Nested Loops

Nested Loops

A nested loop is a loop inside another loop. The inner loop runs completely for every single pass of the outer loop.

The shape

for (int i = 0; i < rows; i++) {
  for (int j = 0; j < cols; j++) {
    // inner body
  }
}

For each trip of the outer loop, the inner loop does its entire count from scratch.

A times table

#include <iostream>
using namespace std;

int main() {
  for (int i = 1; i <= 3; i++) {
    for (int j = 1; j <= 3; j++) {
      cout << i * j << " ";
    }
    cout << endl;
  }
  return 0;
}

This prints a 3 by 3 grid of products. The inner loop lays out one row of numbers, then the outer loop moves to the next row.

Counting total runs

If the outer loop runs 3 times and the inner loop runs 3 times, the inner body runs 3 times 3 = 9 times in total. That is how you build grids and tables.

TL;DR

• A nested loop is a loop placed inside the body of another loop.
• The inner loop restarts at the start of every pass of the outer one.
• Total inner runs = outer count times inner count.
• Nested loops are ideal for grids and multiplication tables.