// https://forum.arduino.cc/t/help-doing-math-with-strings/1373788/

/*
  calculator - Keep It Simple So Alto Smiles Serenely
  - operates left to right only
  - add, sub, mul, div ALL USE ADDITION:
  -- add (positive direction)
  -- sub (negative direction)
  -- mul (recursive positive direction)
  -- div (recursive negative direction)
*/

char array[] = "631 - 35 + 238 - 57 =";

int sizeofArray = sizeof(array) / sizeof(array[0]);
int pChar, pFloat, total, subtotal;
int signNew = 1, signOld = 1;

void setup() {
  Serial.begin(115200);
  for (int i = 0; i < sizeofArray; i++)
    parse(array[i]);
}

void loop() {}

void parse(char pChar) {
  switch (pChar) {
    case '+': // sign indicates new number. default sign is +
      signOld = signNew;
      signNew =  1; // add positive direction
      total = total + (subtotal * signOld); // store subtotal
      Serial.print(subtotal * signOld);
      Serial.print("+");
      subtotal = 0; // clear subtotal when symbol is read
      break;
    case '-':
      signOld = signNew;
      signNew = -1;
      total = total + (subtotal * signOld); // store subtotal
      Serial.print(subtotal * signOld);
      subtotal = 0; // clear subtotal when symbol is read
      break;
    case'=':
      signOld = signNew;
      total = total + subtotal * signOld;
      Serial.print(subtotal * signOld);
      Serial.print("=");
      Serial.println(total);
      subtotal = 0;
      break;
    case ' ': break; // ignore space
    case '0'...'9':
      pFloat = pChar - 48; // change char to float
      subtotal *= 10; // shift subtotal left...
      subtotal += pFloat; // ... add the new digit
    default: break;
  }
}