const byte swPin=2;  //switch pin
const byte ledPin=13;  //built-in LED
boolean ledState=LOW;  //initial state of LED

void setup() {
  pinMode(ledPin, OUTPUT);
  pinMode(swPin, INPUT_PULLUP); //enable pull-up resistor
  digitalWrite(ledPin, ledState); //set LED OFF
  }

void loop() {
  boolean swState=digitalRead(swPin);
  if (swState==LOW) { //switch pressed
    if (ledState==HIGH) {ledState=LOW;} //ON:turn LED OFF
    else {ledState=HIGH;}  //OFF:turn LED ON
    digitalWrite(ledPin, ledState); //toggle LED
    }
  }