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Module 010: Milestone Project — Command-Line Calculator

Phase: 1. Fundamentals | Estimated time: 3 hours | Milestone Project: Yes

Prerequisites

Learning Objectives

By the end of this module, you will be able to:

  • Build a complete, interactive CLI program from scratch
  • Take user input for two numbers and an operator
  • Perform arithmetic based on the operator using conditional logic
  • Handle division by zero gracefully
  • Apply concepts from all prior modules (variables, types, conversion, operators, strings, formatting, math)

Why This Matters

A calculator is the classic "first real program" for a reason: it combines input, output, math, control flow, and error handling into one cohesive project. Completing this milestone proves you can build something useful from scratch — and gives you a template for many future projects.

Project Brief

Build a command-line calculator that:

  1. Prompts the user for two numbers.
  2. Prompts for an operator (+, -, *, /, //, %, **).
  3. Computes and displays the result.
  4. Handles division by zero gracefully (prints an error message instead of crashing).
  5. Formats output cleanly using f-strings.

Example Run

=== Python Command-Line Calculator ===

Enter first number: 10
Enter operator (+, -, *, /, //, %, **): /
Enter second number: 3
10.0 / 3.0 = 3.3333333333333335

Enter first number: 10
Enter operator (+, -, *, /, //, %, **): /
Enter second number: 0
Error: Division by zero is not allowed.

Enter first number: 2
Enter operator (+, -, *, /, //, %, **): **
Enter second number: 10
2.0 ** 10.0 = 1024.0

Enter first number: done
Goodbye!

Concept Application

This project brings together everything from Modules 002–009:

Concept Where Used
print() Output and formatting
input() Getting user input
Variables Storing numbers and operator
Type conversion float() on input strings
Arithmetic operators +, -, *, /, //, %, **
Comparison operators Checking for zero divisor, valid operator
Logical operators Combining conditions
f-strings Formatting output
== and != Comparing operator string
Comments Documenting code

Common Pitfalls

  1. Not converting input to numbers: input() returns strings. Use float() or int().
  2. Division by zero: Check if the second number is 0 before dividing.
  3. Invalid operator: Decide what happens when the user enters something other than the 7 valid operators.
  4. Infinite loop: Make sure there's a way for the user to exit (e.g., typing "done" or pressing Ctrl+C).
  5. Floating-point display: Some results like 0.1 + 0.2 may show many decimal places. round() or format specifiers can help.

Key Takeaways

  • A complete interactive program requires input → processing → output.
  • Always validate input before performing operations.
  • Division by zero must be checked explicitly.
  • A loop lets the user perform multiple calculations without restarting.
  • Break the problem into small steps: get input, validate, compute, display.
  • All the pieces from earlier modules come together here.
  • This project structure (input-loop-compute-display) applies to many CLI tools.

Further Reading

Next Module

Congratulations on completing your first milestone! Continue to Module 011 (Control Flow: if/elif/else) to learn how to make decisions in your programs.