# Circumference and circular area

This C++ code calculates the circumference and area of a circle based on a given radius.

## Code Example

``````#include <iostream>
#include <math.h>
using namespace std;

#define _USE_MATH_DEFINES

int main () {
float r, area, circumference;

cin >> r;

// calculation of circumference
circumference = 2 * M_PI * r;

// calculation of circular area
// pow(base, exponent): Returns base raised to the power exponent
area = M_PI * pow(r, 2);

cout << "Circumference: " << circumference << endl;
cout << "Circular Area: " << area << endl;
}``````

### Output

``````Please enter radius: 7
Circumference: 43.9823
Circular Area: 153.938``````

### Code Explanation

#### 1. Preprocessor Directive:

``#define _USE_MATH_DEFINES``
• This preprocessor directive enables the definition of several mathematical constants in C++. When this line is included before including `cmath` or `math.h`, constants such as `M_PI` (which represents the value of π) become available. This is particularly useful since `M_PI` is used to calculate the circumference and area of the circle.

#### 2. Variable Declaration and User Input:

``````float r, area, circumference;
cin >> r;``````
• Here, three variables of type `float` are declared: `r`, `area`, and `circumference`. The variable `r` will store the radius of the circle.
• The program prompts the user to input the radius of the circle, which is then read and stored in `r` using `cin`.

#### 3. Calculate the Circumference:

``circumference = 2 * M_PI * r;``
• The circumference of the circle is calculated using the formula 2𝜋𝑟2πr. `M_PI` provides the value of π, and `r` is the radius entered by the user. The result is stored in the `circumference` variable.

#### 4. Calculate the Circular Area:

``area = M_PI * pow(r, 2);``
• The area of the circle is calculated using the formula 𝜋𝑟2πr2. The `pow` function is used to square the radius (`pow(r, 2)`), which is then multiplied by π (`M_PI`) to find the area. The result is stored in the `area` variable.

#### 5. Output the Results:

``````cout << "Circumference: " << circumference << endl;
cout << "Circular Area: " << area << endl;``````
• These lines print the calculated values of the circumference and area to the standard output. `endl` is used after each output to insert a newline character and flush the stream, ensuring that all output is immediately displayed.