// this code and design is in the public domain
// designed by Peterthinks Sept 25th 2021
#include <LiquidCrystal_I2C.h>
// include the LCD library
LiquidCrystal_I2C lcd(0x27,20,3);   
// declare globally the type, name and assign a value to the variables.
// I used longs for +, - and * with floats for division
// so the decimal points show without rounding off and
// the numbers do not roll over into the negatives as they
// would with intergers
 long sensorValue3 = 0;
 float FsensorValue1 = 0;
 float FsensorValue3 = 0;
 long sensorValue1 = 0;
// sensorValue2 controls which operation (+,-,*,/) is being performed
// an interger is big enough for this
 int sensorValue2 = 0;
// run the setup once
void setup()
{
// start the LCD and tell the arduino
// how many characters and rows the LCD has
  lcd.begin(16, 2); 
// turn on the LCD backlight
   lcd.backlight();
// declare pinmodes as inputs for the potientiometer wiper pins
  pinMode(A0, INPUT); 
  pinMode(A1, INPUT); 
  pinMode(A2, INPUT); 
}
// begin the loop
void loop()
{
// read the values coming into the arduino and fill the variables
// A0 and A2 are read twice to fill the intergers and float variables
  sensorValue1 = analogRead(A0);
  sensorValue2 = analogRead(A1);
  sensorValue3 = analogRead(A2);
  FsensorValue1 = analogRead(A0);
  FsensorValue3 = analogRead(A2);
// begin the else/if statements
// if sensorValue2 is less than or equal to 256 addition is performed
// on the user input long variables
  if (sensorValue2 <= 256)
  { 
// map the value of A0 and A2 from the raw 0 to 1023 to 0 to 10000 
  sensorValue1 = map(analogRead (A0), 0, 1023, 0, 100);
  sensorValue3 = map(analogRead (A2), 0, 1023, 0, 100);
// set the cursor at the first character of the first line    
  lcd.setCursor(0, 0);
// print sensorValue3 to the LCD starting at the first character on the first line
  lcd.print(sensorValue3);
// print a + symbol with a space on either side of it beside sensorValue3
  lcd.print(" + ");
// print sensorValue1 to the LCD after the + symbol and spaces
  lcd.print(sensorValue1);
// print some blank spaces to clear anything to the right if a longer
// equation was previously shown on the LCD
// this is used instead of lcd.clear(); to prevent flicker on the LCD
  lcd.print("           ");
// set the cursor at the first character of the second line    
  lcd.setCursor(0, 1);
// add the two longs together
  lcd.print(sensorValue3+sensorValue1);
// print some blank spaces to clear anything to the right if a longer
// answer was previously shown on the LCD
  lcd.print("                "); 
  }
// second of the else/if statements
// if sensorValue2 is greater than 256 and less than or equal to 512
// subtraction is performed on the user input long variables 
  else if (sensorValue2 > 256 && sensorValue2 <= 512)
  {
  sensorValue1 = map(analogRead (A0), 0, 1023, 0, 100);
  sensorValue3 = map(analogRead (A2), 0, 1023, 0, 100);
    
  lcd.setCursor(0, 0);  
  lcd.print(sensorValue3);
  lcd.print(" - ");
  lcd.print(sensorValue1);
  lcd.print("           ");
 
  lcd.setCursor(0, 1);  
  lcd.print(sensorValue3-sensorValue1);
  lcd.print("                "); 
  }
// third of the else/if statements
// if sensorValue2 is greater than 512 and less than or equal to 768
// multiplication is performed on the user input long variables   
  else if (sensorValue2 > 512 && sensorValue2 <= 768)
  {
  sensorValue1 = map(analogRead (A0), 0, 1023, 0, 100);
  sensorValue3 = map(analogRead (A2), 0, 1023, 0, 100);
    
  lcd.setCursor(0, 0); 
  lcd.print(sensorValue3);
  lcd.print(" * ");
  lcd.print(sensorValue1);
  lcd.print("           ");
    
  lcd.setCursor(0, 1);   
  lcd.print(sensorValue3*sensorValue1);
  lcd.print("                ");
    
  }
// last of the else/if statements
// if sensorValue2 is any other value
// division is performed on the user input float variables
// FsensorValue1 and FsensorValue3. note that "else" is used
// instead of "if else" since this is a catch all to prevent
// stepping out of the loop and breaking the program
  else
  {
  sensorValue1 = map(analogRead (A0), 0, 1023, 0, 100);
  sensorValue3 = map(analogRead (A2), 0, 1023, 0, 100); 
// only the long data versions are used to display the equation
// since the decimal of the float is only needed in the answer
  lcd.setCursor(0, 0); 
  lcd.print(sensorValue3);
  lcd.print(" / ");
  lcd.print(sensorValue1);
  lcd.print("           ");
    
  lcd.setCursor(0, 1);
  FsensorValue1 = sensorValue1;
 FsensorValue3 = sensorValue3;
// the ,6 at the end of the division equation forces six decimal
// places on the float answer. otherwise it only shows two decimal places
  lcd.print(FsensorValue3/FsensorValue1,6);
  lcd.print ("                ");
  } 
// and go back up to the top of the loop
}