This content originally appeared on DEV Community and was authored by datatoinfinity

Ever stared at a coding pattern problem and wondered, “Where do I even start?”
If yes, this series is for you. We’ll break down pattern-based logic building — from stars to numbers — using simple Python code and clear visuals.

Pattern: Right-Angled Triangle Using Nested Loops in Python

We’re going to build this simple star pattern and understand the logic behind nested loops:

Logic Overview

We’re using two for loops:

• The outer loop controls the rows
• The inner loop controls the columns (stars)

Output:

* 
* * 
* * * 
* * * * 
* * * * *

Step-by-Step Explanation

row = 5
for i in range(1, row + 1):       # Outer loop: for each row (1 to 5)
    for j in range(i):            # Inner loop: print i stars
        print("*", end=" ")
    print()                       # Move to the 
next line after each row

1. Why range(1, row + 1)?

   • range(n) goes from 0 to n-1
   • So range(1, row + 1) gives: 1, 2, 3, 4, 5
   • That means we’ll have 5 rows, as required.

2. Why range(i) for inner loop?
   In each row, the number of stars is equal to the row number:

Row 1 → 1 star
Row 2 → 2 stars
…
Row 5 → 5 stars

Dry Run Table

Notice how the number of * matches the current row number i.

Summary
Outer loop (i) = rows
Inner loop (j) = columns (stars)
print("*", end=" ") prints stars on the same line
print() moves to the next line after each row

Reverse Right-Angled Triangle Pattern in Python

Output:

* * * * * 
* * * * 
* * * 
* * 
* 

Step-by-Step Explanation:

row = 5
for i in range(row, 0, -1):       # Outer loop: from 5 to 1
    for j in range(i):            # Inner loop: print i stars
        print("*", end=" ")
    print()                       # Move to the next line after each row

1. row = 5 We want 5 rows, so we initialize the row variable as 5. You can increase this number for a larger pattern.

2. for i in range(row, 0, -1)

   • This loop goes in reverse from row to 1.
   • range(5, 0, -1) outputs: 5, 4, 3, 2, 1
   • The third parameter -1 is the step; without it, the loop won’t run in reverse.

3. for j in range(i)

   • This inner loop controls how many stars get printed in each row — same as the current value of i. So: Row 1 → 5 stars Row 2 → 4 stars … Row 5 → 1 star

Dry Run Table

Summary:
Outer loop (i): controls the number of rows, decreasing each time
Inner loop (j): prints stars equal to the current row number
end=" " prints stars on the same line
print() moves to the next line after each row

What if we make mirror image of Right-Angled Triangle Using Nested Loops in Python?

This content originally appeared on DEV Community and was authored by datatoinfinity