A basic four-function calculator written in ROBOTC for RCX. Video at http://youtu.be/Wa_-ehRMjs0

calc.c

#pragma config(Sensor, S1,     select,              sensorTouch)
#pragma config(Sensor, S2,     dial,                sensorRotation)
//*!!Code automatically generated by 'ROBOTC' configuration wizard               !!*//

task main()
{
  SensorValue[dial] = 0; //set first number
  bool choosing;
  choosing = true;
  int num1;
  while (choosing == true)
  {
    SetUserDisplay(SensorValue[dial]);
    if (SensorValue[select] == 1)
    {
      num1 = SensorValue[dial];
      choosing = false;
    }
  }
  PlayTone(220,50);
  wait10Msec(50);

  int quarter; //select mathematical operation
  int operation;
  SensorValue[dial] = 0;
  choosing = true;
  while (choosing == true)
  {
    quarter = (SensorValue[dial]/4)%4;
    if (quarter < 0){quarter*=-1;}
    if (quarter == 0)
    {
      SetUserDisplay(1); //add
    }
    if (quarter == 1)
    {
      SetUserDisplay(2); //subtract
    }
    if (quarter == 2)
    {
      SetUserDisplay(3); //multiply
    }
    if (quarter == 3)
    {
      SetUserDisplay(4); //divide
    }
    if (SensorValue[select] == 1)
    {
      operation = quarter+1;
      choosing = false;
    }
  }
  PlayTone(220,50);
  wait10Msec(50);

  SensorValue[dial] = 0; //set second number
  int num2;
  choosing = true;
  while (choosing == true)
  {
    SetUserDisplay(SensorValue[dial]);
    if (SensorValue[select] == 1)
    {
      num2 = SensorValue[dial];
      choosing = false;
    }
  }
  PlayTone(220,50);

  int result; //calculate result
  if (operation == 1) //add
  {
   result = num1+num2;
  }
  if (operation == 2) //subtract
  {
    result = num1-num2;
  }
  if (operation == 3) //multiply
  {
    result = num1*num2;
  }
  if (operation == 4)
  {
    result = num1/num2;
  }
  SetUserDisplay(result);
  
  choosing = true; //waits for button to be pressed before starting again
  wait1Msec(10000);
}