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Program to check if N is a Pentagonal Number
  • Difficulty Level : Easy
  • Last Updated : 12 Apr, 2021

Given a number (N), check if it is pentagonal or not. 
Examples : 
 

Input: 12 
Output: Yes
Explanation: 12 is the third pentagonal number

Input: 19
Output: No
Explanation: The third pentagonal number is 12
while the fourth pentagonal number is 22.
Hence 19 is not a pentagonal number.

Pentagonal numbers are numbers which can be arranged to form a pentagon. If N is a pentagonal number then we can use N dots or points to generate a regular pentagon (Please see figure below).
The first few pentagonal numbers are 1, 5, 12, 22, 35, 51, 70, … 
 

Image Source: Wiki
Method I (Iterative) 
We begin by noting that the nth Pentagonal Number is given by 
P_n = \frac{3*n^2-n}{2}
Follow an iterative process. Consecutively substitute n = 1, 2, 3 … into the formula and store the result in some variable M. Stop, if M >= N. After iteration if M equals N then N must be a pentagonal number. Else if M exceeds N then N cannot be a pentagonal number.
Algorithm 
 

function isPentagonal(N) 
    Set i = 1
    do 
        M = (3*i*i - i)/2
        i += 1
    while M < N
    
    if M == N
        print Yes
    else
        print No

Below is the implementation of the algorithm
 

C++




// C++ program to check
// pentagonal numbers.
#include <iostream>
using namespace std;
 
// Function to determine
// if N is pentagonal or not.
bool isPentagonal(int N)
{
    int i = 1, M;
     
    do {
 
        // Substitute values of i
        // in the formula.
        M = (3*i*i - i)/2;
        i += 1;
    }
    while ( M < N );
     
    return (M == N);
}
 
// Driver Code
int main()
{
    int N = 12;
     
    if (isPentagonal(N))
        cout << N << " is pentagonal " << endl;   
    else
        cout << N << " is not pentagonal" << endl;
     
    return 0;
}

Java




// Java program to check
// pentagonal numbers.
import java.io.*;
 
class GFG {
     
// Function to determine
// if N is pentagonal or not.
static Boolean isPentagonal(int N)
{
    int i = 1, M;
      
    do {
  
        // Substitute values of
        // i in the formula.
        M = (3*i*i - i)/2;
        i += 1;
    }
    while ( M < N );
      
    return (M == N);
}
    public static void main (String[] args) {
    int N = 12;
      
    if (isPentagonal(N))
        System.out.println( N + " is pentagonal " );   
    else
        System.out.println( N + " is not pentagonal");
 
    }
}
 
// This code is contributed by Gitanjali.

Python3




# python3 program to check
# pentagonal numbers.
import math
 
# Function to determine if
# N is pentagonal or not.
def isPentagonal( N ) :
 
    i = 1
    while True:
 
        # Substitute values of i
        # in the formula.
        M = (3 * i * i - i) / 2
        i += 1
     
        if ( M >= N ):
            break
     
    return (M == N)
     
# Driver method
N = 12
if (isPentagonal(N)):
    print(N , end = ' ')
    print ("is pentagonal " )
else:
    print (N , end = ' ')
    print ("is not pentagonal")
 
# This code is contributed by Gitanjali.

C#




// C# program to check pentagonal numbers.
using System;
 
class GFG {
     
// Function to determine
// if N is pentagonal or not.
static bool isPentagonal(int N)
{
    int i = 1, M;
     
    do {
 
        // Substitute values of
        // i in the formula.
        M = (3 * i * i - i) / 2;
        i += 1;
    }
    while ( M < N );
     
    return (M == N);
}
 
// Driver Code
public static void Main ()
{
    int N = 12;
     
    if (isPentagonal(N))
    Console.Write( N + " is pentagonal " );
    else
    Console.Write( N + " is not pentagonal");
 
}
}
 
// This code is contributed by vt_m.

PHP




<?php
// PHP program to check
// pentagonal numbers.
 
// Function to determine
// if N is pentagonal or not.
function isPentagonal(int $N)
{
    $i = 1;
    $M;
     
    do {
 
        // Substitute values of i
        // in the formula.
        $M = (3 * $i * $i - $i) / 2;
        $i += 1;
    }
    while ($M < $N);
     
    return ($M == $N);
}
 
    // Driver Code
    $N = 12;
     
    if (isPentagonal($N))
        echo $N , " is pentagonal " ;
    else
        echo $N ," is not pentagonal" ;
     
// This code is contributed by anuj_67.
?>

Javascript




<script>
// javascript program to check
// pentagonal numbers.
    
// Function to determine
// if N is pentagonal or not.
function isPentagonal(N)
{
    var i = 1, M; 
    do
    {
  
        // Substitute values of
        // i in the formula.
        M = (3 * i * i - i)/2;
        i += 1;
    }
    while ( M < N );
        return (M == N);
}
 
var N = 12;
 
if (isPentagonal(N))
    document.write( N + " is pentagonal " );   
else
    document.write( N + " is not pentagonal");
 
// This code is contributed by Amit Katiyar
</script>

Output: 
 



12 is pentagonal 

The Time complexity of this method is O(n) since we need to compute successive values of pentagonal numbers up to N.
  
