String incomingString = "";
int str_len = 0 ;

void setup() {
  Serial.begin(9600);
  Serial.println("Type something :");
  pinMode(13, OUTPUT);
  pinMode(8, OUTPUT);
}

void loop() {
  if (Serial.available() > 0) {
    incomingString = Serial.readString();
    str_len = incomingString.length();
    Serial.println(str_len); 

    char x[str_len];
    incomingString.toCharArray(x,str_len);
    for(int i=0 ; i< str_len ; i++ ) {
      Serial.println(x[i]);
      switch (x[i]){
        case 'a' :
          digitalWrite(13 , HIGH);
          delay(500);
          digitalWrite(13 , LOW);
          delay(500);
          break;
/*        case 'b':
          digitalWrite( , HIGH);
          delay(500);
          digitalWrite( , LOW);
          delay(500);
          break;  
        case 'c':
          digitalWrite( , HIGH);
          delay(500);
          digitalWrite( , LOW);
          delay(500); 
          break;  */
        case 'd':
          digitalWrite(11 , HIGH);
          delay(500);
          digitalWrite(11 , LOW);
          delay(500);
          break;   
/*        case 'e':
          digitalWrite( , HIGH);
          delay(500);
          digitalWrite( , LOW);
          delay(500);
          break;  */ 
        case 'f':
          digitalWrite(10 , HIGH);
          delay(500);
          digitalWrite(10 , LOW);
          delay(500);
          break;   
        case 'g':
          digitalWrite(9 , HIGH);
          delay(500);
          digitalWrite(9 , LOW);
          delay(500);
          break;   
        case 'h':
          digitalWrite(8 , HIGH);
          delay(500);
          digitalWrite(8 , LOW);
          delay(500);
          break;   
/*        case 'i':
          digitalWrite( , HIGH);
          delay(500);
          digitalWrite( , LOW);
          delay(500);
          break;   */
        case 'j':
          digitalWrite(7 , HIGH);
          delay(500);
          digitalWrite(7 , LOW);
          delay(500);
          break;   
        case 'k':
          digitalWrite(6 , HIGH);
          delay(500);
          digitalWrite(6 , LOW);
          delay(500);
          break;   
        case 'l':
          digitalWrite(5 , HIGH);
          delay(500);
          digitalWrite(5 , LOW);
          delay(500);
          break;   
/*        case 'm':
          digitalWrite( , HIGH);
          delay(500);
          digitalWrite( , LOW);
          delay(500);
          break;   
        case 'n':
          digitalWrite( , HIGH);
          delay(500);
          digitalWrite( , LOW);
          delay(500);
          break;   
        case 'o':
          digitalWrite( , HIGH);
          delay(500);
          digitalWrite( , LOW);
          delay(500);
          break;   
        case 'p':
          digitalWrite( , HIGH);
          delay(500);
          digitalWrite( , LOW);
          delay(500);  
          break; 
        case 'q':
          digitalWrite( , HIGH);
          delay(500);
          digitalWrite( , LOW);
          delay(500);  
          break; 
        case 'r':
          digitalWrite( , HIGH);
          delay(500);
          digitalWrite( , LOW);
          delay(500);  
          break; */
        case 's':
          digitalWrite(12 , HIGH);
          delay(500);
          digitalWrite(12 , LOW);
          delay(500);  
          break; 
/*        case 't':
          digitalWrite( , HIGH);
          delay(500);
          digitalWrite( , LOW);
          delay(500);
          break;   
        case 'u':
          digitalWrite( , HIGH);
          delay(500);
          digitalWrite( , LOW);
          delay(500); 
          break;  
        case 'v':
          digitalWrite( , HIGH);
          delay(500);
          digitalWrite( , LOW);
          delay(500);  
          break; 
        case 'w':
          digitalWrite( , HIGH);
          delay(500);
          digitalWrite( , LOW);
          delay(500);
          break ;
        case 'x':
          digitalWrite( , HIGH);
          delay(500);
          digitalWrite( , LOW);
          delay(500);
          break ;
        case 'y':
          digitalWrite( , HIGH);
          delay(500);
          digitalWrite( , LOW);
          delay(500);
          break ;
        case 'z':
          digitalWrite( , HIGH);
          delay(500);
          digitalWrite( , LOW);
          delay(500);
          break ; */
      }
    }
  }
}