const int ledPin = 9;
const int buttonPin = 5;

int ledState = LOW;

void setup() {
  // put your setup code here, to run once:
  pinMode(ledPin, OUTPUT);
  pinMode(buttonPin, INPUT);
  
  // start with led off
  digitalWrite(ledPin, ledState);
}

void loop() {
  int reading = digitalRead(buttonPin);

  // if button is pressed and led off, then turn led on
  if (reading == LOW and ledState == LOW) {
    ledState = HIGH;
    digitalWrite(ledPin, HIGH);
    
  // if button is pressed and led on, then turn led off
  } else if (reading == LOW and ledState == HIGH) {
    ledState = LOW;
    digitalWrite(ledPin, LOW);
  }
}