Start and stop a loop with push button

[TimMJN]

It's rather straightforward:

void loop {
  if(loopRunning) {
    doYourThingHere;
  }

  buttonState = digitalRead(buttonPin);
  if((buttonState==LOW) && (prevButtonState==HIGH)) {
    loopRunning = !loopRunning;
  }
  prevButtonState=buttonState;
}