Method 2 (Efficient) 
The formula indicates that the n-th pentagonal number depends quadratically on n. Therefore, try to find the positive integral root of N = P(n) equation. 
P(n) = nth pentagonal number 
N = Given Number
Solve for n: 
P(n) = N 
or (3*n*n – n)/2 = N 
or 3*n*n – n – 2*N = 0 … (i)
The positive root of equation (i) 
n = (1 + sqrt(24N+1))/6
After obtaining n, check if it is an integer or not. n is an integer if n – floor(n) is 0.
Implementation of the method is given below : 
 

C++




// C++ Program to check a
// pentagonal number
#include <bits/stdc++.h>
using namespace std;
 
// Function to determine if
// N is pentagonal or not.
bool isPentagonal(int N)
{   
    // Get positive root of
    // equation P(n) = N.
    float n = (1 + sqrt(24*N + 1))/6;
     
    // Check if n is an integral
    // value of not. To get the
    // floor of n, type cast to int.
    return (n - (int) n) == 0;
}
 
// Driver Code
int main()
{
    int N = 19;   
    if (isPentagonal(N))
        cout << N << " is pentagonal " << endl;   
    else
        cout << N << " is not pentagonal" << endl;   
    return 0;
}

Java




// Java program to check
// pentagonal numbers.
import java.io.*;
 
class GFG {
     
// Function to determine if
// N is pentagonal or not.
static Boolean isPentagonal(int N)
{
        // Get positive root of
    // equation P(n) = N.
    double n = (1 + Math.sqrt(24*N + 1))/6;
     
    // Check if n is an integral
    // value of not. To get the
    // floor of n, type cast to int.
    return (n - (int) n) == 0;
}
    public static void main (String[] args) {
    int N = 19;
      
    if (isPentagonal(N))
        System.out.println( N + " is pentagonal " );   
    else
        System.out.println( N + " is not pentagonal");
 
    }
}
 
// This code is contributed by Gitanjali.

Python3




# Python3 code Program to 
# check a pentagonal number
 
# Import math library
import math as m
 
# Function to determine if
# N is pentagonal or not
def isPentagonal( n ):
     
    # Get positive root of
    # equation P(n) = N.
    n = (1 + m.sqrt(24 * N + 1)) / 6
     
 
    # Check if n is an integral
    # value of not. To get the
    # floor of n, type cast to int
    return( (n - int (n)) == 0)
 
# Driver Code
N = 19
 
if (isPentagonal(N)):
    print ( N, " is pentagonal " )
else:
    print ( N, " is not pentagonal" )
 
# This code is contributed by 'saloni1297'

C#




// C# program to check pentagonal numbers.
using System;
 
class GFG {
 
    // Function to determine if
    // N is pentagonal or not.
    static bool isPentagonal(int N)
    {
        // Get positive root of
        // equation P(n) = N.
        double n = (1 + Math.Sqrt(24 * N + 1)) / 6;
 
        // Check if n is an integral
        // value of not. To get the
        // floor of n, type cast to int.
        return (n - (int)n) == 0;
    }
     
    // Driver Code
    public static void Main()
    {
        int N = 19;
 
        if (isPentagonal(N))
            Console.Write(N + " is pentagonal ");
        else
            Console.Write(N + " is not pentagonal");
    }
}
 
// This code is contributed by vt_m.

PHP




<?php
// PHP Program to check
// a pentagonal number
 
// Function to determine if
// N is pentagonal or not.
function isPentagonal($N)
{
    // Get positive root of
    // equation P(n) = N.
    $n = (1 + sqrt(24 * $N + 1)) / 6;
     
    // Check if n is an integral
    // value of not. To get the
    // floor of n, type cast to int.
    return ($n - (int) $n) == 0;
}
 
// Driver Code
$N = 19;
if (isPentagonal($N))
    echo $N . " is pentagonal ";
else
    echo $N . " is not pentagonal";
 
// This code is contributed by mits.
?>

Javascript




<script>
// javascript program to check
// pentagonal numbers.
    
// Function to determine if
// N is pentagonal or not.
function isPentagonal(N)
{
     // Get positive root of
    // equation P(n) = N.
    var n = (1 + Math.sqrt(24*N + 1))/6;
     
    // Check if n is an integral
    // value of not. To get the
    // floor of n, type cast to int.
    return (n - parseInt( n) == 0);
}
 
var N = 19;
 
if (isPentagonal(N))
    document.write( N + " is pentagonal " );   
else
    document.write( N + " is not pentagonal");
 
// This code is contributed by Amit Katiyar
</script>

Output : 
 

19 is not pentagonal

Time and Space Complexities of this method are both O(1).
References : 
1) Wikipedia – Pentagonal Numbers 
2) Wolfram Alpha – Pentagonal Numbers
 

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