Conditional Statements

temperature = 38

# Simple if
if temperature > 37:
    print("You have a fever.")   # runs — 38 > 37 is True

# if / else
age = 16
if age >= 18:
    print("You can vote.")
else:
    print("Too young to vote.")  # runs — 16 >= 18 is False

# if / elif / else — multiple branches
score = 74
if score >= 90:
    print("Grade: A")
elif score >= 75:
    print("Grade: B")
elif score >= 60:
    print("Grade: C")    # runs — 74 is >= 60 but not >= 75
else:
    print("Grade: F")

age       = 22
has_id    = True
is_banned = False

# and — both must be True
if age >= 18 and has_id:
    print("Entry allowed.")

# or — at least one True
if age < 12 or age > 60:
    print("Discounted ticket.")

# not — invert
if not is_banned:
    print("Account is active.")

# Combined — real login check
username = "admin"
password = "secure123"
attempts = 2

if username == "admin" and password == "secure123" and attempts <= 3:
    print("✅ Login successful!")
elif attempts > 3:
    print("🔒 Account locked — too many attempts.")
else:
    print("❌ Wrong username or password.")

# Chained comparison — very Pythonic
bmi = 22.5
if 18.5 <= bmi < 25.0:
    print("Normal weight range.")

is_registered = True
has_paid      = True
seat_number   = 14

if is_registered:
    print("User found in system.")
    if has_paid:
        print("Payment confirmed.")
        if seat_number <= 50:
            print("🎫 Front section. Enjoy the event!")
        else:
            print("🎫 Back section. Enjoy the event!")
    else:
        print("⚠️  Payment not received. Please pay first.")
else:
    print("❌ User not found. Register first.")

# ─────────────────────────────────────────────
# Better approach for simple nesting: use 'and'
if is_registered and has_paid and seat_number <= 50:
    print("🎫 Front section. Enjoy the event!")

# Checking for empty string (Pythonic way)
username = input("Enter username: ").strip()
if username:              # True if not empty
    print(f"Welcome, {username}!")
else:
    print("Username cannot be empty.")

# Checking for None (use 'is')
result = None
if result is None:
    print("No result yet.")

# Checking non-zero
items_in_cart = 3
if items_in_cart:         # True because 3 != 0
    print(f"{items_in_cart} items in your cart.")

# Common pattern: default value when empty
name = input("Name (or press Enter): ").strip()
display_name = name if name else "Anonymous"
print(f"Hello, {display_name}!")

# Traditional if/else
age = 20
if age >= 18:
    status = "adult"
else:
    status = "minor"

# Same thing as a ternary — one line
status = "adult" if age >= 18 else "minor"
print(status)     # adult

# Ternary inside f-string
score = 72
print(f"Result: {'Pass' if score >= 50 else 'Fail'}")  # Pass

# Ternary with absolute value
n = -7
absolute = n if n >= 0 else -n
print(absolute)   # 7

# Pass/fail label for report card
pf = lambda m: "✅ Pass" if m >= 50 else "❌ Fail"
print(pf(85))    # ✅ Pass
print(pf(32))    # ❌ Fail

# ── Pattern 1: Input validation guard ────────
age_raw = input("Enter age: ")
if not age_raw.isdigit():
    print("Age must be a number.")
else:
    age = int(age_raw)
    print(f"Age: {age}")

# ── Pattern 2: Range classification ──────────
bmi = 26.4
if   bmi < 18.5:  cat = "Underweight"
elif bmi < 25.0:  cat = "Normal"
elif bmi < 30.0:  cat = "Overweight"   # 26.4 lands here
else:              cat = "Obese"
print(f"BMI {bmi}: {cat}")

# ── Pattern 3: Membership check ──────────────
role    = "editor"
allowed = ["admin", "editor", "moderator"]
if role in allowed:
    print("Access granted.")
else:
    print("Access denied.")

# ── Pattern 4: Fizz-Buzz (classic interview Q)
n = 15
if   n % 15 == 0: print("FizzBuzz")  # check divisible by BOTH first
elif n % 3  == 0: print("Fizz")
elif n % 5  == 0: print("Buzz")
else:               print(n)