Alternating Number Pattern in C++

Beginner
⏱️ 6 min read
📚 Updated: Aug 2025
🎯 2 Code Examples
Condition + Loops
Alternating number pattern output (12345, 4321, 123, 21, 1) in C++ programming

What You’ll Learn

How to print an alternating ascending-descending number pattern in C++: 12345, 4321, 123, 21, 1.

The trick is to use a simple even/odd check on the row size i to decide the direction of printing.

⭐ Pattern Output

For rows = 5, the pattern looks like this:

Output
12345
4321
123
21
1
1

Complete C++ Program

The outer loop shrinks the row length. If i is odd, print 1..i. If i is even, print i..1.

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

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

    for (i = rows; i >= 1; i--) {
        if (i % 2 == 0) {
            for (j = i; j >= 1; j--) {
                cout << j;
            }
        } else {
            for (j = 1; j <= i; j++) {
                cout << j;
            }
        }
        cout << "\n";
    }

    return 0;
}

🧠 How It Works

1

Choose the row count

int rows = 5; sets the starting row length.

Setup
2

Outer loop (shrink rows)

for (i = rows; i >= 1; i--) reduces the row length from 5 down to 1.

Row control
3

Decide the direction (odd vs even)

if (i % 2 == 0) chooses descending output for even rows and ascending output for odd rows.

Condition
4

Print the row

If odd, print 1..i. If even, print i..1. Then print a newline with cout << "\\n";.

Output
=

Alternating direction pattern

Total printed numbers are still triangular: n(n+1)/2, giving O(n²) time complexity for n rows.

2

Variation — User Input Version

Make the pattern dynamic by reading rows from the user:

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

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

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

    for (i = rows; i >= 1; i--) {
        if (i % 2 == 0) {
            for (j = i; j >= 1; j--) {
                cout << j;
            }
        } else {
            for (j = 1; j <= i; j++) {
                cout << j;
            }
        }
        cout << "\n";
    }

    return 0;
}

💡 Tips for Enhancement

Try These

  • Add spaces after each number for clarity: cout << j << ' ';
  • Swap the parity rule (even ascending, odd descending) to create a new variation
  • Start the outer loop from 1..rows to make the triangle grow instead of shrink
  • Replace numbers with letters to create alternating alphabet patterns
  • Use a boolean toggle (flip each row) instead of parity if you want a different alternation rule

Avoid

  • Forgetting to print a newline after each row
  • Hard-coding 5 everywhere instead of using rows
  • Mixing up the loop bounds (descending loop must stop at 1)
  • Using endl inside loops (unnecessary flushing)

Key Takeaways

1

The outer loop controls the row size by counting down from rows to 1.

2

A simple parity check (i % 2) decides whether to print ascending or descending.

3

Odd rows print 1..i; even rows print i..1.

4

Total work is still triangular: n(n+1)/2 prints in total.

❓ Frequently Asked Questions

Because the program uses i % 2 to choose ascending or descending printing. That makes one row go forward and the next row go backward.
Yes. Use a boolean flag and flip it each row. This can be useful if the row index parity doesn’t match the alternation rule you want.
Print a space after each number: cout << j << ' ';. You can also trim the trailing space if needed.
O(n²) for n rows: you print n+(n-1)+…+1 = n(n+1)/2 values.

Explore More C++ Number Patterns!

Alternate, repeat, align, and experiment—each pattern strengthens your loop thinking.

All Number Patterns →
Did you know?

Parity checks like i % 2 are a simple way to create alternating behavior in patterns without extra counters.

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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