Nested Loops and Patterns
beginner11 min readLesson 19 of 148
Rows and columns: loops inside loops, the grid mental model, and printing shapes.
The grid model
A loop inside a loop is a grid: the outer loop is ROWS, the inner is COLUMNS. The inner loop runs completely for each single step of the outer loop.
for (int row = 1; row <= 3; row++) {
for (int col = 1; col <= row; col++) {
printf("*");
}
printf("\n");
}
Output:
*
**
***
Trace before you run
Say it out loud: row 1 prints 1 star; row 2 prints 2; row 3 prints 3. The inner bound depends on the OUTER variable — that is what makes a triangle, not a rectangle.
Cost intuition
3 rows x 3 columns = 9 inner steps. Doubling the side quadruples the work — loops multiply, and this intuition becomes Big-O thinking later.