Star Patterns — Practicing Nested Loops

The Idea#

A star pattern is a shape — a triangle, a pyramid, a diamond — printed to the console using * characters. They look like a party trick, but they're the cleanest way to internalize nested loops. The outer loop walks down the rows; the inner loop fills each row. Get this right once, and 2D arrays, matrix traversal, and grid problems all start to feel familiar.

The Pattern Anatomy#

Every pattern boils down to two questions: how many rows? and for each row, how many characters? The row index i drives everything.

The Inner-Loop Formula#

Every shape is just a different rule for how many spaces and stars row i gets. Here are the four you'll see most often:

Pattern	Inner loop runs	Example (i=3)
Right Triangle	`i` stars	`***`
Pyramid	`N - i` leading spaces, then `2i - 1` stars	`*****` (N=5)
Inverted Triangle	`N - i + 1` stars	`***` (N=5)
Diamond	Top half: pyramid up to N; bottom half: inverted pyramid	two triangles glued together

The trick is to always express the inner bounds in terms of i. Once that clicks, every pattern is a 5-minute problem.

Right Triangle — The Simplest Pattern#

Start here. Outer loop walks rows 1..N; inner loop prints i stars on row i. That's the whole algorithm.

python

Loading editor…

Pyramid — Centering with Spaces#

A pyramid is a right triangle that's been centered. To center row i, you pad the left with N - i spaces, then print 2i - 1 stars. The spaces are what create the symmetry.

python

Loading editor…

Why Practice with Patterns?#

Nested loops become muscle memory. You stop thinking about for syntax and start thinking about what each loop represents.
Row/column thinking transfers directly to 2D arrays. Matrix traversal, dynamic programming tables, grid BFS — all the same (i, j) mental model.
You build intuition for O(N²) complexity. Seeing a triangle of stars grow quadratically as N increases makes time complexity tangible long before you read the formal definition.

Master the four patterns — right triangle, pyramid, inverted triangle, diamond — and you've earned the right to call yourself comfortable with nested loops.

The Idea#

The Inner-Loop Formula#

Every shape is just a different rule for how many spaces and stars row i gets. Here are the four you'll see most often:

Pattern

Inner loop runs

Example (i=3)

Right Triangle

i stars

***

Pyramid

N - i leading spaces, then 2i - 1 stars

***** (N=5)

Inverted Triangle

N - i + 1 stars

*** (N=5)

Diamond

Top half: pyramid up to N; bottom half: inverted pyramid

two triangles glued together

The trick is to always express the inner bounds in terms of i. Once that clicks, every pattern is a 5-minute problem.

Why Practice with Patterns?#

Nested loops become muscle memory. You stop thinking about for syntax and start thinking about what each loop represents.

Row/column thinking transfers directly to 2D arrays. Matrix traversal, dynamic programming tables, grid BFS — all the same (i, j) mental model.

You build intuition for O(N²) complexity. Seeing a triangle of stars grow quadratically as N increases makes time complexity tangible long before you read the formal definition.

Master the four patterns — right triangle, pyramid, inverted triangle, diamond — and you've earned the right to call yourself comfortable with nested loops.