Hollow Number Triangle Pattern in Python

Beginner
⏱️ 6 min read
📚 Updated: Aug 2025
🎯 2 Code Examples
Edge Printing
Hollow number triangle output in Python (2 2, 3 3, 4 4...)

What You’ll Learn

How to print a hollow number triangle in Python. Each row prints the row number on the left and right edges, with spaces in between.

This is a good exercise for practicing nested loops and conditional printing.

⭐ Pattern Output

For rows = 5, the pattern looks like this:

Output
    1
   2 2
  3   3
 4     4
5       5
1

Complete Python Program

We position the number on the left and right edges using two loops that print either a number or a space based on conditions.

Python
rows = 5

for i in range(1, rows + 1):
    for j in range(rows, 0, -1):
        if i == j:
            print(j, end="")
        else:
            print("", end=" ")
    for k in range(2, rows + 1):
        if i == k:
            print(k, end="")
        else:
            print("", end=" ")
    print()

🧠 How It Works

1

Loop over the rows

for i in range(1, rows + 1) creates one output line per row.

Row control
2

Left edge position

The first inner loop counts down and prints the number when i == j; otherwise it prints spaces.

Left
3

Right edge position

The second loop prints the same row number again when i == k, creating the right edge of the triangle.

Right
=

Hollow edges

The triangle becomes hollow because we print spaces for all non-edge positions.

2

Variation — User Input Version

Let the user choose the number of rows and print the same hollow triangle.

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

for i in range(1, rows + 1):
    for j in range(rows, 0, -1):
        if i == j:
            print(j, end="")
        else:
            print("", end=" ")
    for k in range(2, rows + 1):
        if i == k:
            print(k, end="")
        else:
            print("", end=" ")
    print()

💡 Tips for Enhancement

Try These

  • Print a filled triangle by printing i for every position up to the row width
  • Use * instead of numbers for a hollow star triangle
  • Add fixed-width formatting for multi-digit rows
  • Mirror it to create a hollow pyramid
  • Store rows in a list and join to control spacing

Avoid

  • Mixing 0-based and 1-based indexing (edge conditions will break)
  • Forgetting the newline at the end of each row
  • Using invalid row counts without validation
  • Printing without spaces (pattern becomes unreadable)

Key Takeaways

1

Each row prints the row number on two edge positions.

2

All non-edge positions are spaces, creating the hollow look.

3

The inner gap grows as the row number increases.

4

Runtime is O(r²) for r rows.

❓ Frequently Asked Questions

The first loop places the left edge number with leading spaces. The second loop places the right edge number with the correct spacing after it.
Yes. You can loop over a fixed width and print i only at the two edge column indices; print spaces elsewhere.
On row 1, the two edges overlap, so the number appears only once.
O(r²), because the amount of spacing/printing per row grows with the row index.

Explore More Python Number Patterns!

Hollow shapes are great practice for conditional printing and spacing control.

All Number Patterns →
Did you know?

Hollow patterns are often easier to generate by checking whether a cell is on an edge (first/last column, diagonal, or boundary) and printing spaces otherwise.

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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