Ndtyjky

I'm writing a program to find the Lucas Number using a recurrence relation. I used a map to store the values but when I run the program it'll output the correct values for the input up to an input of 45 roughly. For example 60 returns 18446744073418719042 when the solution for 60 is 3461452808002 After that the output is incorrect. I'm not sure where my precision starts to fail.

 #include <map>

#include <iterator>



using namespace std;



unsigned long long int lucasNumber( unsigned long long int n ){



    static std::map<unsigned long long int,unsigned long long int> values;



    if(n == 0) {

        return 2;

    } else if(n == 1) {

        return 1;

    }



    std::map<unsigned long long int,unsigned long long int>::iterator iter = values.find(n);



    if(iter == values.end()) {

        return values[n] = lucasNumber(n-1) + lucasNumber(n-2);

    } else {

        return iter->second;

    }

Here is the Main function class. Hopefully this will help clarify anything

#include <stdio.h>

#include "csce310homeWork04part01.hpp"

#include <cstdio>

#include <cstdlib>

#include <iostream>

using namespace std;



int main( int argc , char* argv ){

    unsigned long long int n;

    cin >> n;

    try{

        fprintf( stdout , "Lucas number %llu has a value of %llun" , n , lucasNumber( n ) );

    }

    catch( exception e ){

        cerr << "ERROR" << endl;

    }

}

The following listing works fine for me. I only made very small edits so that I could debug it. Examine how I output the entire calculated sequence to file, so we can see where things went wrong, if they went wrong.

#include <iostream>

#include <fstream>

#include <map>



using namespace std;



typedef std::map<unsigned long long, unsigned long long> StorageType;

StorageType g_previouslyCalculatedValues{ {0, 2}, {1, 1} };



// Calculate Lucas Numbers up to the index specified

unsigned long long LucasNumber(unsigned long long n)

{

    StorageType::iterator iter = g_previouslyCalculatedValues.find(n);



    if (iter == g_previouslyCalculatedValues.end()) {

        return g_previouslyCalculatedValues[n] = LucasNumber(n - 1) + LucasNumber(n - 2);

    }



    return iter->second;

}



int main(int argc, char* argv)

{

    unsigned long long int n;

    cout << "Enter the index of Lucas numbers to calculate" << endl;



    if ((cin >> n).fail())

    {

        cout << "Use an unsigned integer next time. Exiting..." << endl;

        return -1;

    }



    try

    {

        StorageType::value_type result = LucasNumber(n);



        // Debug

        ofstream myfile("results.txt");

        for (auto element : g_previouslyCalculatedValues)

        {

            myfile << element.first << "t: " << element.second << endl;

        }

        myfile.close();

    }

    catch (exception e)

    {

        cerr << "ERROR" << endl;

    }

}

I made the storage global rather than static,but tried it both ways and it worked fine. In the real world, it would be a class member in a calculator class. Are you sure you are testing the listing you gave us? It appears fine to me, using VS2017 64 bit!

Expected result: 3461452808002

Actual result: 3461452808002



From command line:

Enter the index of Lucas numbers to calculate

60

The lucas number for 60 is 3461452808002