【求教】小白求助，超声波测距

代码如下：
int trigpin = 2;
int echopin = 3;
int val = 0;
float distance;
void setup(){
Serial.begin(9600);
pinMode(trigpin, OUTPUT);
pinMode(echopin,INPUT);
Serial.println("ready");
delay(1000);
}
void loop(){
digitalWrite(trigpin,LOW);
delayMicroseconds(10);
digitalWrite(trigpin,HIGH);
delayMicroseconds(30);
digitalWrite(trigpin,LOW);
val = pulseIn(echopin,HIGH);
delayMicroseconds(30);
distance = val/58.0;
Serial.print(distance);
Serial.println("cm");
}