🌱

Basic

Arithmetic · Comparison · Logical · f-strings · input()

Week 3 — Operators & I/O

Master Python's arithmetic, comparison, and logical operators, format output with f-strings, and build interactive programs with input().

operatorsf-stringinputformat

Duration

⏱ 2 hours

Level

📊 Beginner

Prerequisite

🎯 Week 2

OUTCOME

Build a simple calculator that reads two numbers and prints results

What you'll learn

1Use all arithmetic operators including // and %
2Compare values with ==, !=, <, >, <=, >=
3Combine conditions with and, or, not
4Format output with f-strings
5Chain augmented assignment operators (+=, -=, *=)

1. Arithmetic Operators

Operator	Name	Example	Result
+	Add	5 + 3	8
-	Subtract	5 - 3	2
*	Multiply	5 * 3	15
/	Divide	7 / 2	3.5
//	Floor divide	7 // 2	3
%	Modulo	7 % 2	1
**	Exponent	2 ** 10	1024

💡

// always returns int (floors toward negative infinity). % returns the remainder — handy for even/odd checks: n % 2 == 0.

2. Comparison & Logical Operators

python

print(5 > 3)        # True
print(5 == 5)       # True
print(5 != 3)       # True
print(True and False)  # False
print(True or False)   # True
print(not True)        # False

3. f-strings

f-strings (f"...") are the modern way to embed expressions inside string literals.

python

name = "Alice"
age = 20
pi = 3.14159
print(f"Hello, {name}!")
print(f"Age: {age}")
print(f"Pi ≈ {pi:.2f}")   # 2 decimal places

4. Augmented Assignment

python

x = 10
x += 5   # x = x + 5 = 15
x -= 3   # x = 12
x *= 2   # x = 24
print(x) # 24

5. Common Mistakes

= vs ==: = assigns, == compares. if x = 5 is a SyntaxError.
7 / 2 → 3.5 (float), not 3. Use 7 // 2 for integer division.
"5" + 5 → TypeError. Convert first: int("5") + 5.

💻 Examples

Run these examples and check the output yourself.

01_arithmetic.py— All arithmetic operators

CODE

a, b = 17, 5
print(f"{a} + {b} = {a+b}")
print(f"{a} - {b} = {a-b}")
print(f"{a} * {b} = {a*b}")
print(f"{a} / {b} = {a/b:.2f}")
print(f"{a} // {b} = {a//b}")
print(f"{a} % {b} = {a%b}")
print(f"{a} ** {b} = {a**b}")

▶ Output

17 + 5 = 22
17 - 5 = 12
17 * 5 = 85
17 / 5 = 3.40
17 // 5 = 3
17 % 5 = 2
17 ** 5 = 1419857

02_fstring.py— f-string formatting

CODE

name = "Alice"
score = 92.567
print(f"Student: {name}")
print(f"Score:   {score:.1f}")
print(f"Passed:  {score >= 60}")

▶ Output

Student: Alice
Score:   92.6
Passed:  True

03_calculator.py— Simple two-number calculator

CODE

a = float(input("First number: "))
b = float(input("Second number: "))
print(f"{a} + {b} = {a+b}")
print(f"{a} - {b} = {a-b}")
print(f"{a} * {b} = {a*b}")
if b != 0:
    print(f"{a} / {b} = {a/b:.4f}")
else:
    print("Division by zero!")

📝 Exercises

Try them yourself first, then open the solution to compare.

Exercise 1

Odd/Even Checker

Goal: Read an integer and print whether it is odd or even.

Requirements

Use input() to read an integer
Use % operator
Print 'even' or 'odd'

Sample I/O

Enter a number: 7
7 is odd

▶Toggle solution

SOLUTION

n = int(input("Enter a number: "))
if n % 2 == 0:
    print(f"{n} is even")
else:
    print(f"{n} is odd")

Exercise 2

BMI Calculator

Goal: Calculate and print BMI from weight and height.

Requirements

Read weight (kg) and height (m) from input
BMI = weight / height**2
Print result with 2 decimal places

Sample I/O

Weight (kg): 65
Height (m): 1.70
BMI: 22.49

▶Toggle solution

SOLUTION

w = float(input("Weight (kg): "))
h = float(input("Height (m): "))
bmi = w / h**2
print(f"BMI: {bmi:.2f}")

Example code / lecture materials

All lecture materials and example code are openly available on GitHub.

View on GitHub ↗