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12GAME PROJECTS · CALCULATOR

Build your own calculator.

You type two numbers and a sign. PyBot picks the right math and shows the answer.

IN REAL LIFE

Calculators are everywhere.

Many machines take numbers, choose an operation, and show a result.

The shop register

It adds up prices and takes away the money you pay.

Sharing a pizza

8 slices for 4 friends: divide, and each one gets 2.

The phone calculator

Type 0 after the ÷ sign and it shows a message, not an answer.

THE PLAN

The calculator in four steps.

Each step uses something you already know.

Ask two numbers

Turn each answer into a number. float keeps decimals like 2.5.

input + float

Ask the sign

The player types +, -, * or /. It stays text.

input

Choose the math

Compare the sign with == and do the matching operation.

if / elif / else

Pack it in a function

calculate gets the numbers and the sign and returns the answer. It checks for 0 first.

def + return

RUN IT

Try the whole calculator.

Guess first. Then run real Python and check.

1

Guess the result

a = float(input("First number? ")) sign = input("Sign (+ - * /)? ") b = float(input("Second number? ")) print("Answer:", calculate(a, sign, b))

PyBot's answers: 8, then *, then 2

What is the last line?

calculator.pyREAL PYTHON
OUTPUT
Ready. Press Run Python.
Python runs here thanks to Pyodide ♥

Pyodide 314.0.3 runs CPython in your browser with WebAssembly.

Open-source under the Mozilla Public License 2.0. License

Now change one thing: Change the answers to 7, / and 0. What does PyBot say?

BUILD IT

Build it step by step.

Fix one part at a time. Run after each step.

0Completed
11Not tried
0Review
01

Step 1: two real numbers

8 + 2 shows 82. Turn both answers into numbers with float().

Goal output
First number? 8
Second number? 2
Sum: 10.0
OUTPUT

          
02
Step 2: choose the math
The calculator knows + and -, but not *. Add an elif for * before else.
Goal output
First number? 6
Sign (+ - * /)? *
Second number? 7
Answer: 42.0
OUTPUT

          
03
Step 3: protect from 0
Dividing by 0 stops the program with an error. Make divide return the message when b is 0.
Goal output
Answer: 4.0
Answer: Oops! You can't divide by 0.
OUTPUT

        

      


      
YOUR TURN
Questions about the calculator.
Try each tiny question. A wrong answer becomes something to review.

        

          
04
Without float()
a = input("First number? ")
b = input("Second number? ")
print(a + b)
PyBot's answers: 3, then 4
What does the last line show?

          
05
Which branch?
if sign == "+":
    print(a + b)
elif sign == "-":
    print(a - b)
else:
    print("?")
a is 9.0, b is 4.0 and sign is "-"
What will Python show?

          
06
Compare the sign
PyBot must check if the sign is a plus.
Which line is right?

          
07
Why check for 0?
if b == 0:
    return "Oops! You can't divide by 0."
return a / b
Why does the calculator check b first?

          
08
What does return do?
print("Answer:", calculate(8, "*", 2))
calculate ends with return a * b. Where does the 16 go?

          
09
A strange sign
In the full calculator, the player types 5, then x, then 3.
What does PyBot answer?

          
10
Half a pizza
print(7 / 2)
What will Python show?

        

        
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!
BIG IDEA
A function can be a machine.
Numbers and a sign go in, if/elif picks the math, and return sends the answer out. Check for 0 before you divide.