Simple calculator

Table of Contents

Planning the Simple Calculator

In this mini project, you will build a text-based calculator that can:

Ask the user which operation to perform
Ask for one or two numbers (depending on the operation)
Perform the calculation
Show the result
Keep running until the user chooses to quit

You will use concepts you already know: variables, input, if/elif/else, loops, and basic functions.

A good way to think about this project is to break it into small steps:

Show a menu of options (add, subtract, multiply, divide, quit).
Get the user’s choice.
If they chose to quit, stop the program.
If they chose a valid operation:

Ask for the numbers.
Perform the calculation.
Show the result.

If they chose something invalid, show an error message.
Repeat from step 1.

You can later improve it by:

Handling invalid number input
Handling division by zero
Organizing code into functions

Basic Version: One Operation at a Time

Start with the simplest version: choose an operation, enter two numbers, get the answer, then the program ends.

print("Simple Calculator")
print("------------------")
print("Choose an operation:")
print("1. Add")
print("2. Subtract")
print("3. Multiply")
print("4. Divide")
choice = input("Enter your choice (1/2/3/4): ")
num1 = float(input("Enter first number: "))
num2 = float(input("Enter second number: "))
if choice == "1":
    result = num1 + num2
    print("Result:", result)
elif choice == "2":
    result = num1 - num2
    print("Result:", result)
elif choice == "3":
    result = num1 * num2
    print("Result:", result)
elif choice == "4":
    result = num1 / num2
    print("Result:", result)
else:
    print("Invalid choice")

Things to notice:

All user input starts as text (strings), so we convert the numbers with float(...) so the calculator can handle decimals.
The operation is chosen using if / elif / else.
There is no loop yet: after one calculation, the program ends.

Adding a Loop: Calculator That Keeps Running

Next, make the calculator repeat until the user chooses to quit. A common pattern:

Use a while True: loop.
Inside, show the menu and handle the choice.
Use break to exit the loop when the user wants to quit.

while True:
    print("\nSimple Calculator")
    print("------------------")
    print("1. Add")
    print("2. Subtract")
    print("3. Multiply")
    print("4. Divide")
    print("5. Quit")
    choice = input("Enter your choice (1/2/3/4/5): ")
    if choice == "5":
        print("Goodbye!")
        break
    if choice not in ("1", "2", "3", "4"):
        print("Invalid choice, please try again.")
        continue
    num1 = float(input("Enter first number: "))
    num2 = float(input("Enter second number: "))
    if choice == "1":
        result = num1 + num2
    elif choice == "2":
        result = num1 - num2
    elif choice == "3":
        result = num1 * num2
    elif choice == "4":
        result = num1 / num2
    print("Result:", result)

New ideas in this version:

Using continue to skip the rest of the loop and start over for an invalid choice.
Moving the repeated menu into the loop so it shows every time.
Using the same variables (num1, num2, result) again on each loop.

Handling Division by Zero

If the user tries to divide by zero, Python will raise an error. You can prevent that with a simple check before dividing.

Modify just the division part:

elif choice == "4":
    if num2 == 0:
        print("Error: Cannot divide by zero.")
        continue
    result = num1 / num2

Full loop with this check included:

while True:
    print("\nSimple Calculator")
    print("------------------")
    print("1. Add")
    print("2. Subtract")
    print("3. Multiply")
    print("4. Divide")
    print("5. Quit")
    choice = input("Enter your choice (1/2/3/4/5): ")
    if choice == "5":
        print("Goodbye!")
        break
    if choice not in ("1", "2", "3", "4"):
        print("Invalid choice, please try again.")
        continue
    num1 = float(input("Enter first number: "))
    num2 = float(input("Enter second number: "))
    if choice == "1":
        result = num1 + num2
    elif choice == "2":
        result = num1 - num2
    elif choice == "3":
        result = num1 * num2
    elif choice == "4":
        if num2 == 0:
            print("Error: Cannot divide by zero.")
            continue
        result = num1 / num2
    print("Result:", result)

Organizing Code with Functions

To make your calculator easier to read and extend, you can split it into functions:

A function to show the menu and get a valid choice.
A function to ask for a number.
Separate functions for each operation (add, subtract, multiply, divide).

This is good practice for structuring slightly larger programs.

def show_menu():
    print("\nSimple Calculator")
    print("------------------")
    print("1. Add")
    print("2. Subtract")
    print("3. Multiply")
    print("4. Divide")
    print("5. Quit")
    return input("Enter your choice (1/2/3/4/5): ")
def get_number(prompt):
    return float(input(prompt))
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:
        print("Error: Cannot divide by zero.")
        return None
    return a / b
while True:
    choice = show_menu()
    if choice == "5":
        print("Goodbye!")
        break
    if choice not in ("1", "2", "3", "4"):
        print("Invalid choice, please try again.")
        continue
    num1 = get_number("Enter first number: ")
    num2 = get_number("Enter second number: ")
    if choice == "1":
        result = add(num1, num2)
    elif choice == "2":
        result = subtract(num1, num2)
    elif choice == "3":
        result = multiply(num1, num2)
    elif choice == "4":
        result = divide(num1, num2)
    if result is not None:
        print("Result:", result)

Here you can clearly see:

The loop is now easier to read.
Each function has a single, clear job.
divide returns None when division is not possible, and the main loop checks for that before printing.

Simple Input Validation (Optional Improvement)

If the user types something that is not a number, float(...) will raise an error. You can handle this with try / except and a loop that keeps asking until a valid number is entered.

Replace get_number with this version:

def get_number(prompt):
    while True:
        value = input(prompt)
        try:
            return float(value)
        except ValueError:
            print("That is not a valid number, please try again.")

Now your calculator is more robust: it will not crash when the user types text instead of a number.

Possible Extensions

Once the basic calculator works, try adding features like:

More operations:

Power: $a^b$ using a ** b
Square root: using a library function

Memory features:

Store the last result and allow using it in the next calculation

Single-number operations:

Example: square a number (only one input)

You don’t need to add everything at once. The important part of this mini project is: