Lesson 8 · 8 of 20

while and do-while · C++

Repeat code in C++ with while as long as a condition holds, with do-while at least once, and steer the loop with break and continue.

C++ · while and do-while in DMarlon Studio: the example's code and its output in the terminal

`while` repeats a block as long as the condition is true. Saving 120 euros a month, the loop goes round until savings reaches 500: after 5 months there are 600. Something inside the loop must eventually change the condition, or it would never end.

The countdown fits on one line: seconds-- uses the current value for printing and then subtracts one. When it hits 0 the condition fails and the program moves on to Lift off!.

`do { ... } while (condition);` checks the condition at the end, so the block runs at least once: the player gets a turn even though energy is 0. Note the semicolon after the while.

`while (true)` would loop forever, but `break` ends it as soon as n goes past 10. `continue` jumps straight to the next round without printing anything, which is how 3, 6 and 9 are skipped.

Code · main.cpp

#include <iostream>int main() {    // while repeats the block as long as the condition is true.    int savings = 0, months = 0;    while (savings < 500) {        savings += 120;        months++;    }    std::cout << "Goal reached in " << months << " months: " << savings << " euros\n";    // A countdown: seconds-- uses the value, then subtracts one.    int seconds = 5;    while (seconds > 0) std::cout << seconds-- << "... ";    std::cout << "Lift off!\n";    // do-while runs the block once before checking the condition.    int energy = 0;    do {        std::cout << "The player gets one turn (energy " << energy << ")\n";    } while (energy > 0);    // break leaves the loop; continue jumps to the next round.    int n = 0;    while (true) {        n++;        if (n % 3 == 0) continue;  // skip multiples of 3        if (n > 10) break;        std::cout << n << ' ';    }    std::cout << '\n';    return 0;}

Output · c++ -std=c++17 main.cpp -o main && ./main

Goal reached in 5 months: 600 euros
5... 4... 3... 2... 1... Lift off!
The player gets one turn (energy 0)
1 2 4 5 7 8 10

Key points

while checks the condition before each round.
do-while checks it afterwards: it runs at least once.
Something inside the loop must change the condition.
break leaves the loop; continue moves on to the next round.

Exercise

A plant is 5 cm tall and grows 20% every week. Use a while to work out how many weeks it takes to pass 30 cm, printing its height each week.