#include <Wire.h>
uint8_t t = 0;
void setup()
{
  Wire.begin(4);                // join i2c bus with address #4
  Wire.onReceive(receiveEvent); // register event
  Serial.begin(9600);           // start serial for output
  Wire.onRequest(requestEvent); // register event
}

void loop()
{
}

// function that executes whenever data is received from master
// this function is registered as an event, see setup()
void receiveEvent(int howMany)
{
  while(1 < Wire.available()) // loop through all but the last
  {
    int c = Wire.read(); // receive byte as a character
    Serial.print((char)c);         // print the character
  }
  int x = Wire.read();    // receive byte as an integer
  Serial.println((char)x);         // print the integer
}

void requestEvent() {
  t++;
  if(t%10 >= 5)  
  Wire.write("hello"); // respond with message of 6 bytes
  else
    Wire.write("world");
  
  Serial.println();  
  Serial.println("requested!");  
}