Lesson 8 · 8 of 20
for, while and do-while · Dart
Repeat code in Dart with for, while and do-while loops, and control each turn with break and continue: a times table, savings and a countdown.
A `for` loop has three parts in brackets, separated by semicolons: the start (int i = 1), the condition to keep going (i <= 5) and the step after each turn (i++). That way it goes through the numbers 1 to 5 and prints the 7 times table.
`while` repeats as long as its condition is true, without knowing in advance how many turns it will take. The savings grow by 10% a year (savings *= 1.10) until they pass 150 euros, and years counts the turns: it takes 5 years.
`do-while` checks the condition at the end, so the block runs at least once. The countdown prints 3, 2 and 1, and stops when countdown reaches 0. Note the semicolon after while (...).
Inside a loop, `continue` jumps straight to the next turn and `break` leaves the loop altogether. The last for goes from 1 to 20, but skips even numbers with continue and stops as soon as n goes past 11 with break.
Code · main.dart
void main() { // for: a start, a condition and a step. for (int i = 1; i <= 5; i++) { print('7 x $i = ${7 * i}'); } // while repeats as long as the condition is true. double savings = 100; int years = 0; while (savings < 150) { savings *= 1.10; // 10% interest each year years++; } print('$years years to reach ${savings.toStringAsFixed(2)} euros'); // do-while checks at the end, so it runs at least once. int countdown = 3; do { print('Countdown: $countdown'); countdown--; } while (countdown > 0); // continue jumps to the next turn; break leaves the loop. String odds = ''; for (int n = 1; n <= 20; n++) { if (n % 2 == 0) continue; if (n > 11) break; odds += '$n '; } print('Odd numbers up to 11: $odds');}Output · dart run main.dart
7 x 1 = 7 7 x 2 = 14 7 x 3 = 21 7 x 4 = 28 7 x 5 = 35 5 years to reach 161.05 euros Countdown: 3 Countdown: 2 Countdown: 1 Odd numbers up to 11: 1 3 5 7 9 11
Key points
for (start; condition; step)when you know how many turns.while (condition)for as long as something holds.do { } while (condition);runs at least once.continuemoves to the next turn;breakleaves.
Exercise
With a while, work out how many times you must fold a sheet of paper (each fold doubles its thickness; start at 0.1 mm) until it is thicker than one metre.