27 — Calculator — a command-line project

A calculator that reads 3 + 4 and answers, then asks again until you type quit, is a complete small project. It uses all three levels in one program: beginner ask for input, plain Python while True: and def for the loop and the math, and NME show for output.

#Steps

1. Save the whole calculator in one file, calculator.nme, and run it:

   # A command-line calculator: type 3 + 4, or quit.
   # Run: nme r calculator
   
   def calculate(parts):
       if parts[1] == "+":
           return int(parts[0]) + int(parts[2])
       elif parts[1] == "-":
           return int(parts[0]) - int(parts[2])
       elif parts[1] == "*":
           return int(parts[0]) * int(parts[2])
       else:
           return int(parts[0]) / int(parts[2])
   
   
   show "Calculator — type a command like 3 + 4, or quit."
   
   while True:
       ask command, "Your command? "
       if command == "quit":
           show "Bye!"
           break
       parts = command.split()
       if len(parts) == 3:
           answer = calculate(parts)
           show f"{command} = {answer}"
       else:
           show "Use the form: number operator number"
   

   copyrun it →

   printf '3 + 4\n10 - 3\n7 * 6\n10 / 4\nquit\n' | nme r calculator
   

   copy

   Calculator — type a command like 3 + 4, or quit.
   Your command? 3 + 4 = 7
   Your command? 10 - 3 = 7
   Your command? 7 * 6 = 42
   Your command? 10 / 4 = 2.5
   Your command? Bye!
   

   copy

2. The math lives in a function. def names it, parts holds the split command, and return sends the answer back. parts[1] is the operator and int(parts[0]) turns the text "3" into the number 3:

   def calculate(parts):
       if parts[1] == "+":
           return int(parts[0]) + int(parts[2])
       elif parts[1] == "-":
           return int(parts[0]) - int(parts[2])
       elif parts[1] == "*":
           return int(parts[0]) * int(parts[2])
       else:
           return int(parts[0]) / int(parts[2])
   

   copyrun it →

   elif chains one branch per operator; the last else is division. This block is advanced Python written directly inside the NME file.

3. split() cuts the command into words. "3 + 4".split() becomes ['3', '+', '4'], so parts[0] is the first number, parts[1] the operator, parts[2] the second number. len(parts) == 3 rejects anything that is not a well-formed command:

   parts = command.split()
   if len(parts) == 3:
       answer = calculate(parts)
       show f"{command} = {answer}"
   else:
       show "Use the form: number operator number"
   

   copyrun it →

   show f"{command} = {answer}" prints the line you typed and the result on one row.

4. The loop is the same shape as the menu in 22: while True: never ends on its own, so quit must break out. Everything between is one turn of the calculator:

   while True:
       ask command, "Your command? "
       if command == "quit":
           show "Bye!"
           break
       parts = command.split()
       if len(parts) == 3:
           answer = calculate(parts)
           show f"{command} = {answer}"
       else:
           show "Use the form: number operator number"
   

   copyrun it →

5. Once the function works, move it into its own module as guide 23 showed. Save the function in calc.nme:

   # calc.nme — the calculate function only
   def calculate(parts):
       if parts[1] == "+":
           return int(parts[0]) + int(parts[2])
       elif parts[1] == "-":
           return int(parts[0]) - int(parts[2])
       elif parts[1] == "*":
           return int(parts[0]) * int(parts[2])
       else:
           return int(parts[0]) / int(parts[2])
   

   copyrun it →

   Then import it at the top of calculator.nme. The main file now only describes the loop; the math stays in calc.nme:

   from "calc.nme" import calculate
   
   show "Calculator — type a command like 3 + 4, or quit."
   
   while True:
       ask command, "Your command? "
       if command == "quit":
           show "Bye!"
           break
       parts = command.split()
       if len(parts) == 3:
           answer = calculate(parts)
           show f"{command} = {answer}"
       else:
           show "Use the form: number operator number"
   

   copy

   nme check calculator checks both files, and nme r calculator runs the import just like before.

6. The Korean twin calculator.ko.nme keeps the same def and writes the loop with 물어봐, 만약, and 말해:

   def calculate(parts):
       if parts[1] == "+":
           return int(parts[0]) + int(parts[2])
   
   
   while True:
       물어봐 command, "명령을 입력하세요? "
       만약 command == "quit":
           말해 "안녕!"
           break
       parts = command.split()
       answer = calculate(parts)
       말해 f"{command} = {answer}"
   

   copyrun it →

   The same piped input produces the same answers in both languages.

#Try it yourself

Add a fifth operator: change the else branch so // means floor division (int(parts[0]) // int(parts[2])), then test 17 // 5. Or make the unknown operator a friendly message instead of dividing.

#What you learned

• def/return package the math into one reusable function.
• command.split() cuts a line into parts; int(parts[0]) reads a number.
• while True: with break on quit makes a repeat-until-quit loop.
• Moving the function to a .nme module keeps the project clean.