Mini Calculator

Build a calculator that can add, subtract, multiply and divide, and that keeps going until you tell it to stop.

Functions
input()
float()
if / elif / else
while loops

🎯 What we're going to build

Computers are amazing at maths. They can add up millions of numbers in the blink of an eye. In this project you will turn Python into your very own pocket calculator. The player types two numbers and a symbol like + or *, and the calculator works out the answer.

Building a calculator is a great way to learn about functions. A function is a little machine with a name: you give it some numbers, it does a job, and it hands back a result. You will write one function for each kind of sum, and then your program will choose the right one based on what the player types.

You will also learn how to handle a tricky situation: dividing by zero! Real programmers always think about what could go wrong, and you will make sure your calculator gives a friendly message instead of crashing.

💡 How to use this guide: work through the steps in order and run your code after each one. Steps marked snippet are pieces that slot into the program you are building, so they won't run on their own. Programs that use input() need a Python editor such as IDLE, Thonny or Replit, where you can type your answers.

👣 Step by step

1
Write your first function
We create a function with the word def, give it a name, and list the values it needs in brackets. The return line sends the answer back to whoever called the function. Then we call add(3, 4) to test it.
🐍 Python
```
def add(a, b):
    return a + b

print(add(3, 4))
print(add(10, 25))
```
▶ Output
```
7
35
```
2
Add the other three functions
Subtracting, multiplying and dividing work in exactly the same way. In Python, * means multiply and / means divide. Notice that dividing always gives a decimal number, like 5.0, even when the answer is a whole number.
🐍 Python
```
def subtract(a, b):
    return a - b

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

def divide(a, b):
    return a / b

print(subtract(10, 4))
print(multiply(6, 7))
print(divide(20, 4))
```
▶ Output
```
6
42
5.0
```
3
Protect against dividing by zero
Nobody can divide a number by zero, not even a computer! If you try, Python stops with an error. We add an if inside divide() to check whether b is 0 first. If it is, we return a friendly message instead of crashing.
🐍 Python
```
def divide(a, b):
    if b == 0:
        return "Oops! You can't divide by zero."
    return a / b

print(divide(9, 3))
print(divide(9, 0))
```
▶ Output
```
3.0
Oops! You can't divide by zero.
```
4
Ask the player for numbers
We use input() to ask for two numbers and a symbol. Because input() gives us text, we use float() to turn the numbers into decimals. That way the calculator works with numbers like 2.5 as well as whole numbers.
🐍 Python
```
first = float(input("First number: "))
symbol = input("Choose + - * or /: ")
second = float(input("Second number: "))

print("You want to work out", first, symbol, second)
```
▶ Output
```
First number: 8
Choose + - * or /: +
Second number: 5
You want to work out 8.0 + 5.0
```
5
Pick the right function
Now we check which symbol the player typed and call the matching function. Each elif checks one more symbol. The final else catches anything we don't understand, like a typo. This piece of code uses the four functions from the earlier steps, so they need to be at the top of your file.
🧩 Snippet: add to your program
```
if symbol == "+":
    answer = add(first, second)
elif symbol == "-":
    answer = subtract(first, second)
elif symbol == "*":
    answer = multiply(first, second)
elif symbol == "/":
    answer = divide(first, second)
else:
    answer = "I don't know that symbol."

print("Answer:", answer)
```

Keep calculating with a loop

A real calculator doesn't switch off after one sum. We put everything inside a while loop that keeps going while keep_going is "yes". At the end of each sum we ask the player if they want another. Using .lower() means "YES" and "Yes" work too.

🐍 Python

keep_going = "yes"

while keep_going == "yes":
    # ...ask for the numbers and work out the answer here...
    keep_going = input("Another sum? (yes/no): ").lower()

print("Thanks for calculating with me! 👋")

▶ Output

Another sum? (yes/no): yes
Another sum? (yes/no): no
Thanks for calculating with me! 👋

✅ The finished program

Here is the whole program with every step put together. If something isn't working in your version, compare it with this one line by line. Check the spelling, the colons at the end of if and while lines, and that the indentation (the spaces at the start of lines) matches.

🐍 Mini Calculator

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 "Oops! You can't divide by zero."
    return a / b

print("🧮 Mini Calculator")

keep_going = "yes"

while keep_going == "yes":
    first = float(input("First number: "))
    symbol = input("Choose + - * or /: ")
    second = float(input("Second number: "))

    if symbol == "+":
        answer = add(first, second)
    elif symbol == "-":
        answer = subtract(first, second)
    elif symbol == "*":
        answer = multiply(first, second)
    elif symbol == "/":
        answer = divide(first, second)
    else:
        answer = "I don't know that symbol."

    print("Answer:", answer)
    keep_going = input("Another sum? (yes/no): ").lower()

print("Thanks for calculating with me! 👋")

🖥️ Sample run

This is what it looks like when someone plays. The words after each question are what the player typed.

▶ Output

🧮 Mini Calculator
First number: 12
Choose + - * or /: *
Second number: 3
Answer: 36.0
Another sum? (yes/no): yes
First number: 10
Choose + - * or /: /
Second number: 0
Answer: Oops! You can't divide by zero.
Another sum? (yes/no): yes
First number: 7.5
Choose + - * or /: +
Second number: 2.5
Answer: 10.0
Another sum? (yes/no): no
Thanks for calculating with me! 👋

🚀 Make it your own

Finished? Brilliant! Real programmers are always improving their programs. Try one of these challenges to make the project truly yours.

⭐Add a power button: if the player types **, work out first ** second (for example 2 ** 3 is 8).
⭐Add a % option that gives the remainder after dividing. Try 17 % 5!
⭐Count how many sums the player did and print the total when they say no.
⭐Use round(answer, 2) so long answers like 3.3333333 are tidied up to 3.33. (Careful: only round numbers, not the divide-by-zero message!)

📚 Lessons that help

Stuck on a step? These lessons explain the Python ideas used in this project.

⚙️Functions ⌨️User Input ➗Operators 🤔If / Else 🔢Data Types

← Back to all projects

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Mini Calculator

🎯 What we're going to build

👣 Step by step

Write your first function

Add the other three functions

Protect against dividing by zero

Ask the player for numbers

Pick the right function

Keep calculating with a loop

✅ The finished program

🖥️ Sample run

🚀 Make it your own

📚 Lessons that help

Mini Calculator

🎯 What we're going to build

👣 Step by step

Write your first function

Add the other three functions

Protect against dividing by zero

Ask the player for numbers

Pick the right function

Keep calculating with a loop

✅ The finished program

🖥️ Sample run

🚀 Make it your own

📚 Lessons that help