Nested Loops¶

The placing of one loop inside the body of another loop is called nesting. When you “nest” two loops, the outer loop takes control of the number of complete repetitions of the inner loop. How this works is that the first pass of the outer loop triggers the inner loop, which executes to completion. Then the second pass of the outer loop triggers the inner loop again. This repeats until the outer loop finishes.

A Nested for loop (in most computer programming languages) takes the generic form of:

FOR counter in range(n)

FOR counter in range(m)

statement(s)

END

or using While loops:

WHILE counter1 <= n :

WHILE counter2 >= m :

statement(s)
…
counter2 = counter2 + 1

END

…

counter1 = counter1 + 1

END

In a flow chart it looks like:

The following code snippet is a nested loop example of a 2 digit odometer:

// Copyright (c) 2019 St. Mother Teresa HS All rights reserved.
//
// Created by: Mr. Coxall
// Created on: Oct 2019
// This program uses a nested loop

#include <iostream>

main() {
    // this function uses a nested loop
    int counter1;
    int counter2;

    // process & output
    for (counter1 = 0; counter1 < 10; counter1++) {
        for (counter2 = 0; counter2 < 10; counter2++) {
            std::cout << "Odometer reading: " << counter1 << counter2 << std::endl;
        }
    }
}

// nested loop

// nested loop

// nested loop

#!/usr/bin/env python3

# Created by: Mr. Coxall
# Created on: Oct 2019
# This program uses a nested loop


def main():
    # this function uses a nested loop

    counter1 = 0
    counter2 = 0

    # process & output
    for counter1 in range(10):
        for counter2 in range(10):
            print("Odometer {0}{1}".format(counter1, counter2))


if __name__ == "__main__":
    main()

// nested loop

// nested loop