C++ Heap Corruption problem

I was writing a simple commandline C++ program in native code to create a list of prime numbers up to a specified number, but I am running into Heap Corruption errors when I try to deallocate memory I no longer need before my sieve() function terminates. The following is my program's code, and the problem is towards the end of the function when I try to release memory that I dynamically allocated to a bool array.

#include <iostream>
#include <sstream>
#include <cmath>

using namespace std;

int * sieve(int limit);

int main()
{
    int x;

    cout << "Please Enter a Number: " << endl;

    cin >> x;

    if (x = 0)
    {
        return 0;
    }

    sieve(x);

    //Output code goes here

    return main();
}

int * sieve(int limit)
{
    //Create bool array, each element is a number, element 0 is 0.
    //Idea for future use; use individual bits instead of
    bool * a = new (nothrow) bool [limit];

    //Set all to true
    for (int x = limit; x >= 0; x--)
    {
        a[x] = true;
    }

    //Set Mutiples of Primes to False
    for (int x = 2, max = (int) sqrt((double)limit); x <= max; x++)
    {
        for (int y = x << 1; y < limit; y += x)
        {
            a[y] = false;
        }
    }

    //Count Primes
    int i = 0;
    for (int x = 2; x < limit; x++)
    {
        if (a[x])
        {
            i++;
        }
    }

    //Create int array to store primes
    int * b = new (nothrow) int [i];

    //Fill array with primes
    for (int x = 2, p = 0; x < limit; x++)
    {
        if (a[x])
        {
            b[p] = x;
            p++;
        }   
    }

    //Release Memory used to store bool array
    //The following two lines do not work
    delete [] a;
    delete a;

    //Return Array of Primes to function call
    return b;
}

Debug Error!

Program: ...

HEAP CORRUPTION DETECTED: after Normal block (#134) at 0x00355DC0. CRT detected that the application wrote to memory after the end of the heap buffer.

Matthew van Eerde wrote:

You're not getting that far.  The heap corruption is here:

bool * a = new (nothrow) bool [limit]; // a[0] to a[limit - 1]

//Set all to true
for (int x = limit; x >= 0; x--)
{
    a[x] = true; // first iteration is a[limit] = true; boom!
}

Shining Arcanine wrote:

    //Release Memory used to store bool array
    //The following two lines do not work
    delete [] a;
    delete a;

Another problem is this piece of code:

Shining Arcanine wrote:


    if (x = 0)
    {
        return 0;
    }

1. The code inside the if-block will never be executed, since (x = 0) evaluates to the value of x, which is 0 after the assignment.
2. x will always be 0 below the block.

Can you please tell me what this line means:

int y = x << 1

especially the << 1....what does that do for the program.

I am fairly new to programming so I would appreciate any help.  I was just trying to understand how this program works.

Thanks.

as400k wrote:

Can you please tell me what this line means:

int y = x << 1

especially the << 1....what does that do for the program.

Rossj wrote:



as400k wrote: Can you please tell me what this line means: int y = x << 1 especially the << 1....what does that do for the program.

It is the bitshift. Shifts the int (x) 1 bit to the left,  multiplying the number by two.  It could be replaced by int y = x * 2; but shifting *maybe/used to be* faster.

You can also shift right by 1 to divide a number by two - although you have to be very careful to ensure you know what you are doing (i.e. what does 1 >> 1 do?).