Alternating Odd/Even Number Triangle in C++

Beginner
⏱️ 5 min read
📚 Updated: Aug 2025
🎯 2 Code Examples
Nested Loops
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 a triangle where odd rows contain odd numbers and even rows contain even numbers:

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

We choose a starting value based on the row index (1 or 2), then keep adding 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

For each row i, start k as 1 (odd row) or 2 (even row), then print i values by doing k += 2.

C++
#include <iostream>
using namespace std;

int main() {
    int rows = 5;
    int i, j, k;

    for (i = 1; i <= rows; i++) {
        k = (i % 2 == 0) ? 2 : 1;
        for (j = 1; j <= i; j++) {
            cout << k << " ";
            k += 2;
        }
        cout << "\n";
    }

    return 0;
}

🧠 How It Works

1

Set the number of rows

int rows = 5; controls how many lines to print.

Setup
2

Outer loop (rows)

for (i = 1; i <= rows; i++) increases the row length from 1 to rows.

Row control
3

Pick odd/even start

k = (i % 2 == 0) ? 2 : 1; sets k to 2 on even rows and 1 on odd rows.

Parity
4

Inner loop (print and add 2)

Printing k and then doing k += 2 keeps the row odd-only or even-only.

Number printing
=

Alternating odd/even triangle

Odd rows build 1,3,5,... and even rows build 2,4,6,... using the same k += 2 rule.

2

Variation — User Input Version

Let the user choose the number of rows at runtime. We keep the same odd/even row logic.

C++
#include <iostream>
using namespace std;

int main() {
    int rows;
    int i, j, k;

    cout << "Enter the number of rows: ";
    cin >> rows;

    if (!cin || rows <= 0) return 0;

    for (i = 1; i <= rows; i++) {
        k = (i % 2 == 0) ? 2 : 1;
        for (j = 1; j <= i; j++) {
            cout << k << " ";
            k += 2;
        }
        cout << "\n";
    }

    return 0;
}

💡 Tips for Enhancement

Try These

  • Remove trailing spaces by printing a space only between values
  • Start even rows from a different even number (like 0) for another variation
  • Right-align the triangle by printing leading spaces before each row
  • Replace the numbers with characters (A/C/E for odd rows, B/D/F for even rows)
  • Convert this into a function to reuse across multiple patterns

Avoid

  • Using endl inside loops (unnecessary flushing)
  • Forgetting to reset k at the start of every row
  • Mixing the row counter (i) and position counter (j) logic
  • Not validating user input before using rows

Key Takeaways

1

Odd rows start from 1 and print odd numbers by adding 2.

2

Even rows start from 2 and print even numbers by adding 2.

3

Resetting k each row is what makes the pattern alternate correctly.

4

Printing \(1+2+\dots+n\) values makes the pattern growth quadratic.

❓ Frequently Asked Questions

It checks the row number i. If i is even, it starts at 2; otherwise it starts at 1.
Adding 2 preserves parity. Starting from 1 keeps it odd-only; starting from 2 keeps it even-only.
Print a space only between values (for example, print a space when j < i).
O(n²) for n rows, because the total printed numbers are 1+2+…+n.

Explore More C++ Number Patterns!

Alternating rules like parity (odd/even) are a fun way to practice nested loops.

All Number Patterns →
Did you know?

Adding 2 repeatedly keeps numbers in the same parity group (all odd or all even). That’s why k += 2 is perfect for this pattern.

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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