In this article, you will learn and get code on finding the area and circumference of a circle based on its radius entered by the user at run-time in C++ programming. Here is the list of approaches used:

• Without using a function, calculate the area of a circle
• Area of a Circle using Function
• Circumference of a circle without using function
• Circumference of a circle using function
• Circle Area and Circumference Using Class

Before starting the program, let's first review the formula used to find the area and circumference of a circle.

Circle Area Formula

To find the area of a circle, use this formula:

3.14 is the value of π. And r denotes the circle's radius. This formula can also be written as:

Circumference of a Circle Formula

To find the circumference of a circle, use this formula:

Now let's move on to the program.

Calculate the Area of a Circle

To calculate the area of any circle in C++ programming, you have to ask the user to enter the radius of the circle, place the radius in a variable, say rad, and then initialize 3.14*rad*rad in a variable that holds the value of the area of the circle, as shown here in the following program.

#include<iostream>
using namespace std;
int main()
{
    float rad, area;
    cout<<"Enter the Radius of Circle: ";
    cin>>rad;
    area = 3.14*rad*rad;
    cout<<"\nArea of Circle = "<<area;
    cout<<endl;
    return 0;
}

This program was built and runs under the Code::Blocks IDE. Here is its sample run:

Now enter the radius of the circle, say 5, and press the ENTER key to see the following output:

You can also replace the variable rad with r to make the above program more meaningful.

In C++, find the area of a circle using a function

Let's create the same-purpose program using a user-defined function, findArea(). This function takes the radius as its argument and returns the area of the circle. The question is: write a program in C++ to find the area of a circle using a user-defined function. The answer to this question is given below:

#include<iostream>
using namespace std;
float findArea(float);
int main()
{
    float rad, area;
    cout<<"Enter the Radius of Circle: ";
    cin>>rad;
    area = findArea(rad);
    cout<<"\nArea of Circle = "<<area;
    cout<<endl;
    return 0;
}
float findArea(float r)
{
    return (3.14*r*r);
}

It will produce the same output as the previous one.

Find the Circumference of a Circle in C++

The question is, "Write a program in C++ to find the circumference of a circle." The answer to this question is:

#include<iostream>
using namespace std;
int main()
{
    float rad, circum;
    cout<<"Enter the Radius of Circle: ";
    cin>>rad;
    circum = 2*3.14*rad;
    cout<<"\nCircumference of Circle = "<<circum;
    cout<<endl;
    return 0;
}

Here is the initial snapshot of the sample run:

Now enter the radius of the circle to see its circumference. Here is the final snapshot of the sample run, supposing that the user enters 5 as the radius of the circle:

Find the circumference of a circle in C++ using Function

This program does the same job as the previous program but uses a function named findCircum().

#include<iostream>
using namespace std;
float findCircum(float);
int main()
{
    float rad;
    cout<<"Enter the Radius of Circle: ";
    cin>>rad;
    cout<<"\nCircumference of Circle = "<<findCircum(rad);
    cout<<endl;
    return 0;
}
float findCircum(float r)
{
    return (2*3.14*r);
}

This program produces the same output as the previous one.

Using a class, calculate the area and circumference in C++

This is the end of this article. This program finds both the area and circumference of a circle using an object-oriented feature of C++, namely classes.

#include<iostream>
using namespace std;
class CodesCracker
{
    public:
        float findArea(float r)
        {
            return (3.14*r*r);
        }
        float findCircum(float r)
        {
            return (2*3.14*r);
        }
};
int main()
{
    CodesCracker c;
    float rad, area, circum;
    cout<<"Enter the Radius of Circle: ";
    cin>>rad;
    area = c.findArea(rad);
    circum = c.findCircum(rad);
    cout<<"\nArea of Circle = "<<area;
    cout<<"\nCircumference of Circle = "<<circum;
    cout<<endl;
    return 0;
}

Here is the sample run of the above program with user input of radius 5:

The above program can also be written as:

#include<iostream>
using namespace std;
class CodesCracker
{
    private:
        float r;
    public:
        void getData();
        float findArea();
        float findCircum();
};
void CodesCracker::getData()
{
    cout<<"Enter the Radius of Circle: ";
    cin>>r;
}
float CodesCracker::findArea()
{
    return (3.14*r*r);
}
float CodesCracker::findCircum()
{
    return (2*3.14*r);
}
int main()
{
    CodesCracker c;
    c.getData();
    cout<<"\nArea of Circle = "<<c.findArea();
    cout<<"\nCircumference of Circle = "<<c.findCircum();
    cout<<endl;
    return 0;
}

The same program in different languages

• C Find Area & Circumference
• Java Find Area & Circumference
• Python Find Area & Circumference
• Python Find Circumference

C++ Quiz

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