Thread: A problem I can't figure out at all!

Originally Posted by oogabooga

This seems to be the proper test for pentagonal numbers:

Code:

#include <iostream>
#include <cmath>

typedef unsigned long ulong;

bool checkPentagonal(ulong n)
{
    double x = (sqrt(24.0 * n + 1) + 1) / 6;
    return floor(x) == x;
}

int main()
{
    for (size_t i = 1; i < 2000; i++)
        if (checkPentagonal(i))
            std::cout << i << '\n';
}

Aww yeah!! The solution came correct when I replaced my checkPentagonal() function with yours.
Thanks a bunch dude.

Output: 1533776805

The new code:

Code:

#include <iostream>
#include <cmath>
 
using namespace std;
 
bool checkPentagonal(unsigned long somenum);
unsigned long genTriNum(long n);
 
int main()
{
    unsigned long answer = 0;
    for(int i = 287; answer < 1; i += 2)
    {
        unsigned long cur_tn = genTriNum(i);
        if(checkPentagonal(cur_tn))
        {
            answer = cur_tn;
            break;
        }
    }
    cout << "Answer: " << answer << endl;
    cin.get();
    cin.ignore();
    return 0;
}
 
bool checkPentagonal(unsigned long somenum)
{
    double x = (sqrt(24.0 * somenum + 1) + 1) / 6;
    return floor(x) == x;
}
 
unsigned long genTriNum(long n)
{
    unsigned long nth_term = n*((n+1)/2);
    return nth_term;
}

And thank you all for trying to help me .

EDIT:
Just a quick question. What does floor() do?