Check if N is a Pentagonal Number in C++

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

$$p(n)=\frac{\text{3}*n^2-n}{\text{2}}$$

Where, n is the number of points pentagonal will have

Example

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

Algorithm

Start
Step 1 -> declare function to Check N is pentagonal or not
   bool check(int n)
      declare variables as int i = 1, a
      do
         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
   End
   Else
      Print it is not pentagonal
   End
Stop

Example

#include <iostream>
using namespace std;
// check N is pentagonal or not.
bool check(int n){
   int i = 1, a;
   do{
      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;
   else
      cout << n << " is not pentagonal" << endl;
   return 0;
}

Output

22 is pentagonal