Nested loops and patterns

A loop inside a loop

• You can put one loop inside another. The inner loop runs fully for each step of the outer loop.
• This is how you work with rows and columns, or compare every pair of items.
• If the outer loop runs R times and the inner runs C times, the inner body runs R × C times.

Rows and columns

• Think of the outer loop as the row and the inner loop as the column.
• For a block of *, the outer loop picks a row; the inner loop prints the stars in that row.
• After the inner loop finishes a row, print a newline \n to start the next row.

Building a line, then a block

• printf("*") prints one star without moving to a new line.
• Print all the stars for one row first, then one printf("\n") to end the row.
• Repeat for every row, and the rows stack into a block.

#include <stdio.h>

int main(void) {
    for (int row = 0; row < 3; row++) {
        for (int col = 0; col < 3; col++) {
            printf("*");
        }
        printf("\n");      // end the row
    }
    return 0;
}

Counting with nested loops

• Nested loops also count pairs. To count pairs (i, j) with i < j, start the inner loop at i + 1.
• A counter inside the inner loop adds up the matches.
• Watch the start and end values — off-by-one mistakes are common here.

Common mistakes

• The inner loop runs fully for each single turn of the outer loop.
• Check the bounds so a pattern has no extra or missing row.

Now you try

• Use an inner printf without \n to build a row, then a printf("\n") to end it.
• For the function task, do not write a main — the checker provides one.