Program to check if N is a Pentagonal Number in C++

C++Server Side ProgrammingProgramming

Given with a number N the task is to check whether the number is a pentagonal number or not. Numbers that can be arranged to form a pentagon is a pentagonal number as these numbers can be used as points to form a pentagon. For example, some of pentagonal numbers are 1, 5, 12, 22, 35, 51....

We can use formula to check whether the number is a pentagonal number or not


Where, n is the number of points pentagonal will have


Input-: n=22
Output-: 22 is pentagonal number
Input-: n=23
Output-: 23 is not a pentagonal number


Step 1 -> declare function to Check N is pentagonal or not
   bool check(int n)
      declare variables as int i = 1, a
         set a = (3*i*i - i)/2
         set i += 1
      while ( a < n );
      return (a == n);
Step 2 -> In main()
   Declare int n = 22
   If (check(n))
      Print is pentagonal
      Print it is not pentagonal


#include <iostream>
using namespace std;
// check N is pentagonal or not.
bool check(int n){
   int i = 1, a;
      a = (3*i*i - i)/2;
      i += 1;
   while ( a < n );
   return (a == n);
int main(){
   int n = 22;
   if (check(n))
      cout << n << " is pentagonal " << endl;
      cout << n << " is not pentagonal" << endl;
   return 0;


22 is pentagonal
Published on 23-Sep-2019 11:35:24