Control Flow

score = 83

if score >= 90:
    grade = "A"
elif score >= 80:
    grade = "B"
elif score >= 70:
    grade = "C"
elif score >= 60:
    grade = "D"
else:
    grade = "F"

print(grade)  # B
 
# Short-circuit: only first True branch runs
x = 15
if x > 10:
    print("Greater than 10")     # prints this
elif x > 5:                      # NOT evaluated (first was True)
    print("Greater than 5")
else:
    print("5 or less")

# match/case: Python 3.10+
point = (5, 0)

match point:
    case (0, 0):
        result = "Origin"
    case (x, 0):
        result = f"On x-axis at {x}"
    case (0, y):
        result = f"On y-axis at {y}"
    case (x, y):
        result = f"Point at ({x}, {y})"

print(result)  # On x-axis at 5
 
# match with guards (if condition)
value = "hi"

match value:
    case int(n) if n < 0:
        classification = "negative int"
    case int(n) if n == 0:
        classification = "zero"
    case int(n):
        classification = "positive int"
    case str(s):
        classification = f"string of length {len(s)}"
    case _:              # wildcard: matches anything
        classification = "other"
 
print(classification)  # string of length 2

# Basic usage
x = 10
result = "positive" if x > 0 else "non-positive"
print(result)   # positive
 
# In assignment
score = 85
grade = "Pass" if score >= 60 else "Fail"
print(grade)    # Pass
 
# Inside a print call argument
print(f"Status: {'Active' if True else 'Inactive'}")   # Status: Active
 
# In f-string
n = 7
print(f"{n} is {'odd' if n % 2 else 'even'}")     # 7 is odd
 
# In list comprehension
data = [-3, -1, 0, 2, 5, -4, 8]
abs_vals = [x if x >= 0 else -x for x in data]
print(abs_vals)   # [3, 1, 0, 2, 5, 4, 8]
 
# Lazy evaluation: unchosen branch not evaluated
safe = 1/0 if False else 99   # no ZeroDivisionError!
print(safe)   # 99
 
# Nested ternary (valid, but avoid for readability)
x = 0
sign = "positive" if x > 0 else ("negative" if x < 0 else "zero")
print(sign)   # zero
 
# PITFALL: or vs ternary for None check
val = 0
print(val or "default")              # "default"  (0 is falsy!)
print(val if val is not None else "default")  # 0  (correct None-check)

# Checking empty containers the Pythonic way
lst = []
if not lst:           # Pythonic: rely on truthiness
    print("empty")
 
# Anti-pattern (verbose)
if len(lst) == 0:     # works but not idiomatic
    print("empty")
 
# PITFALL: "0" is truthy (non-empty string)
s = "0"
if s:
    print("truthy!")  # prints! "0" is NOT the int 0
 
# PITFALL: [0] is truthy (non-empty list)
L = [0]
if L:
    print("truthy!")  # prints! list is non-empty
 
# PITFALL: {False} is truthy (non-empty set)
st = {False}
if st:
    print("truthy!")  # prints! set is non-empty
 
# None vs falsy: use 'is None' to distinguish
a, b = 10, 0
result = a / b if b else None   # None on zero
 
if result is None:          # correct None check
    print("Division by zero")
if not result:              # WRONG: also catches result=0.0
    print("Would also catch 0.0!")

# 1. Without end="" (Default - prints downward)
print("Apple")
print("Banana")
# Output:
# Apple
# Banana

# 2. With end="" (Glues things together side-by-side)
print("Apple", end="")
print("Banana", end="")
# Output: AppleBanana

# 3. Adding a space inside end=" " (Keeps things neat in a row)
for i in range(3):
    print("Hi", end=" ")
# Output: Hi Hi Hi 

# 4. Adding a symbol inside end="-"
for i in range(3):
    print("Go", end="-")
# Output: Go-Go-Go-

# 1. Counting up to 5 (stops before 5)
for i in range(5):
    print(i, end=" ")   # 0 1 2 3 4
 
# 2. Skipping numbers (count by twos)
for i in range(2, 8, 2):
    print(i, end=" ")   # 2 4 6
 
# 3. Rocket countdown!
for i in range(5, 0, -1):
    print(i, end=" ")   # 5 4 3 2 1
 
# 4. Spell a word letter by letter
for letter in "CAT":
    print(letter, end=" ")   # C A T
 
# 5. Looking at keys in a dictionary
toy_box = {"car": "red", "ball": "blue"}
for toy in toy_box:
    print(toy, end=" ")   # car ball
 
