Question

I’m currently building a prime number finder, and am having a memory problem:

This may be due to a corruption of the heap, which indicates a bug in PrimeNumbers.exe or any of the DLLs it has loaded.

PS. Please don’t say to me if this isn’t the way to find prime numbers, I want to figure it out myself!

Code:

// PrimeNumbers.cpp : main project file.

#include "stdafx.h"
#include <vector>

using namespace System;
using namespace std;

int main(array<System::String ^> ^args)
{
Console::WriteLine(L"Until what number do you want to stop?");
signed const int numtstop = Convert::ToInt16(Console::ReadLine());
bool * isvalid = new bool[numtstop];


    int allattempts = numtstop*numtstop; // Find all the possible combinations of numbers

    for (int currentnumb = 0; currentnumb <= allattempts; currentnumb++) // For each number try to find a combination
    {
        for (int i = 0; i <= numtstop; i++)
        {
            for (int tnumb = 0; tnumb <= numtstop; tnumb++)
            {
                if (i*tnumb == currentnumb)
                {
                    isvalid[currentnumb] = false;
                    Console::WriteLine("Error");
                }
            }
        }
    }

    Console::WriteLine(L"\nAll prime number in the range of:" + Convert::ToString(numtstop));

    for (int pnts = 0; pnts <= numtstop; pnts++)
    {
        if (isvalid[pnts] != false)
        {
            Console::WriteLine(pnts);
        }
    }

return 0;
}

I don’t see the memory problem.

Please help.

Answer 1

You are allocating numtstop booleans, but you index that array using a variable that ranges from zero to numtstop*numtstop. This will be severely out of bounds for all numstop values greater than 1.

You should either allocate more booleans (numtstop*numtstop) or use a different variable to index into isvalid (for example, i, which ranges from 0 to numstop). I am sorry, I cannot be more precise than that because of your request not to comment on your algorithm of finding primes.

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P.S. If you would like to read something on the topic of finding small primes, here is a link to a great book by Dijkstra. He teaches you how to construct a program for the first 1000 primes on pages 35..49.