#include "Wire.h"

byte x = 0;

void setup()
{
  // join i2c bus (address optional for master)
  Wire.begin();
}

void loop()
{
  Wire.beginTransmission(4); // transmit to device #4
  Wire.write("x is ");       // write five bytes
  Wire.write(x);             // write one byte  
  Wire.endTransmission();    // stop transmitting

  x++;
  delay(500);
}

#include "Wire.h"

void setup()
{
  // join i2c bus with address #2
  Wire.begin(2);   
  // register event  
  Wire.onRequest(requestEvent);
}

void loop()
{
  delay(100);
}

// function that executes whenever data is requested by master
// this function is registered as an event, see setup()
void requestEvent()
{
  // respond with message of 6 bytes
  // as expected by master
  Wire.write("hello ");  
}