Reading from a light sensor

gistfile1.c

int sensorValue = 0;
int outputValue = 0;
int previousValue = 0;
const int threshold = 30;
const int analogInPin = A0;

void setup() {
  // initialise serial:
  Serial.begin(9600);
}

void loop() {
  sensorValue = analogRead(analogInPin);            
  outputValue = map(sensorValue, 4, 70, 0, 255);
  if (outputValue - previousValue > threshold || outputValue - previousValue < -threshold) { 
    previousValue = outputValue;    
    Serial.println(outputValue);
    delay(100);
  }
}