Arduino一个简单的if判断的问题

Question

问题遇到的现象和发生背景

不太明白这里为什么不按下按钮的时候，输入为HIGH。

电路及问题相关代码

/*
  Input Pull-up Serial

  This example demonstrates the use of pinMode(INPUT_PULLUP). It reads a digital
  input on pin 2 and prints the results to the Serial Monitor.

  The circuit:
  - momentary switch attached from pin 2 to ground
  - built-in LED on pin 13

  Unlike pinMode(INPUT), there is no pull-down resistor necessary. An internal
  20K-ohm resistor is pulled to 5V. This configuration causes the input to read
  HIGH when the switch is open, and LOW when it is closed.

  created 14 Mar 2012
  by Scott Fitzgerald

  This example code is in the public domain.

  https://www.arduino.cc/en/Tutorial/BuiltInExamples/InputPullupSerial
*/

void setup() {
  //start serial connection
  Serial.begin(9600);
  //configure pin 2 as an input and enable the internal pull-up resistor
  pinMode(2, INPUT_PULLUP);
  pinMode(13, OUTPUT);

}

void loop() {
  //read the pushbutton value into a variable
  int sensorVal = digitalRead(2);
  //print out the value of the pushbutton
  Serial.println(sensorVal);

  // Keep in mind the pull-up means the pushbutton's logic is inverted. It goes
  // HIGH when it's open, and LOW when it's pressed. Turn on pin 13 when the
  // button's pressed, and off when it's not:
  if (sensorVal == HIGH) {
    digitalWrite(13, LOW);
  } else {
    digitalWrite(13, HIGH);
  }
}

按下按钮，2接地，输入为LOW，所以根据if语句13变为HIGH这个我理解。
我不理解的是没有按按钮的时候，2也没有电源输入啊，为什么输入为HIGH呢？

Answer 1

2 脚被设置成 INPUT_PULLUP 就是输入，上拉电阻模式。在该脚悬空的时候，输入为高。