Project: Calculator

Build a CLI calculator with functions, a looped menu, and solid input validation.

~50 min4 sections2 exercisesQuiz · pass 4/5

On this page

01Project spec
02Step 1 — operation functions
03Step 2 — safe number input
04Step 3 — menu loop
05Ship the calculator
06Try It
07Quiz

Project spec

Build a calculator that:

Shows a menu: add, subtract, multiply, divide, quit
Asks for two numbers for each operation
Prints the result
Rejects invalid numbers and division by zero without crashing
Loops until the user quits

Sample session

=== Calculator ===
1) add
2) subtract
3) multiply
4) divide
5) quit
Choose: 1
a: 12
b: 5
Result: 17.0

Step 1 — operation functions

Write one function per operation. Keep them tiny and testable.

divide should guard against b == 0 (return None or raise a clear error you catch later).

Core operations

def add(a, b):
    return a + b

def subtract(a, b):
    return a - b

def multiply(a, b):
    return a * b

def divide(a, b):
    if b == 0:
        return None
    return a / b

print(add(2, 3), divide(10, 2), divide(10, 0))

Returning None for illegal divide keeps the UI layer in charge of the message.

Step 2 — safe number input

Wrap float(input(...)) in try/except ValueError and keep asking until the user types a valid number.

This helper removes duplication from every menu action.

read_number helper

def read_number(prompt):
    while True:
        raw = input(prompt)
        try:
            return float(raw)
        except ValueError:
            print("Please enter a valid number.")

# In Try It, call read_number once or twice to test

Step 3 — menu loop

Print the menu, read a choice, dispatch to the right function, then loop.

Use if/elif or a dict mapping choices to functions—both are fine at this level.

Menu skeleton

def menu():
    print("1) add  2) subtract  3) multiply  4) divide  5) quit")

# Pseudocode:
# while True:
#     menu()
#     choice = input("Choose: ").strip()
#     if choice == "5":
#         print("Bye!")
#         break
#     if choice not in {"1", "2", "3", "4"}:
#         print("Invalid choice")
#         continue
#     a = read_number("a: ")
#     b = read_number("b: ")
#     ... compute and print result ...

continue skips the rest of the loop when the choice is invalid—clean control flow.

Practice

Ship the calculator

Assemble the full app: helpers + operations + menu loop. Support all four operations, validate numbers, handle divide-by-zero, and quit cleanly. Extra credit: remember the last result and allow using it as the next a via typing "ans".

challenge starter

def add(a, b):
    return a + b

def subtract(a, b):
    return a - b

def multiply(a, b):
    return a * b

def divide(a, b):
    if b == 0:
        return None
    return a / b

def read_number(prompt):
    while True:
        try:
            return float(input(prompt))
        except ValueError:
            print("Please enter a valid number.")

# TODO: menu loop wiring everything together

Interactive

Try It in the browser

Implement add/subtract/multiply/divide as above. Print results for (8,2) on all four ops, and show that divide(8,0) is None.

Output

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Errors

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