Replace serial monitor input with matrix keypad

[J-M-L]

@infiman12

if (key >= '0' && key <= '9') {     // only act on numeric keys ==> HOW DO YOU ENTER NEGATIVE VALUES?
      keypadInput += key;               // append new character to input string
      lcd.print(key);
    }

This part adds the user input to the String (the char the user types in) but also prints to the LCD the last character. So when you empty the string you also want to refresh the lcd display by calling lcdEnterValue();