Bluetooth Controlled Wheeled Robot

//This code is tested only for the provided RobotCo App You may need to change the alphabet for the control of the direction of robot.
//Connection: Digital pin 4,5,6,7 to IN1,IN2,IN3,IN4 respectively.

char state;
void stp();
void forward();
void left();
void right();
void backward();
void setup()
{
    pinMode(4,OUTPUT);
    pinMode(5,OUTPUT);
    pinMode(6,OUTPUT);
    pinMode(7,OUTPUT);
    Serial.begin(9600);
}
void loop() {
    if(Serial.available() > 0)      // Ckeck for command Recieved
    {
      state = Serial.read();
      Serial.println(state);
      flag=0;
    }
    if (state == 's')     // Checking Command from User
    {
        stp();
    }
    else if (state == 'f')
    {
        right();
    }
    else if (state == 'b')
    {
        left();
    }
    else if (state == 'l')
    {
        backward();
    }
   else if (state == 'r')
  {
        forward();
    }
}                                           //loop() ends here
void forward()
{
  digitalWrite(4,HIGH);
  digitalWrite(5,LOW);
  digitalWrite(6,HIGH);
  digitalWrite(7,LOW);
}
void backward()
{
  digitalWrite(4,LOW);
  digitalWrite(5,HIGH);
  digitalWrite(6,LOW);
  digitalWrite(7,HIGH);
}
void left()
{
  digitalWrite(4,LOW);
  digitalWrite(5,HIGH);
  digitalWrite(6,HIGH);
  digitalWrite(7,LOW);
}
void right()
{
  digitalWrite(4,HIGH);
  digitalWrite(5,LOW);
  digitalWrite(6,LOW);
  digitalWrite(7,HIGH);
}
void stp()
{
  digitalWrite(4,LOW);
  digitalWrite(5,LOW);
  digitalWrite(6,LOW);
  digitalWrite(7,LOW);
}