return value: This is the result that the function returns, if applicable. The return statement is used to return a value.

Basic example

Let’s look at a basic example of a function in C++ that adds two numbers:

#include <iostream>

int Add(int num1, int num2) {
    return num1 + num2;
}

int main() {
    int result = Add(5, 3);
    std::cout << "The sum is: " << result << std::endl;
    return 0;
}

In this example:

• The function Add takes two parameters (num1 and num2), performs the addition operation, and returns the result.
• In main, the function Add is called with values 5 and 3, and the result 8 is printed to the console.

Calling functions

To call a function, you simply use its name followed by parentheses containing the necessary arguments (if any). Here’s an example of calling the function Add:

int result = Add(5, 3);

This line:

• Calls the function Add with the values 5 and 3
• Stores the result in the variable result

Parameters

Functions in C++ can accept parameters that are values passed to the function for it to process. These parameters are defined in the function declaration and are used within the function to perform calculations or manipulations.

For example, the following function calculates the area of a rectangle given its width and height:

double CalculateRectangleArea(double width, double height) {
    return width * height;
}

In this function, width and height are the parameters used to calculate the area of the rectangle.

Return value

Functions can return a value using the return keyword. The type of the returned value must match the return type specified in the function declaration.

For example, the following function calculates the area of a circle given its radius:

#include <cmath> // For the constant M_PI

double CalculateCircleArea(double radius) {
    return M_PI * radius * radius;
}

The function CalculateCircleArea returns the area of the circle using the constant M_PI from the cmath library.

Functions without return

If a function does not return any value, the return type void is used. These functions are generally used to perform actions without the need to return a result.

For example, a function that prints a message to the console:

void PrintMessage(const std::string& message) {
    std::cout << message << std::endl;
}

Here, PrintMessage does not return any value; it simply prints the provided message.

Practical examples

Function to determine if a number is even or odd

This function determines if a number is even or odd:

std::string EvenOdd(int number) {
    if (number % 2 == 0) {
        return "even";
    } else {
        return "odd";
    }
}

Function to calculate the area of a circle

This function calculates the area of a circle given its radius:

#include <cmath> // For the constant M_PI

double CalculateCircleArea(double radius) {
    return M_PI * radius * radius;
}

Function to determine the maximum of two numbers

This function returns the greater of two numbers:

int Maximum(int a, int b) {
    if (a > b) {
        return a;
    } else {
        return b;
    }
}

Function to convert Celsius to Fahrenheit

This function converts a temperature from degrees Celsius to degrees Fahrenheit:

double CelsiusToFahrenheit(double celsius) {
    return (celsius * 9 / 5) + 32;
}

Function to check if a number is prime

This function determines if a number is prime:

bool IsPrime(int number) {
    if (number <= 1) return false;
    for (int i = 2; i < number; i++) {
        if (number % i == 0) return false;
    }
    return true;
}

Function to calculate the sum of an array of integers

This function calculates the sum of all elements in an array of integers:

#include <vector>

int SumArray(const std::vector<int>& numbers) {
    int sum = 0;
    for (int number : numbers) {
        sum += number;
    }
    return sum;
}