State machine: what the program remembers

A sensor only tells you what is true now. To close a gate three seconds after a car has gone, the program must remember which state it is in: closed, open, or waiting to close. A program built that way is a state machine, and engineers draw it as a state chart: a box for each state, an arrow for each thing that makes it change.

Keep the state in a variable (like gateOpen), change it only when something happens, and act on it every loop.

More: state chart or flow chart, and counting something once

A flow chart shows the steps of the program one after the other. A state chart shows the states and what moves the machine from one to the next; it does not care how many lines of code that takes. Traffic lights, washing machines, lifts, games (playing, paused, game over) and robots are all state machines.

The same idea counts a car once. The loop runs ten times a second, so a car that stands in front of the sensor for two seconds is "near" twenty times. Remember what the sensor said last time: the car has driven past when it was near last time and is not near now. That is a tiny state machine too, with two states: no car, car.

bool carWasNear = false;      // the state of the sensor last time round
int cars = 0;

void loop() {
  bool near = sensor.measureDistanceCm() < 15;
  if (carWasNear && !near) {   // near before, not near now: a car drove past
    cars = cars + 1;
  }
  carWasNear = near;          // remember for the next loop
  delay(100);
}

The same gate as a flow chart: the steps, not the states. Draw both for your program and compare.