C Nested Loops

A loop inside a loop

Nested loops means the body of one loop contains another loop. The outer loop runs once per row. For each of those passes, the inner loop runs all the way through. If the outer loop has 3 passes and the inner loop has 4, the inner body runs 12 times.

That product is the point. One loop walks a line. Two loops walk a rectangle: seats in a theater, cells in a table, pairs of numbers. Keep the counters on different names. This tutorial uses row andcol, or i and j.

Print every cell of a grid

This program does not store a table. It only prints coordinates. The outer loop picks the row. The inner loop prints every column on that row, then a newline starts the next line. Read the output as a 3 by 4 grid.

#include <stdio.h>

int main(void) {
  int row;
  int col;
  for (row = 0; row < 3; row++) {
    for (col = 0; col < 4; col++) {
      printf("(%d,%d) ", row, col);
    }
    printf("\n");
  }
  return 0;
}

A multiplication table

Each cell is the product of its row number and its column number. The outer loop is the left factor. The inner loop is the right factor. A newline after the inner loop finishes one row of the table.

#include <stdio.h>

int main(void) {
  int row;
  int col;
  for (row = 1; row <= 5; row++) {
    for (col = 1; col <= 5; col++) {
      printf("%4d", row * col);
    }
    printf("\n");
  }
  return 0;
}

The loops count from 1 this time because people read a times table that way. Array indexes still start at 0; this chapter is about nested repetition, not storage. %4d prints each product in a four-character field so the columns line up.

Read the counters the same way every time

Declare each counter in its own loop. Then col is the inner name and row is the outer name. Reusing one name for both loops is a common source of off-by-one bugs.

A triangle of stars

The inner loop does not have to run a fixed number of times. Here it runs once on the first row, twice on the second, and so on. The inner bound depends on the outer counter.

#include <stdio.h>

int main(void) {
  int row;
  int col;
  for (row = 1; row <= 5; row++) {
    for (col = 1; col <= row; col++) {
      printf("*");
    }
    printf("\n");
  }
  return 0;
}

break leaves only the inner loop

break ends the loop it sits in. Inside the inner loop, it stops that row and the outer loop continues with the next row. It does not jump out of both loops. If you need to stop the whole nest, set a flag and test it in the outer condition, or return from a function.

Nested loops print a table. They also fill a 2D array and read one. Next: const, so names you only mean to read stay that way.