// constants used to set pin numbers:
const int buttonPin = 3;     // Pushbutton pin
const int ledPin =  7;      // LED pin

// variables:
int buttonState = 0;         // variable for reading the Pushbutton state

void setup() {
  // initialize the LED pin as an output:
  pinMode(ledPin, OUTPUT);
  // initialize the Pushbutton pin as an input:
  pinMode(buttonPin, INPUT);
}

void loop() {
// read the state of the Pushbutton :
  buttonState = digitalRead(buttonPin);

// check if the Pushbutton is pressed. If it is, the buttonState is HIGH:
  if (buttonState == HIGH) {
    // turn LED on:
    digitalWrite(ledPin, LOW);
  } else {
    // turn LED off:
    digitalWrite(ledPin, HIGH);
  }
}