This is a sample code for Line Following Robot using Arduino.

RBTLineFollowing.ino

#define BUTTON  2
#define PIEZO   8

#define L293N_ENA 5
#define L293N_ENB 6
#define L293N_IN1 7
#define L293N_IN2 9
#define L293N_IN3 10
#define L293N_IN4 11

#define IR_LEFT   A0
#define IR_RIGHT  A1

#define NOTE_G4  392
#define NOTE_C5  523
#define NOTE_G5  784
#define NOTE_C6  1047

int startMelody[] = {NOTE_G5, NOTE_C6};
int startNoteDurations[] = {12, 8};

int stopMelody[] = {NOTE_C6, NOTE_G5};
int stopNoteDurations[] = {12, 8};

#define playStartMelody() playMelody(startMelody, startNoteDurations, 2)
#define playStopMelody() playMelody(stopMelody, stopNoteDurations, 2)

char inChar;
String inString;

void setup()
{
  pinMode(BUTTON, INPUT_PULLUP);
  pinMode(L293N_ENA, OUTPUT);
  pinMode(L293N_ENB, OUTPUT);
  pinMode(L293N_IN1, OUTPUT);
  pinMode(L293N_IN2, OUTPUT);
  pinMode(L293N_IN3, OUTPUT);
  pinMode(L293N_IN4, OUTPUT);
  pinMode(IR_LEFT, INPUT);
  pinMode(IR_RIGHT, INPUT);

  Serial.begin(9600);

  delay(1000);
}

void loop()
{
  if (digitalRead(BUTTON) == LOW) {
    playStartMelody();

    while (true) {
      if (digitalRead(IR_LEFT) == LOW &&
          digitalRead(IR_RIGHT) == LOW) {
        robotForward(150, 150);
      }
      else if (digitalRead(IR_LEFT) == HIGH &&
               digitalRead(IR_RIGHT) == LOW) {
        robotForward(0, 150);
      }
      else if (digitalRead(IR_LEFT) == LOW &&
               digitalRead(IR_RIGHT) == HIGH) {
        robotForward(150, 0);
      }
      else if (digitalRead(IR_LEFT) == HIGH &&
               digitalRead(IR_RIGHT) == HIGH) {
        Serial.println("stop");
        robotBreak();
        playStopMelody();
        break;
      }
    }
  }
}

void playMelody(int *melody, int *noteDurations, int notesLength)
{
  pinMode(PIEZO, OUTPUT);

  for (int thisNote = 0; thisNote < notesLength; thisNote++) {
    int noteDuration = 1000 / noteDurations[thisNote];
    tone(PIEZO, melody[thisNote], noteDuration);
    int pauseBetweenNotes = noteDuration * 1.30;
    delay(pauseBetweenNotes);
    noTone(PIEZO);
  }
}

void robotStop()
{
  digitalWrite(L293N_ENA, LOW);
  digitalWrite(L293N_IN1, LOW);
  digitalWrite(L293N_IN2, LOW);

  digitalWrite(L293N_ENB, LOW);
  digitalWrite(L293N_IN3, LOW);
  digitalWrite(L293N_IN4, LOW);
}

void robotBreak()
{
  digitalWrite(L293N_ENA, HIGH);
  digitalWrite(L293N_IN1, HIGH);
  digitalWrite(L293N_IN2, HIGH);

  digitalWrite(L293N_ENB, HIGH);
  digitalWrite(L293N_IN3, HIGH);
  digitalWrite(L293N_IN4, HIGH);
}

void robotForward(int motorLeftSpeed, int motorRightSpeed)
{
  analogWrite(L293N_ENA, motorRightSpeed);
  digitalWrite(L293N_IN1, LOW);
  digitalWrite(L293N_IN2, HIGH);

  analogWrite(L293N_ENB, motorLeftSpeed);
  digitalWrite(L293N_IN3, HIGH);
  digitalWrite(L293N_IN4, LOW);
}

void robotReverse(int motorSpeed)
{
  analogWrite(L293N_ENA, motorSpeed);
  digitalWrite(L293N_IN1, HIGH);
  digitalWrite(L293N_IN2, LOW);

  analogWrite(L293N_ENB, motorSpeed);
  digitalWrite(L293N_IN3, LOW);
  digitalWrite(L293N_IN4, HIGH);
}

void robotTurnRight(int motorSpeed)
{
  analogWrite(L293N_ENA, motorSpeed);
  digitalWrite(L293N_IN1, HIGH);
  digitalWrite(L293N_IN2, LOW);

  analogWrite(L293N_ENB, motorSpeed);
  digitalWrite(L293N_IN3, HIGH);
  digitalWrite(L293N_IN4, LOW);
}

void robotTurnLeft(int motorSpeed)
{
  analogWrite(L293N_ENA, motorSpeed);
  digitalWrite(L293N_IN1, LOW);
  digitalWrite(L293N_IN2, HIGH);

  analogWrite(L293N_ENB, motorSpeed);
  digitalWrite(L293N_IN3, LOW);
  digitalWrite(L293N_IN4, HIGH);
}