In this lesson, we’ll build a simple calculator using Python. It will perform basic arithmetic operations: addition, subtraction, multiplication, and division.

Let’s walk through the project.

🛠️ Goal: Create a simple calculator that takes two numbers and an operation as input, then displays the result.

🧱 Step-by-Step Breakdown

✅ Step 1: Accept Two Numbers as Input

In Python, we use the input() function to collect user data. But input is always taken as a string, and we can’t do math with strings!

That’s why we wrap it with int() to convert the input to an integer:

num1 = int(input("Enter first number: "))

Then, we collect the second number later after asking for the operation:

num2 = int(input("Enter Second number: "))

Why after? You’ll see shortly — it keeps the logic clear when we handle division.

✅ Step 2: Ask the User Which Operation They Want

The user should be able to choose one of the following:

  • + for addition
  • - for subtraction
  • * for multiplication
  • / for division

We ask like this:

operation = input("Choose operation(+,-,*,/):")

This input remains a string (no conversion needed), and we’ll use it to decide which math to perform.

✅ Step 3: Use if...elif...else to Perform the Operation

Here comes the core of the logic. Python checks what the user typed and does the correct calculation:

if operation == "+":
    print("The sum is:")
    print(num1 + num2)

We use elif for the other operations:

elif operation == "-":
    print("The answer is:")
    print(num1 - num2)

Same for multiplication and division:

elif operation == "*":
    print("The answer is:")
    print(num1 * num2)

⚠️ Step 4: Handle Division Carefully

Division is tricky because you can’t divide by zero. That would crash your program. So we check for that before doing the math:

elif operation == "/":
    if num2 != 0:
        print("The answer is:")
        print(num1 / num2)
    else:
        print("Error: Cannot divide by zero!")

🧠 This is your first nested if statement — an if inside an elif block. Very useful!

❌ Step 5: Catch Invalid Inputs

What if the user types ^, %, or something else? We need a fallback:

else:
    print("Invalid Operation Selected!")

This catches anything not handled above and gives the user feedback — no crashing!

🧪 The Full Code on Edublocks

🧾 The Full Code

num1 = int(input("Enter first number: "))
operation = input("Choose operation(+,-,*,/):")
num2 = int(input("Enter Second number: "))

if operation == "+":
    print("The sum is:")
    print(num1 + num2)
elif operation == "-":
    print("The answer is:")
    print(num1 - num2)
elif operation == "*":
    print("The answer is:")
    print(num1 * num2)
elif operation == "/":
    if num2 != 0:
        print("The answer is:")
        print(num1 / num2)
    else:
        print("Error: Cannot divide by zero!")
else:
    print("Invalid Operation Selected!")

🧠 Key Concepts Covered

Concept Description
input() Get data from the user
int() Convert string to number
if / elif / else Control flow to handle choices
print() Display output
Nested if Used to prevent division by zero

🧪 Sample Test Run

Input:

Enter first number: 8  
Choose operation(+,-,*,/): *  
Enter second number: 5

Output:

The answer is:  
40

📝 Review & Practice Questions

  1. Why do we use int(input(...)) instead of just input()?
  2. What will happen if a user enters / and the second number is 0?
  3. How does the program decide which operation to perform?
  4. What’s the purpose of the final else block?
  5. How would you modify this program to allow the user to do multiple calculations without restarting it?
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