Ultrasonic ranging on an LCD, on an Arduino Uno

// Ultrasonic ranging: hold the button, read the distance on the LCD.
//
//   HC-SR04 TRIG -> pin 9    ECHO -> pin 3    VCC -> 5V    GND -> GND
//   LCD RS -> pin 2    E -> A3    D4 -> pin 8    D5 -> pin 7    D6 -> A2    D7 -> pin 4
//   button -> A0 and GND (INPUT_PULLUP)
//
// These are the pins of a ready-made sensor shield, which is why the LCD's six
// lines are scattered over digital and analog pins: A2 and A3 work as plain
// digital pins, and LiquidCrystal does not mind which pins it gets.
//
// While the button is held, the sketch measures four times a second and
// writes the distance on the second line. Let go and the last reading stays.

#include <LiquidCrystal.h>
#include "SR04.h"

const int TRIG_PIN = 9;
const int ECHO_PIN = 3;
const int BUTTON = A0;

LiquidCrystal lcd(2, A3, 8, 7, A2, 4);
SR04 sonar = SR04(ECHO_PIN, TRIG_PIN);

void setup() {
  pinMode(BUTTON, INPUT_PULLUP);
  lcd.begin(16, 2);
  lcd.print("Hold the button");
  Serial.begin(9600);
  Serial.println("Mokxi Uno: hold the button on A0 to measure");
}

void loop() {
  if (digitalRead(BUTTON) == LOW) {
    long cm = sonar.Distance();
    lcd.clear();
    lcd.print("Distance:");
    lcd.setCursor(0, 1);
    if (cm == 0) {
      lcd.print("out of range");
    } else {
      lcd.print(cm);
      lcd.print(" cm");
    }
    Serial.print(cm);
    Serial.println(" cm");
    delay(250);
  }
}