8. if문으로 의사결정하기

if condition:
    # condition이 True일 때 실행할 코드
    statement1
    statement2
    # ... 더 많은 문장들

# temperature_check.py
temperature = 30
 
if temperature > 25:
    print("It's a warm day!")
    print("Consider wearing light clothing.")
 
print("Have a great day!")  # 이 줄은 항상 실행됩니다

It's a warm day!
Consider wearing light clothing.
Have a great day!

# temperature_check_cool.py
temperature = 18
 
if temperature > 25:
    print("It's a warm day!")
    print("Consider wearing light clothing.")
 
print("Have a great day!")  # 이 줄은 항상 실행됩니다

Have a great day!

# voting_eligibility.py
age = 16
 
if age >= 18:
    print("You are eligible to vote.")
# 16 >= 18은 False이므로 아무것도 출력되지 않습니다
 
score = 85
 
if score >= 60:
    print("You passed the exam!")  # Output: You passed the exam!

# user_access.py
username = "alice"
banned_users = ["bob", "charlie", "dave"]
 
if username not in banned_users:
    print(f"Welcome, {username}!")  # Output: Welcome, alice!
 
favorite_color = "blue"
 
if "u" in favorite_color:
    print("Your favorite color contains the letter 'u'.")  # Output: Your favorite color contains the letter 'u'.

# weather_check.py
is_raining = True
 
if is_raining:
    print("Don't forget your umbrella!")  # Output: Don't forget your umbrella!

# name_input.py
user_input = input("Enter your name: ")
 
if user_input:  # 빈 문자열은 falsy이고, 비어 있지 않으면 truthy입니다
    print(f"Hello, {user_input}!")
else:
    print("You didn't enter a name.")

# bank_withdrawal.py
balance = 1500
withdrawal = 200
 
if withdrawal <= balance:
    balance = balance - withdrawal  # balance에서 withdrawal을 뺍니다
    print(f"Withdrawal successful!")
    print(f"Your new balance is: ${balance}")
    print("Thank you for your transaction.")
 
print("Transaction complete.")  # 항상 실행됩니다

Withdrawal successful!
Your new balance is: $1300
Thank you for your transaction.
Transaction complete.

# bank_withdrawal_insufficient.py
balance = 1500
withdrawal = 2000
 
if withdrawal <= balance:
    balance = balance - withdrawal
    print(f"Withdrawal successful!")
    print(f"Your new balance is: ${balance}")
    print("Thank you for your transaction.")
 
print("Transaction complete.")  # 항상 실행됩니다

Transaction complete.

if condition:
    # condition이 True일 때 실행할 코드
    statement1
    statement2
else:
    # condition이 False일 때 실행할 코드
    statement3
    statement4

# age_verification.py
age = int(input("Enter your age: "))
 
if age >= 18:
    print("You are an adult.")
    print("You can vote in elections.")
else:
    print("You are a minor.")
    print("You cannot vote yet.")
 
print("Thank you for providing your age.")

You are an adult.
You can vote in elections.
Thank you for providing your age.

You are a minor.
You cannot vote yet.
Thank you for providing your age.

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

# password_verification.py
stored_password = "python123"
entered_password = input("Enter password: ")
 
if entered_password == stored_password:
    print("Access granted.")
    print("Welcome to the system!")
else:
    print("Access denied.")
    print("Incorrect password.")

if condition1:
    # condition1이 True이면 실행
    block1
elif condition2:
    # condition1이 False이고 condition2가 True이면 실행
    block2
elif condition3:
    # condition1과 condition2가 False이고 condition3이 True이면 실행
    block3
else:
    # 모든 조건이 False일 때 실행
    block4

# grade_calculator.py
score = int(input("Enter your exam score (0-100): "))
 
if score >= 90:
    grade = "A"
    print("Excellent work!")
elif score >= 80:
    grade = "B"
    print("Good job!")
elif score >= 70:
    grade = "C"
    print("Satisfactory.")
elif score >= 60:
    grade = "D"
    print("You passed, but consider reviewing the material.")
else:
    grade = "F"
    print("Unfortunately, you did not pass.")
 
print(f"Your grade is: {grade}")

Good job!
Your grade is: B

Unfortunately, you did not pass.
Your grade is: F

# INCORRECT ORDER - Don't do this!
score = 85
 
if score >= 60:  # This will match first!
    grade = "D"
elif score >= 70:
    grade = "C"
elif score >= 80:
    grade = "B"
elif score >= 90:
    grade = "A"
else:
    grade = "F"
 
print(f"Your grade is: {grade}")  # Output: Your grade is: D

# CORRECT ORDER
score = 85
 
if score >= 90:  # Most specific
    grade = "A"
elif score >= 80:
    grade = "B"
elif score >= 70:
    grade = "C"
elif score >= 60:
    grade = "D"
else:  # Least specific
    grade = "F"
 
print(f"Your grade is: {grade}")  # Output: Your grade is: B

# weekend_checker.py
day = input("Enter a day of the week: ").lower()
 
if day == "saturday" or day == "sunday":
    print("It's the weekend! Time to relax.")
elif day == "friday":
    print("It's Friday! The weekend is almost here.")
elif day in ["monday", "tuesday", "wednesday", "thursday"]:
    print("It's a weekday. Time to work or study.")
# else 절 없음 - 이들 중 어느 것도 일치하지 않으면 아무 일도 일어나지 않습니다
 
print("Have a great day!")

