Functions

 

A function is a block of code that performs a specific task.

Suppose we need to create a program to create a circle and color it. We can create two functions to solve this problem:

• a function to draw the circle
• a function to color the circle

Dividing a complex problem into smaller chunks makes our program easy to understand and reusable.

There are two types of function:

1. Standard Library Functions: Predefined in C++
2. User-defined Function: Created by users

 

C++ User-defined Function

C++ allows the programmer to define their own function.

A user-defined function groups code to perform a specific task and that group of code is given a name (identifier).

When the function is invoked from any part of the program, it all executes the codes defined in the body of the function.

 

C++ Function Declaration

The syntax to declare a function is:

returnType functionName (parameter1, parameter2,...) {
    // function body   
}

Here's an example of a function declaration.
 

// function declaration
void greet() {
    cout << "Hello World";
}

Here,

• the name of the function is greet()
• the return type of the function is void
• the empty parentheses mean it doesn't have any parameters
• the function body is written inside {}
•  

• Calling a Function

  In the above program, we have declared a function named greet(). To use the greet() function, we need to call it.

  Here's how we can call the above greet() function.

  int main() {
       
      // calling a function   
      greet(); 
  
  }

   Hello World

  Function Parameters

  As mentioned above, a function can be declared with parameters (arguments). A parameter is a value that is passed when declaring a function.

  
  

  For example, let us consider the function below:

  void printNum(int num) {
      cout << num;
  }

   


   

  Benefits of Using User-Defined Functions

• Functions make the code reusable. We can declare them once and use them multiple times.
• Functions make the program easier as each small task is divided into a function.
• Functions increase readability.

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C++ Library Functions

Library functions are the built-in functions in C++ programming.

Programmers can use library functions by invoking the functions directly; they don't need to write the functions themselves.

Some common library functions in C++ are sqrt(), abs(), isdigit(), etc.

In order to use library functions, we usually need to include the header file in which these library functions are defined.

For instance, in order to use mathematical functions such as sqrt() and abs(), we need to include the header file cmath.

 

Example 5: C++ Program to Find the Square Root of a Number

#include <iostream>
#include <cmath>
using namespace std;

int main() {
    double number, squareRoot;
    
    number = 25.0;

    // sqrt() is a library function to calculate the square root
    squareRoot = sqrt(number);

    cout << "Square root of " << number << " = " << squareRoot;

    return 0;
}

Output

Square root of 25 = 5

In this program, the sqrt() library function is used to calculate the square root of a number.

The function declaration of sqrt() is defined in the cmath header file. That's why we need to use the code #include <cmath> to use the sqrt() function.