C++ value of double is changed when returned

Question

New to C++ and learning. This program returns the correct output. I changed the function prototype to void to isolate and make sure function is providing correct output.

#include <iostream>
#include <fstream>

void ArraySortToMedian(int x[], int numElem); 

using namespace std;

int main() 
{
    ifstream infile;
    infile.open("numbers.txt");

    const int SIZE = 6;
    int array[SIZE];
    int i;

    if(!infile)
    {
        cout << "couldn't find 'numbers.txt'";
        return 1;   
    }

    while(i < SIZE && infile >> array[i])
        i++;

    infile.close();

    for(i = 0; i < SIZE; i++)
        cout << array[i] << "\n"; 

    ArraySortToMedian(array, SIZE);

    return 0;
}

void ArraySortToMedian(int x[], int numElem)
{
    bool swap;
    int temp, i;
    double m;

    do
    {
        swap = false;
        for(i = 0;i < (numElem - 1); i++)
        {
            if( x[i] > x[i + 1] )
            {
                temp = x[i];
                x[i] = x[i + 1];
                x[i + 1] = temp;
                swap = true;
            }
        }
    }
    while (swap);
    cout << "\n";
    for(i = 0; i < numElem; i++)
        cout << x[i] << "\n";

    m = (x[numElem/2] + x[numElem/2]-1)/(double)2;
    cout << "\n" << m;
}

Output:

6
5
3
1
2
4

1
2
3
4
5
6

3.5

When I remove void and replace it with double to return to main() the median value like this.

#include <iostream>
#include <fstream>

double ArraySortToMedian(int x[], int numElem); 

using namespace std;

int main() 
{
    ifstream infile;
    infile.open("numbers.txt");

    const int SIZE = 6;
    int array[SIZE];
    int i;
    double median;

    if(!infile)
    {
        cout << "couldn't find 'numbers.txt'";
        return 1;   
    }

    while(i < SIZE && infile >> array[i])
        i++;

    infile.close();

    for(i = 0; i < SIZE; i++)
        cout << array[i] << "\n"; 

    median=ArraySortToMedian(array, SIZE);

    cout<< "\n" << median << "\n";
    return 0;
}

double ArraySortToMedian(int x[], int numElem)
{
    bool swap;
    int temp, i;
    double m;

    do
    {
        swap = false;
        for(i = 0;i < (numElem - 1); i++)
        {
            if( x[i] > x[i + 1] )
            {
                temp = x[i];
                x[i] = x[i + 1];
                x[i + 1] = temp;
                swap = true;
            }
        }
    }
    while (swap);
    cout << "\n";
    for(i = 0; i < numElem; i++)
        cout << x[i] << "\n";

    m = (x[numElem/2] + x[numElem/2]-1)/(double)2;
    return(m);
}

Distorted Output:

1
6
5
3
1
2

1
1
2
3
5
6

2.5

When returned its moving the elements of the array that are generated in main() when before when I simply outputted from ArraySortToMedian(). I assume that it has to do with me referencing the beginning address of the array first element. Probably very simple but with my limited experience I'm at a loss with this behaviour. Any help so that I can learn what I'm doing wrong will be appreciated. Thank you.

Answer 1

The problem is your input loop:

int i;
// ... snip ...
while(i < SIZE && infile >> array[i])
    i++;

You're never initializing i, so this is undefined behavior. Maybe it works maybe it doesn't.

---

If you used a std::vector instead of an array, this would be easier:

std::vector<int> values;
int next;
while (infile >> next) {
    values.push_back(next);
}

And now you're neither size-limited nor do you have to worry about keeping track of an index.