Can't sort arrays with even numbers followed by odd numbers

October 14, 2022

I first wrote this: (which works as expected)

#include<iostream>
using namespace std;

int main() {
    int a[5],cpy[5],ctr = 0;

    for (int i = 0 ; i<5 ; i++) {
        cout<<"Enter Value for index "<<i<<": ";
        cin>>a[i];
    }

    for (int i = 0 ; i<5 ; i++)
        if (a[i]%2==0) {
            cpy[ctr]=a[i];
            ctr++;
        }
    for (int i = 0 ; i<5 ; i++)
        if (a[i]%2!=0) {
            cpy[ctr]=a[i];
            ctr++;
        }

    for (int i = 0 ; i<5 ; i++)
        cout<<cpy[i]<<" ";

    return 0;
}

Wanted to make it more condensed/cleaner by improving my logic,
this is what I came up with:

#include<iostream>
using namespace std;

int main() {
    int a[5],cpy[5],ctr = 0;

    for (int i = 0 ; i<5 ; i++) {
        cout<<"Enter Value for index "<<i<<": ";
        cin>>a[i];
    }

    for (int i = 0 ; i<5 && a[i]%2==0 ; i++,ctr++)
            cpy[ctr]=a[i];
    for (int i = 0 ; i<5 && a[i]%2!=0 ; i++,ctr++)
            cpy[ctr]=a[i];

    for (int i = 0 ; i<5 ; i++)
        cout<<cpy[i]<<" ";

    return 0;
}

Expected Result:

Enter Value for index 0: 1
Enter Value for index 1: 2
Enter Value for index 2: 3
Enter Value for index 3: 4
Enter Value for index 4: 5
2 4 1 3 5

What i get after running 2nd version:

Enter Value for index 0: 1
Enter Value for index 1: 2
Enter Value for index 2: 3
Enter Value for index 3: 4
Enter Value for index 4: 5
1 0 24 0 0

Can you suggest where I am wrong in the 2nd block of code. The first block works correctly.

>Solution :

The problem here is that you will never enter the first loop. The counter is incremented only if the condition is satisfied, otherwise the loop is broken. You should not implement a condition like this.

I suggest you to try the following with std::vector :

#include<iostream>
#include<vector>
using namespace std;

int main() {
    vector<int> a, cpy;

    for (int i = 0 ; i<5 ; i++) {
        a.push_back(i+1);
    }

    for (int i = 0 ; i<5; i++) {
        if (a[i]%2 == 0)
            cpy.push_back(a.at(i));
    }
    for (int i = 0 ; i<5 ; i++) {
        if (a[i]%2 != 0)
            cpy.push_back(a.at(i));
    }

    for (int i = 0 ; i<5 ; i++)
        cout<<cpy[i]<<" ";

    return 0;
}

It works as expected, and in a more condensed manner.