Simple Calculator (Python)

Goals

• Define a function with multiple parameters
• Use if/elif/else to select the operation
• Guard against division by zero
• Handle unknown operators with a clear message

Prerequisites

• Functions and return values
• if/elif/else statements
• input() and float()

Steps

2. 1

   Write the calculate function

   Define calculate(a, op, b) that branches on op and returns the result. Use if/elif/else for the four operators, and return an error string for unknown ops.

   CodeCopy

   def calculate(a, op, b):
       if op == "+":
           return a + b
       elif op == "-":
           return a - b
       elif op == "*":
           return a * b
       elif op == "/":
           if b == 0:
               return "Cannot divide by zero"
           return a / b
       else:
           return "Unknown operator"

   Expected result

   calculate(4, "+", 5) returns 9; calculate(5, "/", 0) returns the error message.

   Show hint
4. 2

   Read input from the user

   Ask the user for two numbers and an operator. Convert the numbers with float() so decimal input works; leave the operator as a string.

   CodeCopy

   a = float(input("First number: "))
   op = input("Operator (+, -, *, /): ")
   b = float(input("Second number: "))

   Expected result

   Three values read from the user, ready to pass to calculate.

6. 3

   Call the function and print the result

   Pass the inputs to calculate and print the result. Keeping the math in a function means the CLI is just input → call → output.

   CodeCopy

   result = calculate(a, op, b)
   print(f"{a} {op} {b} = {result}")

   Expected result

   Entering 4, +, 5 prints "4.0 + 5.0 = 9.0"; entering 5, /, 0 prints the error message.

8. 4

   Test edge cases

   Run the script with a few edge cases: division by zero, an unknown operator, and negative numbers. Confirm each returns a sensible result or message.

   CodeCopy

   # Test cases (run mentally or in a REPL)
   print(calculate(4, "+", 5))   # 9
   print(calculate(10, "/", 0))  # Cannot divide by zero
   print(calculate(3, "%", 2))   # Unknown operator
   print(calculate(-6, "*", 7))  # -42

   Expected result

   Four lines: 9, the zero-division message, the unknown-operator message, and -42.

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