Why is my function not working properly

Question

My function that checks if numbers in the vector are even or odd doesn't work properly. This function prints the numbers that i have typed in the vector and puts them in 2 categories EVENS/ODDS. But if i have a negative number there is a problem with the odds not printing it but evens work. problematic code is here:

void printEvensAndOddsVector(const vector <int>& new_v1)
{
    cout << "Vector Evens: ";
    for (unsigned int i = 0; i < new_v1.size(); ++i)
    {
        if (new_v1.at(i) % 2 == 0)
        {
            cout << new_v1.at(i) << " ";
        }
    }
    cout << " \n";

    cout << "Vector Odds: ";
    for (unsigned int i = 0; i < new_v1.size(); ++i)
    {
        if (new_v1.at(i) % 2 == 1) // Here is the problem.
        {
            cout << new_v1.at(i) << " ";
        }
    }
    cout << " \n";
}

Or more exactly this line: if (new_v1.at(i) % 2 == 1) It prints the numbers that are odd but only the positive ones not the negative ones. But if i change it to if (new_v1.at(i) % 2 != 0) then it works correctly. Why does that happen and is there a problem with the equal operator? If yes then why are the evens getting printed even if they are negatives while still using the equal operator?

Code here for reference.

clude <iostream>
#include <vector>

using namespace std;

void fillVector(vector <int>&);

void printVector(const vector <int>&);

void printEvensAndOddsVector(const vector <int>&); // Prints Evens and Odds.

int main()
{
    vector <int> v1;

    fillVector(v1);
    printVector(v1);
    printEvensAndOddsVector(v1);

    cout << "\n\n\n";
    system( "pause");
    return 0;
}

// Function Definitions
void fillVector(vector <int>& new_v1)
{
    int number;
    cout << "Type in numbers and type -100 to stop: ";
    cin >> number;

    while (number != -100)
    {
        new_v1.push_back(number);
        cin >> number;
    }
}

void printVector(const vector <int>& new_v1)
{
    cout << "\nVector: ";
    for (unsigned int i = 0; i < new_v1.size(); ++i)
    {
        cout << new_v1.at(i) << " ";
    }
    cout << " \n";
}

void printEvensAndOddsVector(const vector <int>& new_v1)
{
    cout << "Vector Evens: ";
    for (unsigned int i = 0; i < new_v1.size(); ++i)
    {
        if (new_v1.at(i) % 2 == 0)
        {
            cout << new_v1.at(i) << " ";
        }
    }
    cout << " \n";

    cout << "Vector Odds: ";
    for (unsigned int i = 0; i < new_v1.size(); ++i)
    {
        if (new_v1.at(i) % 2 == 1)
        {
            cout << new_v1.at(i) << " ";
        }
    }
    cout << " \n";
}

Answer 1

This line: if (new_v1.at(i) % 2 == 1) . It prints the numbers that are odd but only the positive ones not the negative ones.

For negative n , n % 2 returns either 0 or -1 in C++. It never returns 1 , so the condition cannot be true for negative inputs.

As you've discovered, comparing to zero would work.

See Modulo operator with negative values