Topic 02: Expressions, Variables & Types

🎯 Learning Objectives

Use arithmetic operators: +, -, *, /, //, %, **
Understand operator precedence (order of operations)
Convert between types using int(), float(), str()
Perform string concatenation and repetition
Use augmented assignment operators: +=, -=, etc.

📚 Key Concepts

Arithmetic Operators

Operator	Meaning	Example	Result
`+`	Addition	`7 + 3`	`10`
`-`	Subtraction	`7 - 3`	`4`
`*`	Multiplication	`7 * 3`	`21`
`/`	Division (float)	`7 / 2`	`3.5`
`//`	Floor division (int)	`7 // 2`	`3`
`%`	Modulo (remainder)	`7 % 2`	`1`
`**`	Exponentiation	`2 ** 8`	`256`

Operator Precedence (PEMDAS)

Highest → Lowest: ** → * / // % → + -

Same-level operators evaluate left to right. Use parentheses () to control order.

2 + 3 * 4      # = 14, NOT 20
(2 + 3) * 4    # = 20
2 ** 3 ** 2    # = 512 (right to left for **)

Integer vs Float Division

/ always returns a float. // returns an integer (floors the result).

10 / 3    # 3.3333... (float)
10 // 3   # 3 (int — drops decimal)
10 % 3    # 1 (remainder after dividing)

Trick: Use % to check if a number is even: n % 2 == 0

Type Conversion

Explicitly convert between types with built-in functions:

int("42")        # → 42
int(3.9)         # → 3 (truncates, does NOT round)
float("3.14")    # → 3.14
float(5)         # → 5.0
str(42)          # → "42"
bool(0)          # → False (0, "", None, [] are falsy)
bool(1)          # → True (everything else is truthy)

Augmented Assignment

x = 10
x += 3   # same as x = x + 3  → 13
x -= 2   # same as x = x - 2  → 11
x *= 4   # same as x = x * 4  → 44
x //= 5  # same as x = x // 5 → 8

String Operations

Strings support concatenation (+) and repetition (*). You cannot add a string and a number directly.

"Hello" + " " + "World"   # "Hello World"
"Ha" * 3                   # "HaHaHa"
"Age: " + str(25)          # "Age: 25"  (must convert number)

# This causes a TypeError:
"Age: " + 25               # ERROR!

Mixed-Type Arithmetic

When you mix int and float in an expression, Python automatically promotes the result to float.

5 + 2.0   # 7.0 (float, not int)
5 * 2.0   # 10.0

💻 Code Examples

Example 1 — Basic arithmetic

a = 17
b = 5
print(a + b)   # 22
print(a - b)   # 12
print(a * b)   # 85
print(a / b)   # 3.4
print(a // b)  # 3
print(a % b)   # 2
print(a ** b)  # 1419857

Example 2 — Temperature converter

celsius    = float(input("Enter Celsius: "))
fahrenheit = celsius * 9 / 5 + 32
print("Fahrenheit:", fahrenheit)

Example 3 — Even/odd check with modulo

n = int(input("Enter a number: "))
remainder = n % 2
print(remainder)   # 0 if even, 1 if odd

⚠️ Exam Focus

Trace expressions manually — know that 7 // 2 = 3 and 7 % 2 = 1.
Know that / gives float, // gives int — often tested.
Type conversion: int(3.9) gives 3, not 4 — it truncates, not rounds.
String + number causes a TypeError — you must use str(n) first.
Know precedence: ** before *// before +/-.

❌ Common Mistakes

Assuming 10 / 2 = 5 (int) — in Python 3 it gives 5.0 (float). Use 10 // 2 for 5.
Confusing int(3.9) = 3 (truncate) with round(3.9) = 4 (round).
Trying "Score: " + score without converting — always use str(score).
Missing parentheses: a + b / c divides first — use (a + b) / c if needed.

⚡ Quick Recap

/ = float division, // = integer division, % = remainder.
Operator precedence: ** → * / // % → + -. Use () to override.
int() truncates (chops decimal), float() adds decimal, str() converts to text.
Strings use + for joining and * for repeating.
int + float → float (automatic type promotion).

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