Question

I have an integer array sorting program here but I have a problem: whenever I run the program I sometimes get a “The stack around the variable ‘numbers’ was corrupted” message and sometimes it just repeatedly prints out the number 8. Here’s my code (compiled in Visual C++ 2010):

#include <iostream>
#include <cstdlib>
using std::cout;
using std::endl;

void swap(int *x, int *y)
{
    int tmp=0;
    tmp = *x;
    *x  = *y;
    *y  = tmp;
    tmp = 0;
}

int main()
{
    int numbers[13] = {8,16,23,487,2,301,48,0,13,10,644,12};

    int size = sizeof(numbers) / sizeof(int);

    //sort

    int i = 0;
    int* a = &numbers[0];
    int* b = &numbers[1];


    while(i < size){

        if(*a > *b){
            swap(a, b);
        }

        *a++;
        *b++;
         i++;
    }

    //Print our results
    int loopIterator = 0;
    int numToPrint = 0;
    while(loopIterator < size){
        cout << numbers[numToPrint] << endl;
        loopIterator++;
    }


    system("PAUSE");

}

Answer 1

First, you never increment numToPrint, therefore you are never going to print more than the value of numbers[0]. At the very least change your code to:

while(loopIterator < size){
    cout << numbers[numToPrint++] << endl;
    loopIterator++;
}

Secondly, since your while loop uses the test i < size, you are, on the last iteration of the loop, going to be accessing memory outside of numbers for your b pointer, and possibly swapping that value into the last slot of numbers (i.e., where a is pointing to). You want to change your test to i < (size - 1) to avoid that scenario. For instance, if at i == 0 you have a = &numbers[0] and b = &numbers[1], then by the time that i == 12, you are going to end up with a = &numbers[12] and b = &numbers[13] … the value that b is pointing to in this instance is past the end of the array. Depending on how your compiler has setup the stack, and the way you’ve allocated numbers on the stack, this could actually play some havoc with your program should you end up with b pointing into the activation record data-structures for your main() function, and in-turn corrupting it.