02: Operații și expresii

Work with numbers and text — arithmetic operations, f-strings, and type conversions.

python operators arithmetic f-strings conversions

Lesson 02 · Operations and expressions

What you'll learn

  • Arithmetic operators and the order of operations
  • String operations
  • F-strings — the modern way to format text
  • Conversion between types: int(), float(), str()

Arithmetic operators

Operator Operation Example Result
+ addition 3 + 4 7
- subtraction 10 - 3 7
* multiplication 3 * 4 12
/ division (float result) 7 / 2 3.5
// integer division 7 // 2 3
% remainder (modulo) 7 % 2 1
** exponentiation 2 ** 8 256 
print(10 + 3)   # 13
print(10 - 3)   # 7
print(10 * 3)   # 30
print(10 / 3)   # 3.3333...
print(10 // 3)  # 3
print(10 % 3)   # 1
print(2 ** 10)  # 1024

Order of operations

Python follows standard math rules: ** → * / // % → + -

print(2 + 3 * 4)    # 14 (not 20!)
print((2 + 3) * 4)  # 20 — parentheses change the order

The % operator — what's it for?

% returns the remainder of division. Useful for checking parity:

print(10 % 2)   # 0 — even
print(11 % 2)   # 1 — odd
print(15 % 5)   # 0 — divisible by 5

String operations

Concatenation with +

first_name = "Anna"
last_name = "Pop"
print(first_name + " " + last_name)   # Anna Pop

Repetition with *

print("Ha" * 3)    # HaHaHa
print("-" * 30)    # ------------------------------

Useful functions

message = "  Welcome!  "
print(len(message))           # 12
print(message.upper())        # "  WELCOME!  "
print(message.lower())        # "  welcome!  "
print(message.strip())        # "Welcome!"
print(message.strip().upper()) # "WELCOME!"

F-strings

F-strings let you insert variables directly into text:

name = "Michael"
age = 14
print(f"My name is {name} and I am {age} years old.")
# My name is Michael and I am 14 years old.

Number formatting

pi = 3.14159265
print(f"Pi ≈ {pi:.2f}")   # Pi ≈ 3.14
print(f"Pi ≈ {pi:.4f}")   # Pi ≈ 3.1416

Calculations inside f-strings

width = 4
length = 7
print(f"Rectangle area: {width * length} m²")
# Rectangle area: 28 m²

Type conversions

Function What it does
int(x) converts to whole number
float(x) converts to decimal number
str(x) converts to text 
print(int(3.9))      # 3  (truncates decimals, doesn't round!)
print(int("42"))     # 42
print(float(5))      # 5.0
print(float("3.14")) # 3.14
print(str(100))      # "100"

Invalid conversions

int("hello")    # ValueError
int("3.14")     # ValueError — use int(float("3.14"))

Exercises

Exercise 1 — Calculate

Without running the code, calculate:

print(17 % 5)
print(2 ** 3 + 1)
print(15 // 4)
Answer
  • 17 % 5 → 2
  • 2 ** 3 + 1 → 9
  • 15 // 4 → 3

Exercise 2 — F-string

product = "notebook"
price = 3.5
quantity = 4
Display: 4 notebooks cost 14.0 lei.

Solution 
print(f"{quantity} {product}s cost {price * quantity} lei.")

Exercise 3 — Even or odd?

Display the remainder of dividing 47 by 2.

Solution 
print(47 % 2)   # 1 — so it's odd

Mini-project: Temperature converter

Convert 100 °C to Fahrenheit and Kelvin.

Formulas: F = C × 9/5 + 32 and K = C + 273.15

Output: 

Temperature: 100 °C
In Fahrenheit: 212.0 °F
In Kelvin: 373.15 K

Solution 
celsius = 100
fahrenheit = celsius * 9 / 5 + 32
kelvin = celsius + 273.15
print(f"Temperature: {celsius} °C")
print(f"In Fahrenheit: {fahrenheit} °F")
print(f"In Kelvin: {kelvin} K")

Summary

  • Arithmetic operators: +, -, *, /, //, %, **
  • % returns the remainder — useful for parity
  • F-strings f"text {variable}" — modern formatting
  • Conversions: int(), float(), str()

Next step: → Lesson 03: Input and Output

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