Star Pattern Programs using Nested For Loops (Python, C, Java)

Star patterns are the first thing most students learn to understand nested loops. In this post you will learn how the outer loop and inner loop work together, with code in Python, C and Java, a dry run, and an interactive tool where you can watch the loops run step by step.

What is a Nested For Loop?

A nested for loop is a loop placed inside another loop. The outer loop controls the number of rows. For each row, the inner loop runs completely and prints the characters of that row.

How the Logic Works

Example 1: Right Triangle Pattern

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

Python

python

n = 5
for i in range(1, n + 1):
    for j in range(i):
        print("*", end="")
    print()

C

c

#include <stdio.h>
int main() {
    int n = 5;
    for (int i = 1; i <= n; i++) {
        for (int j = 0; j < i; j++) {
            printf("*");
        }
        printf("\n");
    }
    return 0;
}

Java

java

public class Pattern {
    public static void main(String[] args) {
        int n = 5;
        for (int i = 1; i <= n; i++) {
            for (int j = 0; j < i; j++) {
                System.out.print("*");
            }
            System.out.println();
        }
    }
}

Dry Run (n = 3)

Example 2: Pyramid Pattern

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

Here each row needs two inner loops: one for spaces (n - i) and one for stars (2*i - 1).

python

n = 5
for i in range(1, n + 1):
    for j in range(n - i):
        print(" ", end="")
    for j in range(2 * i - 1):
        print("*", end="")
    print()

Quick Formula Table

Common Mistakes

1. Forgetting print() after the inner loop, so all stars come in one line.
2. Starting i at 0 and getting an extra or missing row.
3. Using the same variable name for both loops.
4. Wrong indentation in Python, which puts print() inside the inner loop.