IR Calculator without any calculator bugs

/*
This is an IR Remote Controlled Calculator.
I have tried to get rid of all the bugs that I have seen
in other calculators made using an arduino. This calculator
can be used like a normal calculator, you can make 
simultaneous calculations with this, add points, etc.
The LCD Display used here is a RG1602A,
you may refer to this article to understand how to use the display,
https://create.arduino.cc/projecthub/najad/interfacing-lcd1602-with-arduino-764ec4
All you need to do is connect the wires in the corresponding
pins written down below and edit the IR Codes:
|------#------Arduino Pins------LCD RG1602A--------------------|
|----- 1 ----     GND     ----- VSS,V0 (With 2K ohm),RW,K -----|
|----- 2 ----     5V      ----- VDD, A (With 220 ohm)     -----|
|----- 3 ----     12      ----- RS                        -----|
|----- 4 ----     11      ----- E                         -----|
|----- 5 ----     5       ----- D4                        -----|
|----- 6 ----     4       ----- D5      	          -----|
|----- 7 ----     3       ----- D6                        -----|
|----- 8 ----     2       ----- D7                        -----|     
|--------------------------------------------------------------|
|------#------Arduino Pins------IR Transmitter-----------------|
|----- 1 ----     10      ----- Out                       -----|
|----- 2 ----     GND     ----- GND                       -----|
|----- 3 ----     5V      ----- Power                     -----|
|--------------------------------------------------------------|
Check it out on Github: https://github.com/Jaagrav/IRcalculator/
*/

#include <IRremote.h>
#include <LiquidCrystal.h>
#include <math.h>

// Digital Pin Connections to your LCD
const int 
  rs = 12, 
  en = 11, 
  d4 = 5, 
  d5 = 4, 
  d6 = 3, 
  d7 = 2;

LiquidCrystal lcd(rs, en, d4, d5, d6, d7);

// Digital Pin Connection to your IR Receiver
IRrecv irrecv(10);
decode_results results;

String 
  number1 = "0",
  number2 = "0",
  optr = "=",
  sixteenString = "                ";

void setup() {
  Serial.begin(9600);
  
  lcd.begin(16, 2);
  irrecv.enableIRIn();
}

void loop() {
  if (irrecv.decode(&results)) {
    unsigned int result = results.value;
    String val = String(result);
    acceptInput(val.toInt());
    irrecv.resume(); 
  } 
  lcd.setCursor(0,0);
  lcd.print(optr + "  " + sixteenString.substring(number1.length() + 3) + number1);
  lcd.setCursor(0,1);
  lcd.print(sixteenString.substring(number2.length()) + number2);
}

void calculate(String op) {
  double no1 = number1.toDouble();
  double no2 = number2.toDouble();
  double calcVal = 0.0;
  
  if(optr == "+")
    calcVal = (no1 + no2);
  else if(optr == "-")
    calcVal = (no1 - no2);
  else if(optr == "x")
    calcVal = (no1 * no2);
  else if(optr == "/")
    calcVal = (no1 / no2);
  
  number1 = toString(calcVal);
  number2 = "0";
  optr = op;
}

String toString(double num) {
  return String(num);
}

void function(String e) { 
  if(number1 != "0" && number2 != "0") {
    calculate(e);
  }
  else if(number1 == "0") {  
    number1 = number2;
    number2 = "0";
  }
  optr = e;
}

void concatNumbers(String num) {
  if(optr == "=")
    number1 = "0";
  if(num != "."){
    if(number2.length() == 1 && number2 == "0")
       number2 = num;
     else
       number2 += num;  
  }
  else {
    if(number2.charAt(number2.length()-1) != '.' && number2.indexOf('.') == -1)
      number2 += num;
  }
}

void backSpace() {
  number2 = number2.substring(0, number2.length() - 1);
    if(number2 == "")
      number2 = "0";
}

/*
In the below switch-case replace the numbers with the IR codes
from your remote. Make sure you write the code that gets printed
in your serial monitor from line 142.
*/

void acceptInput(int character) {
  Serial.println(character);
  switch(character) {
  	case 2222: 
      concatNumbers("1");
    break;
    case -31092:
      concatNumbers("2");
    break;
    case 18888:
      concatNumbers("3");
    break;
    case 10000:
      concatNumbers("4");
    break;
    case -22203:
      concatNumbers("5");
    break;
    case 26666:
      concatNumbers("6");
    break;
    case 6333:
      concatNumbers("7");
    break;
    case -25537:
      concatNumbers("8");
    break;
    case 22222:
      concatNumbers("9");
    break;
    case 12222:
      concatNumbers("0");
    break;
    case 28888:
      concatNumbers(".");
    break;
    case 255:
      number1 = "0";
      number2 = "0";
      optr = "=";
    break;
    case 32222:
      function("+");
    break;
    case -28870:
      function("-");
    break;
    case 24444:
      function("/");
    break;
    case 8444:
      function("x");
    break;
    case 45555:
      if(optr != "=")
        calculate("=");
    break;
    case 4333:
      backSpace();
    break;
    default: 
    Serial.println("Invalid Input");
  }
}