int length = 26; // the number of notes
char notes[] = "cdfdaagcdfdggfcdfdfgedccgf "; // a space represents a rest
int beats[] = { 1, 1, 1, 1, 3, 3, 5, 1, 1, 1, 1, 3, 3, 5,  1, 1, 1, 1, 4, 2, 3, 1, 4, 2, 4, 8 };
int tempo = 120;

FULL CODE BELOW

/* Melody
 * (cleft) 2005 D. Cuartielles for K3
 *
 * This example uses a piezo speaker to play melodies.  It sends
 * a square wave of the appropriate frequency to the piezo, generating
 * the corresponding tone.
 *
 * The calculation of the tones is made following the mathematical
 * operation:
 *
 *       timeHigh = period / 2 = 1 / (2 * toneFrequency)
 *
 * where the different tones are described as in the table:
 *
 * note     frequency     period     timeHigh
 * c             261 Hz             3830     1915    
 * d             294 Hz             3400     1700    
 * e             329 Hz             3038     1519    
 * f             349 Hz             2864     1432    
 * g             392 Hz             2550     1275    
 * a             440 Hz             2272     1136    
 * b             493 Hz             2028    1014   
 * C            523 Hz            1912     956
 *
 * http://www.arduino.cc/en/Tutorial/Melody
 */

 
int speakerPin = 9;

int length = 26; // the number of notes
char notes[] = "cdfdaagcdfdggfcdfdfgedccgf "; // a space represents a rest
int beats[] = { 1, 1, 1, 1, 3, 3, 5, 1, 1, 1, 1, 3, 3, 5,  1, 1, 1, 1, 4, 2, 3, 1, 4, 2, 4, 8 };
int tempo = 120;

/*
 * Twinkle Twinkle
 * int length = 15; // the number of notes
 * char notes[] = "ccggaagffeeddc "; // a space represents a rest
 * int beats[] = { 1, 1, 1, 1, 1, 1, 2, 1, 1, 1, 1, 1, 1, 2, 4 };
 * int tempo = 300;
 */

void playTone(int tone, int duration) {
  for (long i = 0; i < duration * 1000L; i += tone * 2) {
    digitalWrite(speakerPin, HIGH);
    delayMicroseconds(tone);
    digitalWrite(speakerPin, LOW);
    delayMicroseconds(tone);
  }
}

void playNote(char note, int duration) {
  char names[] = { 'c', 'd', 'e', 'f', 'g', 'a', 'b', 'C' };
  int tones[] = { 1915, 1700, 1519, 1432, 1275, 1136, 1014, 956 };
 
  // play the tone corresponding to the note name
  for (int i = 0; i < 8; i++) {
    if (names[i] == note) {
      playTone(tones[i], duration);
    }
  }
}

void setup() {
  pinMode(speakerPin, OUTPUT);
}

void loop() {
  for (int i = 0; i < length; i++) {
    if (notes[i] == ' ') {
      delay(beats[i] * tempo); // rest
    } else {
      playNote(notes[i], beats[i] * tempo);
    }
   
    // pause between notes
    delay(tempo / 2);
  }
}