while, break and continue

A for loop is the right tool when you know how many times to go round: six notes, ten readings, five rows.

A while loop is for when you do not.

while (something is true) {
  keep doing this
}

Keep going until the button is released. Keep going until the sensor settles. Keep going until the answer is found. There is no count, only a condition — and something inside the loop has to eventually make that condition false, or the sketch hangs there for ever.

That is the danger, and it is worth being blunt about it: an Arduino has no way out of an infinite loop. No error, no message, no way to interrupt it. The board sits there apparently working, doing nothing, until you unplug it.

break and continue are the two escape hatches. break leaves the loop immediately. continue skips the rest of this pass and goes round again. Both work in for loops too.

No wiring. Read everything in the Serial Monitor at 9600 baud.

while

int count = 0;

while (count < 5) {
  Serial.println(count);
  count++;          // without this line, it never ends
}

Prints 0 to 4 — the same as the equivalent for. The difference is that the counter is declared outside and stepped by hand, so nothing forces you to remember it.

That count++ is the whole safety of the loop. Delete it and the sketch stops responding for ever.

for or while?

Use for when	Use while when
you know how many passes	you do not
walking an array	waiting for something to change
counting to a number	repeating until a result is good enough

A for loop keeps its start, its condition and its step in one place where you can see all three. That is why it is safer, and why you should reach for it first.

An infinite loop

int count = 0;
while (count < 5) {
  Serial.println(count);   // no count++
}

This prints 0 for ever. The board is not crashed — it is doing exactly what you said, endlessly. Nothing else in the sketch will ever run again.

There is no way to interrupt it. Upload a different sketch to recover.

One infinite loop is deliberate and legitimate:

while (true) {
  // never ends — used to halt a sketch on purpose
}

break

Guess the number, properly.

The sketch picks a secret with random(1, 101) and then guesses it itself, using a while loop and halving the range each time — the strategy from ARD-PG-10.

Keep a low of 1 and a high of 100. Each pass, guess the middle, print it, and move low or high depending on whether the guess was too small or too large. Stop when it matches.

Count and print the number of guesses.

Run it ten times with different secrets. What is the worst number of guesses you see?

Think about it: counting from 1 needed up to 100 guesses. This needs 7. Both loops look about the same size on the page. Where did the saving actually come from — and what did the loop have to know that the counting version did not?