Keypad multi String

/* Input Multi keypad
Created by
papermindvention.blogspot.com
19 March 2018 */

#include <Keypad.h>

String inString="";
const byte ROWS = 4;
const byte COLS = 4;
char keys[ROWS][COLS] = {
  {'1','4','7','*'},
  {'2','5','8','0'},
  {'3','6','9','#'},
  {'A','B','C','D'} };
byte rowPins[ROWS] = {5, 4, 3, 2}; //connect to the row pinouts of the keypad
byte colPins[COLS] = {9, 8, 7, 6}; //connect to the column pinouts of the keypad
Keypad keypad = Keypad( makeKeymap(keys), rowPins, colPins, ROWS, COLS );

void setup(){ Serial.begin(9600); }

void loop(){
  char inChar = keypad.getKey();
  if (inChar){
    /* periksa apakah nilai yang diinputkan berupa angka */
  if (isDigit(inChar)) {  inString += inChar;  }
    /*periksa apakah nilai yang diinputkan berupa huruf Alphabet */
  if (isAlpha(inChar)) {  inString += inChar;  }
    /* jika tombol * ditekan, maka tampilkan nilai yang diinputkan. */
    if (inChar == '*') {
      Serial.print("nilai yang anda memasukkan :");
      Serial.println(inString);
      inString = "";
  }
}
}