Simple button state from low to high, no resistor used in hook up

BtnLowToHigh

// constants won't change. They're used here to set pin numbers:
const int BUTTON_PIN = 7; // the number of the pushbutton pin

// Variables will change:
int lastState = HIGH; // the previous state from the input pin
int currentState;    // the current reading from the input pin

void setup() {
  // initialize serial communication at 9600 bits per second:
  Serial.begin(9600);
  // initialize the pushbutton pin as an pull-up input
  // the pull-up input pin will be HIGH when the switch is open and LOW when the switch is closed.
  pinMode(BUTTON_PIN, INPUT_PULLUP);
}

void loop() {
  // read the state of the switch/button:
  currentState = digitalRead(BUTTON_PIN);

  if(lastState == LOW && currentState == HIGH)
    Serial.println("The state changed from LOW to HIGH");

  // save the last state
  lastState = currentState;
}