how to use a button on arduino code example

Example 1: how to connect a button arduino

// constants won't change. They're used here to
// set pin numbers:
const int buttonPin = 2;     // the number of the pushbutton pin
const int ledPin =  13;      // the number of the LED pin
// variables will change:
int buttonState = 0;         // variable for reading the pushbutton status
void setup() {
  Serial.begin(9600);
  // initialize the LED pin as an output:
  pinMode(ledPin, OUTPUT);
  // initialize the pushbutton pin as an input:
  pinMode(buttonPin, INPUT);
}
void loop() {
  // read the state of the pushbutton value:
  buttonState = digitalRead(buttonPin);
  // Show the state of pushbutton on serial monitor
  Serial.println(buttonState);
  // check if the pushbutton is pressed.
  // if it is, the buttonState is HIGH:
  if (buttonState == HIGH) {
    // turn LED on:
    digitalWrite(ledPin, HIGH);
  } else {
    // turn LED off:
    digitalWrite(ledPin, LOW);
  }
  // Added the delay so that we can see the output of button
  delay(100);
}

Example 2: what does the button do on the arduino uno

The reset button does pretty much the same as unplugging the board and
plugging it back in. It restarts your program from the beginning.

The same thing happens when you program the board - the USB interface
presses the reset button for you. That then enters the bootloader for
a second or two so it can try and program it.