1.如果不存在電阻，輸入引腳就不連線任何東西（懸空），因此電壓是浮動的，Why?
援引資料講解:


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: