Alternating Odd/Even Number Triangle in C#

Beginner
⏱️ 6 min read
📚 Updated: Aug 2025
🎯 2 Code Examples
Modulo Operator
Alternating odd/even number triangle output (1, 2 4, 1 3 5, 2 4 6 8, 1 3 5 7 9) in C# programming

What You’ll Learn

How to print an alternating odd/even triangle in C#. Odd rows print odd numbers (1 3 5 ...) and even rows print even numbers (2 4 6 ...).

The key idea is to pick a starting value (k) based on row parity using i % 2, and then increment by 2.

⭐ Pattern Output

For rows = 5, the pattern looks like this:

Output
1
2 4
1 3 5
2 4 6 8
1 3 5 7 9
1

Complete C# Program

Set k to 1 for odd rows and 2 for even rows, then print k and increment it by 2 across the row.

C#
using System;

namespace MyApp
{
    class Program
    {
        static void Main(string[] args)
        {
            int rows = 5;
            int i, j, k;

            for (i = 1; i <= rows; i++)
            {
                if (i % 2 == 0)
                    k = 2;
                else
                    k = 1;

                for (j = 1; j <= i; j++)
                {
                    Console.Write(k + " ");
                    k += 2;
                }
                Console.WriteLine();
            }
        }
    }
}

🧠 How It Works

1

Outer loop controls the row

i runs from 1 to rows. Each row prints i numbers.

Row control
2

Pick odd/even start using i%2

If i is even, set k = 2 (even numbers). Otherwise set k = 1 (odd numbers).

Parity
3

Inner loop prints and increments by 2

We print k, then update k += 2 so it stays odd or even while increasing.

Sequence
=

Alternating odd/even rows

Odd rows print odd numbers; even rows print even numbers.

2

Variation — User Input Rows

Let the user decide the row count:

C#
using System;

namespace MyApp
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.Write("Enter the number of rows: ");
            int rows = Convert.ToInt32(Console.ReadLine());

            for (int i = 1; i <= rows; i++)
            {
                int k = (i % 2 == 0) ? 2 : 1;

                for (int j = 1; j <= i; j++)
                {
                    Console.Write(k + " ");
                    k += 2;
                }
                Console.WriteLine();
            }
        }
    }
}

💡 Tips for Enhancement

Try These

  • Remove trailing spaces by printing a space only between numbers
  • Swap odd/even rows by flipping the condition
  • Try starting values like 5 (odd rows) and 6 (even rows) to shift the sequences

Avoid

  • Forgetting to reset k for each row
  • Using k++ (that mixes odd and even numbers)

Key Takeaways

1

Row parity (i % 2) decides whether to print odd or even numbers.

2

k += 2 keeps the sequence odd-only or even-only.

3

The inner loop prints exactly i numbers on row i.

4

Total numbers printed is triangular: \(1+2+\dots+n\).

❓ Frequently Asked Questions

Because row 2 is even, we set k = 2 and then increment by 2 for each print.
Yes. Change the odd-row start from 1 to 3. Just keep using k += 2 to stay odd.
O(n²) for n rows since total prints are n(n+1)/2.

Explore More C# Number Patterns!

Try combining parity logic with different loop directions to discover new patterns.

All Number Patterns →
Did you know?

Checking i % 2 is a simple example of parity. Parity checks are also used in hashing, checksums, and error detection.

About the author

Mari Selvan M P
Mari Selvan M P 🔗

Developer, cloud engineer, and technical writer

  • Experience 12 years building web and cloud systems
  • Focus Full Stack Development, AWS, and Developer Education

I write practical tutorials so students and working developers can learn by doing—from databases and APIs to deployment on AWS.

AWS Certified Developer – Associate (Credly)

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