Class3_photocell auto calibrate

int sensorPin = A0;    // select the input pin for the potentiometer
int ledPin = 11;      // select the pin for the LED
int sensorValue = 0;  // variable to store the value coming from the sensor
int lowestVal = 1023;
int highestVal = 0;
int pwmVal = 0;

void setup() {
  // declare the ledPin as an OUTPUT:
  pinMode(ledPin, OUTPUT);
  Serial.begin(9600);
  for(int i = 0; i < 3000; i++){
    // read the current value from the photocell
    sensorValue = analogRead(sensorPin);
    //check to see if the value is greater than highestVal ...
    if( sensorValue > highestVal){
     highestVal = sensorValue;
    }
    // or less than lowestVal
    if( sensorValue < lowestVal){
     lowestVal = sensorValue;
    }
    delay(1);
  }
}

void loop() {
  // read the value from the sensor:
  sensorValue = analogRead(sensorPin);
  Serial.println( sensorValue );
  sensorValue = constrain(sensorValue, lowestVal, highestVal);
  pwmVal = map( sensorValue, lowestVal, highestVal, 0, 255);
  analogWrite(ledPin, pwmVal);
}