#include <Keypad.h> 
const byte numRows = 4;
const byte numCols = 4;
byte rowPins[numCols] = {5, 4, 3, 2};
byte colPins[numRows] = {A3, A2, A1, A0};
char keys[numRows][numCols] = {
  {'1', '2', '3', '+'},
  {'4', '5', '6', '-'},
  {'7', '8', '9', '*'},
  {'.', '0', '=', '/'}
};
// connect to the column pinouts of the keypad
char inputArray[4];                  // array to store the user input
int index = 0;                       // array index counter
float result;                        // final result after calculation
void setup()
{
  Serial.begin(9600);
  for (int i = 0; i < numRows; i++)
  {
    pinMode(rowPins[i], INPUT_PULLUP);
  }
  for (int j = 0; j < numCols; j++)
  {
    pinMode(colPins[j], OUTPUT);
    digitalWrite(colPins[j], LOW);
  }
}
void loop()
{
  char keyPress = readKeypad();
  if (keyPress != NO_KEY)
  {
    if (keyPress == '#')
    {
      // if user press # key
      inputArray[index] = 0; // store 0 in array
      index++;               // increment array index counter
      if (index == 4)
      {                             // when all inputs are entered
        result = calculateResult(); // call
        calculateResult();
        Serial.println("Result is : " + String(result));
        resetIndex();
      }
    }
    else
    {
      inputArray[index] = keyPress - 48;
      Serial.println("You pressed : " + String(keyPress));
      index++;
    }
  }
}
float calculateResult()
{
  float sum = 0;
  for (int i = 0; i < 4; i++)
  {
    sum += inputArray[i];
  }
  return sum * 1.5;
}
void resetIndex() { index = 0; }
char readKeypad()
{
  char keypress = NO_KEY;
  for (int j = 0; j < numCols; j++)
  {
    digitalWrite(colPins[j], HIGH);
    delay(10);
    for (int i = 0; i < numRows; i++)
    {
      if (digitalRead(rowPins[i]) == LOW)
      {
        keypress = keys[i][j];
        delay(50);
        break;
      }
    }
    digitalWrite(colPins[j], LOW);
  }
  return keypress;
}