Why does digitalRead randomly return HIGH?

Question

Arduino

I wrote the first program on arduino (mega 2560): close the wires - the LED is on, open it - it does not light.

int ledPin = 13;
int switchPin = 10;


void setup()
{
  pinMode(switchPin, INPUT);
  pinMode(ledPin, OUTPUT);
}

void loop()
{
  digitalWrite(ledPin, digitalRead(switchPin));
  delay(250);
}

So, when I close it, everything is fine - it glows, but when I open it, the LED starts blinking.
Why does digitalRead sometimes return true when there is actually no signal?

Answer 1

Use pull-down/pull-up resistors to avoid picking up interference.
pulling:
Pressing: Vout = Vcc
No pressing: Vout = 0
pulling:
Pressing: Vout = 0
No pressing: Vout = Vcc

Answer 2

In Atmega, the port is very sensitive, and accepts noise on the line as a change in the logic signal at the input.
To avoid this, you can connect a resistor, for example, 10 kOhm, between the input and + power supply, i.e. "pull up to power", or use the programmable "pull-up" resistors "built-in" in the microcontroller.
For this you need to write:

void setup()
{
  pinMode(switchPin, INPUT_PULLUP);
}