Do-While Loops

Integer attempts = 0;

do {
    attempts++;
    System.debug('Attempt number ' + attempts);
} while (attempts < 3);

1. Integer attempts = 0; — starts at zero.
2. (blank line)
3. do { — opens the loop body — notice the condition comes after the body here, not before.
4. attempts++; — increments the counter.
5. System.debug('Attempt number ' + attempts); — logs the current attempt.
6. } while (attempts < 3); — the condition, checked after the body already ran. This means the body always runs at least once, even if the condition would have been false from the very start.

Integer x = 10;

while (x < 5) {
    System.debug('This never runs.');
}

do {
    System.debug('This runs exactly once.');
} while (x < 5);

1. Integer x = 10; — a value that already fails the condition below.
2. (blank line)
3. while (x < 5) { — false immediately (10 is not less than 5).
4. System.debug('This never runs.'); — correctly never runs.
5. } — closes the while loop, having run zero times.
6. (blank line)
7. do { — starts a do-while with the same condition.
8. System.debug('This runs exactly once.'); — runs anyway, because a do-while body always executes before the condition is ever checked.
9. } while (x < 5); — checked only now — false, so the loop stops after that one run.

A while loop checks first, so if the condition starts false, the body never executes at all. A do-while loop always runs the body once, checking only afterward — that's the entire distinction.

Integer attempt = 0;
Boolean readingReceived = false;

do {
    attempt++;
    System.debug('Attempting to read smart meter, try #' + attempt);
    // in real code: readingReceived would be set based on an actual API response
} while (! readingReceived && attempt < 3);

1. Integer attempt = 0; — attempt counter.
2. Boolean readingReceived = false; — whether the read has succeeded yet.
3. (blank line)
4. do { — opens the retry loop.
5. attempt++; — counts this attempt.
6. System.debug('Attempting to read smart meter, try #' + attempt); — logs it.
7. // in real code: readingReceived would be set based on an actual API response — a comment marking where real logic would update the Boolean.
8. } while (! readingReceived && attempt < 3); — keeps retrying while the reading is still missing (! readingReceived) and fewer than 3 attempts have been made.

A retry loop is the classic case for do-while: you always want to attempt the read at least once before deciding whether to retry — checking "should I retry?" before the first attempt wouldn't make sense.