Project 3 — Calculator

Build a command-line calculator that parses infix expressions (+, -, *, /, **, %) and handles division-by-zero and invalid input gracefully.

input parsingtry/exceptfunctionseval-safe

Duration

⏱ 1.5 hours

Level

📊 Basic Applied

Prerequisite

🎯 Basic Week 9

OUTCOME

A robust CLI calculator with history and safe expression evaluation

What you'll learn

1Parse and evaluate arithmetic expressions
2Handle errors gracefully (division by zero, invalid input)
3Store and display calculation history
4Support multi-step calculations (ANS reference)

Project Overview

Evaluate expressions like: 3 + 4 * 2, (10 - 3) ** 2
Commands: history, clear, quit
ANS refers to the previous result
Never use eval() — parse manually or use a safe alternative

Sample Session

text

Calculator (type 'quit' to exit)
> 3 + 4 * 2
= 11
> (10 - 3) ** 2
= 49
> ANS + 1
= 50
> 10 / 0
Error: division by zero
> history
  3 + 4 * 2 = 11
  (10 - 3) ** 2 = 49
  ANS + 1 = 50

💻 Examples

Run these examples and check the output yourself.

solution.py— Safe calculator with history

CODE

import re

def safe_eval(expr, ans):
    expr = expr.replace("ANS", str(ans))
    # Allow only numbers, operators, parens, spaces
    if not re.fullmatch(r'[\d.+\-*/%(). \*\*]+', expr):
        raise ValueError("Invalid characters in expression")
    try:
        return eval(compile(expr, "<string>", "eval"),
                    {"__builtins__": {}}, {})
    except ZeroDivisionError:
        raise ZeroDivisionError("division by zero")

history, ans = [], 0
print("Calculator (type 'quit' to exit)")
while True:
    line = input("> ").strip()
    if line.lower() in ("quit", "exit"): break
    if line.lower() == "history":
        for h in history: print(f"  {h}")
        continue
    if line.lower() == "clear":
        history.clear(); ans = 0; continue
    try:
        result = safe_eval(line, ans)
        print(f"= {result}")
        history.append(f"{line} = {result}")
        ans = result
    except Exception as e:
        print(f"Error: {e}")

📝 Exercises

Try them yourself first, then open the solution to compare.

Exercise 1

Build the calculator

Goal: Implement expression evaluation, history, ANS, and error handling.

Requirements

Evaluate +, -, *, /, **, %
ANS references previous result
Division by zero handled gracefully
history command shows calculation log

Example code / lecture materials

All lecture materials and example code are openly available on GitHub.

View on GitHub ↗