# 6. Using enumerate to get numbers
fruits = ["apple", "banana"]
for index, fruit in enumerate(fruits):
    print(f"Item {index} is {fruit}")
# Item 0 is apple
# Item 1 is banana
 
# 7. Using zip to pair friends with their favorite colors
names  = ["Alice", "Bob"]
colors = ["Red", "Blue"]
for name, color in zip(names, colors):
    print(f"{name} likes {color}")
# Alice likes Red
# Bob likes Blue
 
# 8. zip stops when the shorter list is empty
numbers = [1, 2, 3, 4, 5]
letters = ["A", "B"]
print(list(zip(numbers, letters)))    # [(1, 'A'), (2, 'B')]
 
# 9. zip_longest keeps going to the end
from itertools import zip_longest
print(list(zip_longest(numbers, letters, fillvalue="Empty")))
# [(1, 'A'), (2, 'B'), (3, 'Empty'), (4, 'Empty'), (5, 'Empty')]

# 1. Adding up numbers (Piggy bank)
coins = [5, 2, 10, 1]
total = 0
for coin in coins:
    total += coin
print(total)   # 18
 
# 2. Loop inside a loop (Making a multiplication grid)
for row in range(1, 3):
    for col in range(1, 3):
        print(f"{row}x{col}={row*col}", end="  ")
    print() # Moves to the next line
# 1x1=1  1x2=2  
# 2x1=2  2x2=4  
 
# 3. Reading dictionary keys and values together
pet_sounds = {"dog": "woof", "cat": "meow"}
for pet, sound in pet_sounds.items():
    print(f"The {pet} goes {sound}")
 
# 4. Making a new list using a loop
numbers = [1, 2, 3]
doubled = []
for x in numbers:
    doubled.append(x * 2)
print(doubled)   # [2, 4, 6]

# 1. Simple countdown
candies = 3
while candies > 0:
    print(f"Eating a candy! {candies} left.")
    candies -= 1  # We eat one, so count goes down
# Eating a candy! 3 left.
# Eating a candy! 2 left.
# Eating a candy! 1 left.
 
# 2. Keep guessing until it's right
secret_number = 7
user_guess = 0  # Start with 0 so the loop begins
 
# Simulating guesses: 3, then 5, then 7
guesses = iter([3, 5, 7])
 
while user_guess != secret_number:
    user_guess = next(guesses)
    print(f"Guessed: {user_guess}")
    if user_guess == secret_number:
        print("You got it!")
    else:
        print("Wrong! Try again.")
 
# 3. THE INFINTIE LOOP PITFALL (What NOT to do!)
# item_count = 5
# while item_count > 0:
#     print("Stuck forever!") 
#     # Uh oh! We forgot to do item_count -= 1, so it never stops!

# 1. break: Stop searching when we find what we want
lucky_numbers = [1, 2, 9, 4, 5]
for num in lucky_numbers:
    if num == 9:
        print("Found the 9! Stopping the search.")
        break
    print(f"Checking: {num}")
# Checking: 1
# Checking: 2
# Found the 9! Stopping the search.
 
# 2. continue: Skip bad items (Skip even numbers)
for num in range(1, 6):
    if num % 2 == 0:
        continue # Skip this number!
    print(num, end=" ") # 1 3 5
print()
 
# 3. pass: Just a placeholder so Python doesn't get confused
for num in range(3):
    if num == 1:
        pass # I will write code here tomorrow!
    else:
        print(num, end=" ") # 0 2

# Look for a monster in the closet
closet = ["shirt", "pants", "socks"]
 
for item in closet:
    if item == "monster":
        print("Ahhh! Found a monster!")
        break
else:
    # This runs only if we checked EVERYTHING and found NO monster
    print("Yay! No monsters found in the closet.")
 
# Output: Yay! No monsters found in the closet.