Odd, even

Odd numbers are not even. With modulo division, we can see if the number is evenly divisible by 2. If it is not, it must be odd.

We demonstrate the IsOdd method and then the IsEven method. It is important to test methods like these—one can be implemented with the negation of the other.

Odd example

The IsOdd static method performs a modulo division on the parameter, which returns the remainder of a division operation.

using System;

class Program
{
    static void Main()
    {
        for (int i = 0; i <= 100; i++)
        {
            if (IsOdd(i))
            {
                Console.WriteLine(i);
            }
        }
    }

    public static bool IsOdd(int value)
    {
        return value % 2 != 0;
    }
}
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Even

All even numbers are divisible by two. This means we can use the modulo division operator to see if there is any remainder when the number is divided by two.

using System;

class Program
{
    static void Main()
    {
        for (int i = 0; i <= 100; i++)
        {
            if (IsEven(i))
            {
                Console.WriteLine(i);
            }
        }
    }

    public static bool IsEven(int value)
    {
        return value % 2 == 0;
    }
}
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Notes, IsOdd

You could implement IsOdd by using the IsEven method and returning its boolean opposite. In other words, you could simply return "!IsEven."

A surprising number of programs require that you test for even and odd numbers. For example, if data must be entered in pairs, its count will always be even.