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In this article, we've created some programs in Python to find and print area of a circle using its radius value entered by user at run-time. Here are the list of programs:

- Find Area of Circle without Function
- Find Area of Circle using User-defined
**Function** - Using
**Class**

To find area of a circle with given value of radius, use following formula:

a = 3πr^{2}= 3*3.14*r*r

Here **a** indicates to *area value*, **r** indicates to *radius value* of circle.

**Note - **The value of **π** is **3.14**.

This program receives radius of circle as input from user at run-time, then find and prints its area. The question
is **write a Python program to compute area of a circle.** Here is its answer:

print("Enter the Radius of Circle: ") r = float(input()) a = 3*3.14*r*r print("\nArea = ", a)

Here is its sample run:

Now supply the input say **5** as radius of circle, press `ENTER`

key to find and print the area
of circle based on given radius value as shown in the snapshot given below:

The question is, **write a Python program to calculate area of circle if diameter is inputted.** The answer to this question is given below:

print("Enter the Diameter of Circle: ", end="") d = float(input()) r = d/2 a = 3*3.14*r*r print("\nArea = ", a)

Here is its sample run with user input **10** as diameter:

**Note - **Diameter is the length of line passing through the center of circle, from one side to other.

**Note - **Radius is the length of line from center of circle to any outside edge.

**Note - **Diameter to Radius relation is, __Radius's length is one-half of Diameter's Length__. Or __Diameter is Double the length of Radius__

This program does the same job as of very first program of this article. The only difference is, its approach. That is,
this program uses user-defined function named **areaOfCircle()**, that find and returns area based on the radius
passed as its argument.

def areaOfCircle(rad): ar = 3*3.14*rad*rad return ar print("Enter the Radius of Circle: ", end="") r = float(input()) a = areaOfCircle(r) print("\nArea = ", a)

Here is its sample run with user input, **10** as radius of circle:

This program uses class and object, an object-oriented feature of Python to find and print area of a circle.

class CodesCracker: def areaOfCircle(self, rad): ar = 3*3.14*rad*rad return ar print("Enter the Radius of Circle: ", end="") r = float(input()) CCobj = CodesCracker() a = CCobj.areaOfCircle(r) print("\nArea = ", a)

This program produces the same output as of previous program. In this program, using the statement,
**CCobj = CodesCracker()**, all properties of class **CodesCracker** gets assigned to this object (**CCobj**).
Now we can use this object to access the member function (**areaOfCircle()**) to find area of circle in similar
way as done in previous program using user-defined function.

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