Alternating 1 and 0 Triangle in Python

Beginner
⏱️ 5 min read
📚 Updated: Aug 2025
🎯 2 Code Examples
Modulo
Alternating 1 and 0 triangle output in Python (11111, 0000, 111, 00, 1)

What You’ll Learn

How to print an alternating 1 and 0 pattern in Python where odd rows contain 1s and even rows contain 0s.

The pattern uses modulo (i % 2) and a decreasing inner-loop range to shorten each row.

⭐ Pattern Output

For 5 rows, the pattern looks like this:

Output
11111
0000
111
00
1
1

Complete Python Program

Row i prints i % 2 repeatedly. The inner loop range gets smaller each row.

Python
rows = 5

for i in range(1, rows + 1):
    for _ in range(i, rows + 1):
        print(i % 2, end="")
    print()

🧠 How It Works

1

Choose the number of rows

rows = 5 controls the triangle height and the first row width.

Setup
2

Outer loop (rows)

for i in range(1, rows + 1) iterates row by row.

Row control
3

Inner loop (decreasing width)

for _ in range(i, rows + 1) prints 5 digits on row 1, then 4, 3, 2, and 1.

Width
4

Modulo decides 1 or 0

i % 2 is 1 for odd rows and 0 for even rows, so the rows alternate automatically.

Logic
=

Alternating rows

Total printed digits are 1+2+…+n = n(n+1)/2, so runtime is O(n²) for n rows.

2

Variation — User Input Version

Let the user choose how many rows to print:

Python
rows = int(input("Enter the number of rows: "))

for i in range(1, rows + 1):
    for _ in range(i, rows + 1):
        print(i % 2, end="")
    print()

💡 Tips for Enhancement

Try These

  • Validate input (reject rows < 1) before printing
  • Flip the pattern by printing 1 - (i % 2)
  • Add spaces between digits for readability
  • Center-align the triangle by printing leading spaces
  • Turn it into a checkerboard by alternating per column too (use (i + j) % 2)

Avoid

  • Forgetting print() after each row
  • Hardcoding row counts (use rows variable)
  • Confusing % with division (it gives remainder)
  • Assuming user input is always valid (wrap int() if needed)

Key Takeaways

1

i % 2 is an easy way to alternate between 1 and 0 by row.

2

The inner loop range controls the decreasing row length.

3

You can flip the output by printing 1 - (i % 2).

4

Total prints follow n(n+1)/2, so runtime is O(n²).

❓ Frequently Asked Questions

Because i % 2 alternates between 1 (odd i) and 0 (even i).
Change the print to print(i % 2, end=" ").
Print 1 - (i % 2) instead of i % 2.
O(n²), because the total printed characters grow like \(1+2+\dots+n\).

Explore More Python Number Patterns!

From pyramids to Floyd’s triangle and beyond—practice nested loops with progressively richer patterns.

All Number Patterns →
Did you know?

The modulo operator % is commonly used for pattern problems because it’s a simple way to alternate states (odd/even, on/off, 1/0).

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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