Alternating Ascending/Descending Rows in Python

Beginner
⏱️ 7 min read
📚 Updated: Aug 2025
🎯 2 Code Examples
Row Parity
Alternating ascending and descending number rows output in Python

What You’ll Learn

How to print a number pattern in Python where each row alternates order: odd rows print numbers in ascending order and even rows print in descending order, while keeping the numbers consecutive overall.

This pattern is a great way to practice using a running counter and a simple odd/even row check.

⭐ Pattern Output

For rows = 5, the pattern looks like this:

Output
1
3 2
4 5 6
10 9 8 7
11 12 13 14 15
1

Complete Python Program

We keep a counter k that increases for every printed number. For even rows, we compute the row end first and print in reverse.

Python
k = 1

for i in range(1, 6):
    m = k + i - 1  # last value in this row
    for _ in range(i):
        if i % 2 == 1:
            print(k, end=" ")
        else:
            print(m, end=" ")
            m -= 1
        k += 1
    print()

🧠 How It Works

1

Start a counter

k = 1 is the next number to be printed.

Setup
2

Row loop decides the length

Row i prints exactly i numbers.

Row control
3

Odd rows print forward

If i is odd, we print k and keep increasing it.

Ascending
4

Even rows print backward

For even rows, we compute m = k + i - 1 (the last value in the row) and print m, m-1, ....

Descending
=

Zigzag rows

Total printed numbers are 1+2+…+r, so runtime is O(r²) for r rows.

2

Variation — User Input Version

Let the user choose the number of rows. The same logic works for any size.

Python
rows = int(input("Enter number of rows: "))
if rows < 1:
    raise ValueError("rows must be at least 1")

k = 1
for i in range(1, rows + 1):
    m = k + i - 1
    for _ in range(i):
        if i % 2 == 1:
            print(k, end=" ")
        else:
            print(m, end=" ")
            m -= 1
        k += 1
    print()

💡 Tips for Enhancement

Try These

  • Print the pattern in a fixed-width grid with padding
  • Replace numbers with letters to create an alternating alphabet pattern
  • Start from 0 instead of 1 to see a different sequence
  • Keep odd rows descending and even rows ascending to invert the zigzag
  • Store each row in a list and reverse it for even rows (alternative approach)

Avoid

  • Forgetting to compute m before printing an even row
  • Incrementing k in the wrong place (sequence breaks)
  • Using rows < 1 without validation
  • Printing without spaces (multi-digit numbers become unreadable)

Key Takeaways

1

Row i contains exactly i consecutive numbers.

2

Odd rows print in ascending order; even rows print in descending order.

3

A single counter maintains continuity across rows.

4

Total prints are triangular, so runtime is O(r²).

❓ Frequently Asked Questions

If the next number is k and the row length is i, then the last value is k+i-1. We store it as m.
Yes. Append numbers to a list and reverse it for even rows, then print the list.
Resetting would repeat numbers. A counter ensures numbers continue across rows: 1, then 2-3, then 4-6, and so on.
O(r²), because the total printed values are 1+2+…+r.

Explore More Python Number Patterns!

Zigzag patterns are a stepping stone to snake-matrix and spiral problems.

All Number Patterns →
Did you know?

This alternating-row idea is similar to how a snake traversal works in a matrix: left-to-right on one row, then right-to-left on the next.

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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