same program produces different output every other run

Question

This is part of a quick&dirty program I wrote to do some statistics calculations. Sometimes I gives me the expected output, which is:

42 1 1
48 13 14
54 12 26
60 11 37
66 6 43
72 13 56
78 23 79
84 14 93
90 6 99
96 0 99
102 0 99
108 1 100

But sometimes (about 50% of the times I run the program) the last line of the output differs and looks like this:

108 2 101

I'm compiling with gcc 7.3.0 on Debian 64bit with no compiler flags except -o

I know, the program could be written more elegantly but I don't see where this strange behavior comes from.

#include <iostream> //cout, endl
#include <algorithm> //sort
#include <iomanip> // setw(), setfill(), left, right

using namespace std;

void classes(int, int, int[]);

static int sizeOfData = 0;

int main()
{
    int data[] ={
                    80, 71, 57, 80, 75, 77, 60, 86, 77, 56, 81, 50, 89, 54, 90, 73, 60, 83, 65, 82, 84, 54, 85, 58, 79,
                    57, 88, 68, 76, 78, 74, 85, 75, 65, 76, 58, 91, 50, 87, 48, 93, 54, 86, 53, 78, 52, 83, 60, 87, 49,
                    80, 60, 92, 43, 89, 60, 84, 69, 74, 71, 108, 50, 77, 57, 80, 61, 82, 48, 81, 73, 62, 79, 54, 80, 73,
                    81, 62, 81, 71, 79, 81, 74, 59, 81, 66, 87, 53, 80, 50, 87, 51, 82, 58, 81, 49, 92, 50, 88, 62, 93
                    };

    sizeOfData = sizeof(data)/sizeof(data[0]);
    sort(data, data + sizeOfData);

    classes(42, 6, data);
}

void classes(int start, int step, int data[])
{
    int counter = 0;
    int classCounter = 0;

    const char separator = ' ';
    int tabWidth = 8;

    cout << endl;
    cout << left << setw(tabWidth) << setfill(separator) << "class";
    cout << left << setw(tabWidth) << setfill(separator) << "#";
    cout << left << setw(tabWidth) << setfill(separator) << "# cumulative";
    cout << endl << endl;

    int i = 0;
    while(i < sizeOfData)
    {
        classCounter = 0;
        while(data[i] < start + step)
        {
            classCounter++;
            counter++;
            i++;
        }
        cout << left << setw(tabWidth) << setfill(separator) << start;
        cout << left << setw(tabWidth) << setfill(separator) << classCounter;
        cout << left << setw(tabWidth) << setfill(separator) << counter;
        cout << endl;
        start = start + step;
    }
}

Your life would be a lot easier if instead of using these really clunky calculations for array size you just use a Standard Library container like std::vector. — tadman
– tadman, Commented Apr 14, 2018 at 21:32

miradulo · Accepted Answer · 2018-04-14 21:46:45Z

5

Your program has undefined behavior on the last iteration. If i is 99 on the last iteration, you happily increment beyond the bounds of the array in the inner loop, after which data[i] is undefined behavior.

To fix this, the simplest thing to do would be to check that i is not larger than sizeOfData in the interior condition as well.

while((i < sizeOfData) && (data[i] < start + step))

It would be worthwhile to consider picking up an STL container like std::vector or std::array - it would simplify your handling of the array size, and you could make your classes function more generic.

edited Apr 14, 2018 at 21:46

answered Apr 14, 2018 at 21:37

miradulo

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LogicStuff · Accepted Answer · 2018-04-14 21:37:11Z

3

You don't check if i < sizeOfData in the nested loop. That's the problem. It can go out of bounds - undefined behaviour.

answered Apr 14, 2018 at 21:37

LogicStuff

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AAEM · Accepted Answer · 2018-04-14 22:53:04Z

0

The counter i in the for loop is exceeding the boundary of the sizeOfData that you use in your code. Try check the i value to avoid this.

answered Apr 14, 2018 at 22:53

AAEM

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