int a=2;
int b=3;
int c=4;
int d=5;
int trigPin=9;
int echoPin=8;
float duration , distance;   //float use for decimal number.
void setup() {
  pinMode(a, OUTPUT);
  pinMode(b, OUTPUT);
  pinMode(c, OUTPUT);
  pinMode(d, OUTPUT);
  pinMode(trigPin, OUTPUT);
  pinMode(echoPin, INPUT);
  Serial.begin(9600);
}

void loop() {
 digitalWrite(trigPin, HIGH);
 delayMicroseconds(10);
 digitalWrite(trigPin, LOW);

 if(distance>=300){
  digitalWrite(a, HIGH);
 }else{
  digitalWrite(a, LOW);
 }

 if(distance>200 && distance<300){              // && is AND operation.
  digitalWrite(b, HIGH);
 }else{
  digitalWrite(b, LOW);
 }

 if(distance>100 && distance<200){
  digitalWrite(c, HIGH);
}else{
  digitalWrite(c, LOW);
}

 if(distance<=100){
  digitalWrite(d, HIGH);
}else{
  digitalWrite(d, LOW);
}

 duration=pulseIn(echoPin,HIGH);
 distance=0.017*duration;              // 340/1000=0.017
 // 340m/s is the speed of ultrasonic waves.
 Serial.print("Distance:");
 Serial.print(distance);
 Serial.println("cm");
 delay(500);
}