Arduino 学习上拉电阻和下拉电阻遇到的问题

1.如果不存在电阻，输入引脚就不连接任何东西（悬空），因此电压是浮动的，Why?

援引资料讲解:





Fig 5.3 Our two alternative switch wiring possibilities

For example, in these schematics we can connect and disconnect pin 2 to 5V, or we can connect and disconnect pin 2 to ground. In both cases, as long as the button is held down, the pin is connected to a valid input voltage. When the button is released, though, pin 2 is not connected to anything. This is called a floating input voltage. Basically, it’s invalid input!

Try building up one of these schematics, and trying out the switch testing sketch above. When the button is held down you should definately get the right printout. When its released, it may keep the old value, or it may change, but its certainly not reliable!

Above correct reference the below pics: