Sorting an array— issue

Question

I'm asking for help with a problem implying an array sorting in the following manner: all even numbers must be in front of the odd ones. I've partially made the problem, but I did the sorting in the opposite manner and I can not manage to fix it.

#include "stdafx.h"
#include <iostream>
using namespace std;

int main()
{
    int v[100], n, i, aux = 0, inv;
    cout << "Number of elements: ";
    cin >> n;
    for (i = 0; i < n; i++)
    {
        cout << "v[" << i << "]=";
        cin >> v[i];
    }

    do
    {   
        inv = 0;
        for (i = 0; i < n; i++)
        {
            if (v[i] % 2 == 1 && v[i + 1] % 2 == 0)
            {
                inv = 1;
                aux = v[i];
                v[i] = v[i + 1];
                v[i + 1] = aux;
            }
        }
    } while (inv != 0);
    cout << endl;

    for (i = 0; i < n; i++)
        cout << v[i] << " ";
    cout << endl;
    system("pause");
    return 0;
}

The output for this would be:

n = 8
1 3 2 4 7 8 4 2

Result: 2 4 8 4 2 -858993460 1 3

This seems like a good time to learn how to debug your programs. — Some programmer dude
– Some programmer dude, Commented Oct 22, 2018 at 9:09
Alright. I've checked on my program, but my guess is that, because of the size of the array, [100], the free spaces would get the value `-858993460'. I'm not so common with the dynamic alloc. of memory, so is there any possibility I can initialize the array with no number of elements and give it the number while inputing the values for its elements? Or any tips of dynamic alloc.? — Angel
– Angel, Commented Oct 22, 2018 at 9:15

P.W · Accepted Answer · 2018-10-22 09:12:38Z

1

In the expression v[i + 1], you access v[n] when i = n - 1, this will result in an out-of-bounds memory access which results in undefined behaviour. You should change the for loop to this:

for (i = 0; i < n - 1; i++)

The output for the given input is:

a.exe
Number of elements: 8
v[0]=1
v[1]=3
v[2]=2
v[3]=4
v[4]=7
v[5]=8
v[6]=4
v[7]=2

2 4 8 4 2 1 3 7

answered Oct 22, 2018 at 9:12

P.W

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3 Comments

Angel Over a year ago

Oh, that's right! I forgot that I'm checking, simultaneously, the first element and the next one. Yeah, you are right. Thank you!

Angel Over a year ago

I'm aware of that, but the portal won't let me accept before 5 minutes, I'm afraid. I appreciate the answer. I have, still, a question about the program's efficiency (or complexity), if I'm speaking correctly?

P.W Over a year ago

You can use stl algorithms for swapping, iterating over elements etc. These are considered very efficient.

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