C# Odd and Even

Odd numbers are not even. With the C# modulo division operator, we can see if the number is evenly divisible by two; if it is not, it must be odd. In this example, we demonstrate the IsOdd method and then the IsEven method.

This C# article tests for odd numbers with a simple method. It uses modulo division.

Odd

This program introduces the custom IsOdd static method. It performs a modulo division on the parameter, which returns the remainder of a division operation. If the remainder not 0, then the number must be odd because the remainder would be 0 if it was divisible by two.

Program that finds odd numbers [C#]

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;
    }
}

Output

1
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Not even. You could actually implement IsOdd by using the IsEven method and returning its boolean opposite. In other words, you could simply "return !IsEven(value);". For the source code to the IsEven method, check out the parallel article on Dot Net Perls.

Correction: There was something odd about the first implementation of IsOdd on this page: it did not handle negative numbers correctly. Fortunately, Joshua Goodman wrote in with the bug report and I corrected the problem.

Even

Because all even numbers are divisible by two, we can use the modulo division operator to see if there is any remainder when the number is divided by two. If there is no remainder (it returns zero) then the number is definitely even. Zero is an even number—this method is correct for zero.

Program that finds even numbers [C#]

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;
    }
}

Output

0
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100

Note. The IsEven method shown here is very simple; it could be inlined into the locations you call it without too much loss of clarity. To learn more about the parity of zero, check out Wikipedia.

Summary

A surprising number of programs require that you test for even and odd numbers. For example, if data must be entered in pairs, it will always be even, so if it is odd an error would have occurred.