Loops: for, while, do-while

Teach the computer to repeat work for you

Big Idea: A loop lets you run the same block of code many times without copying it over and over. Instead of writing cout five times, you write it once inside a loop and tell the loop how many times to repeat.

By the end you will be able to:

write a for, a while, and a do-while loop,
choose the right loop for the job,
control a loop with break and continue.

1. What is a loop?

Imagine you are told: "Do ten push-ups." You do not get ten separate instructions. You get one instruction plus a rule: repeat until you reach ten. That is exactly what a loop is - one block of code plus a rule that says how long to keep repeating.

Every loop needs three things to be safe: a start (where do we begin?), a condition (should we keep going?), and a change (something that moves us toward stopping). If the change is missing, the condition never becomes false and the loop runs forever - an infinite loop.

2. The for loop - when you know the count

A for loop packs all three parts onto one line: for (start; condition; change). Use it when you know how many times to repeat.

#include <iostream>
using namespace std;

int main() {
    for (int i = 1; i <= 5; i++) {
        cout << i << " ";
    }
    cout << endl;
    return 0;
}

Output:

1 2 3 4 5

Read the header for (int i = 1; i <= 5; i++) as three steps:

int i = 1 - the start: create a counter and set it to 1 (runs once).
i <= 5 - the condition: keep looping while this is true.
i++ - the change: add 1 to i after each pass.

Loops: for, while, do-while