Find the Diameter or Longest chord of a Circle

Given a circle with radius ‘r’ is given, the task is to find the diameter or longest chord of the circle.
Examples:

Input: r = 4
Output: 8

Input: r = 9
Output: 18

Proof that the Longest chord of a circle is its Diameter:

• Draw circle O and any chord AB on it.
• From one endpoint of the chord, say A, draw a line segment through the centre. That is, draw a diameter.
• Now draw a radius from centre O to B.
• By the triangle inequality, 
  

AB < AO + OB
 = r + r
 = 2r
 = d

• So, any chord that is not a diameter will be smaller than a diameter.
• So the largest chord is a diameter

Approach:

• The Longest chord of any circle is its diameter.
• Therefore, the diameter of a circle is twice the radius of it.
  

Length of the longest chord or diameter = 2r

Below is the implementation of the above approach: 

C++

// C++ program to find
// the longest chord or diameter
// of the circle whose radius is given

#include <bits/stdc++.h>
using namespace std;

// Function to find the longest chord
void diameter(double r)
{
    cout << "The length of the longest chord"
         << " or diameter of the circle is "
         << 2 * r << endl;
}

// Driver code
int main()
{

    // Get the radius
    double r = 4;

    // Find the diameter
    diameter(r);

    return 0;
}

Java

// Java program to find
// the longest chord or diameter
// of the circle whose radius is given
class GFG
{

// Function to find the longest chord
static void diameter(double r)
{
    System.out.println("The length of the longest chord"
        + " or diameter of the circle is "
        + 2 * r);
}

// Driver code
public static void main(String[] args)
{

    // Get the radius
    double r = 4;

    // Find the diameter
    diameter(r);
}
}

// This code contributed by Rajput-Ji

Python3

# Python3 program to find
# the longest chord or diameter
# of the circle whose radius is given

# Function to find the longest chord
def diameter(r):

    print("The length of the longest chord"
        ," or diameter of the circle is "
        ,2 * r)


# Driver code

# Get the radius
r = 4

# Find the diameter
diameter(r)

# This code is contributed by mohit kumar

C#

// C# program to find
// the longest chord or diameter
// of the circle whose radius is given
using System;

class GFG
{

// Function to find the longest chord
static void diameter(double r)
{
    Console.WriteLine("The length of the longest chord"
        + " or diameter of the circle is "
        + 2 * r);
}

// Driver code
public static void Main(String[] args)
{

    // Get the radius
    double r = 4;

    // Find the diameter
    diameter(r);
}
}

// This code has been contributed by 29AjayKumar

PHP

<?php
// PHP program to find
// the longest chord or diameter
// of the circle whose radius is given

// Function to find the longest chord
function diameter($r)
{
    echo "The length of the longest chord"
        ," or diameter of the circle is "
        ,2 * $r << "\n";
}

    // Driver code
    // Get the radius
    $r = 4;

    // Find the diameter
    diameter($r);

    // This code is contributed by Ryuga

?>

Javascript

<script>

// javascript program to find
// the longest chord or diameter
// of the circle whose radius is given

// Function to find the longest chord
function diameter(r)
{
    document.write("The length of the longest chord"
        + " or diameter of the circle is "
        + 2 * r);
}

// Driver code

// Get the radius
var r = 4;

// Find the diameter
diameter(r);

// This code contributed by Princi Singh

</script>

Output:

The length of the longest chord or diameter of the circle is 8

Time Complexity: O(1)

Auxiliary Space: O(1)