Have a great day!

if outer_condition:
    # outer_condition이 True일 때 이 코드가 실행됩니다
    if inner_condition:
        # 두 조건이 모두 True일 때 이 코드가 실행됩니다
        inner_block
    else:
        # outer_condition은 True지만 inner_condition이 False일 때 실행됩니다
        inner_else_block
else:
    # outer_condition이 False일 때 실행됩니다
    outer_else_block

# account_login.py
has_account = input("Do you have an account? (yes/no): ").lower() == "yes"
 
if has_account:
    print("Welcome back!")
    password = input("Enter your password: ")
    
    if password == "secret123":
        print("Login successful!")
        print("Access granted to your dashboard.")
    else:
        print("Incorrect password.")
        print("Access denied.")
else:
    print("Please create an account first.")
    print("Visit our registration page.")

Welcome back!
Login successful!
Access granted to your dashboard.

Welcome back!
Incorrect password.
Access denied.

Please create an account first.
Visit our registration page.

# loan_eligibility.py
age = int(input("Enter your age: "))
 
if age >= 18:
    print("Age requirement met.")
    income = float(input("Enter your annual income: $"))
    
    if income >= 30000:
        print("Income requirement met.")
        credit_score = int(input("Enter your credit score: "))
        
        if credit_score >= 650:
            print("Congratulations! You are eligible for a loan.")
            print("Please proceed to the application form.")
        else:
            print("Sorry, your credit score is too low.")
            print("Minimum required: 650")
    else:
        print("Sorry, your income is too low.")
        print("Minimum required: $30,000")
else:
    print("Sorry, you must be 18 or older to apply.")

# 너무 깊게 중첩됨 - 따라가기 어렵습니다
if condition1:
    if condition2:
        if condition3:
            if condition4:
                if condition5:
                    print("All conditions met!")

# 더 나음: 'and'로 조건을 결합하기(9장에서 자세히 설명합니다)
# 지금은 여러 조건이 모두 참일 때만 True가 된다고만 기억하세요
if condition1 and condition2 and condition3 and condition4 and condition5:
    print("All conditions met!")

# 함수에서는 일찍 return할 수 있습니다
def check_eligibility(age, income, credit_score):
    if age < 18:
        return "Too young"
    if income < 30000:
        return "Income too low"
    if credit_score < 650:
        return "Credit score too low"
    return "Eligible"

# 구간을 검사하는 중첩 if 대신
if score >= 90:
    grade = "A"
elif score >= 80:
    grade = "B"
elif score >= 70:
    grade = "C"
# ... 등등

# INCORRECT - IndentationError
age = 20
 
if age >= 18:
print("You are an adult.")  # 들여쓰지 않았습니다!

  File "example.py", line 4
    print("You are an adult.")
    ^
IndentationError: expected an indented block

# CORRECT
age = 20
 
if age >= 18:
    print("You are an adult.")  # 올바르게 들여쓰기됨

# INCORRECT - 공백과 탭을 섞거나, 공백 수가 일관되지 않음
if temperature > 30:
    print("It's hot!")
      print("Stay hydrated!")  # 공백이 더 많음 - 들여쓰기가 일관되지 않음

# CORRECT
if temperature > 30:
    print("It's hot!")
    print("Stay hydrated!")  # 같은 들여쓰기 레벨

# INCORRECT - 들여쓰기 레벨이 잘못됨
if has_account:
    print("Welcome!")
    if password == "correct":
    print("Login successful!")  # 더 들여써야 합니다

# CORRECT
if has_account:
    print("Welcome!")
    if password == "correct":
        print("Login successful!")  # 중첩 블록에 맞게 올바르게 들여쓰기됨

# INCORRECT - SyntaxError: missing colon
if age >= 18
    print("Adult")

  File "example.py", line 2
    if age >= 18
                ^
SyntaxError: invalid syntax

# CORRECT
if age >= 18:
    print("Adult")

# CORRECT - 모든 콜론이 포함됨
if score >= 90:
    grade = "A"
elif score >= 80:
    grade = "B"
else:
    grade = "F"

# INCORRECT - age에 18을 대입할 뿐, 비교하지 않습니다!
age = 20
 
if age = 18:  # SyntaxError
    print("You are 18")

# CORRECT
age = 20
 
if age == 18:
    print("You are 18")

# INCORRECT - elif는 절대 실행될 수 없습니다
score = 85
 
if score >= 60:
    print("You passed!")
elif score >= 80:  # 절대 실행되지 않습니다!
    print("You got a B!")

# CORRECT
score = 85
 
if score >= 80:
    print("You got a B!")
elif score >= 60:
    print("You passed!")

# INCORRECT - SyntaxError
if temperature > 30:
    # 나중에 여기 코드를 추가할게요
 
print("Done")

  File "example.py", line 3
    print("Done")
    ^
IndentationError: expected an indented block

# CORRECT - pass를 플레이스홀더로 사용
if temperature > 30:
    pass  # TODO: 나중에 여기 코드를 추가
 
print("Done")

if condition:
    pass  # 지금은 아무 것도 하지 않음

# INCORRECT - 이것은 오류를 발생시킵니다
if age >= 18:
    # TODO: Add adult logic here
 
print("Done")

  File "example.py", line 3
    print("Done")
    ^
IndentationError: expected an indented block

# CORRECT - pass를 플레이스홀더로 사용
if age >= 18:
    pass  # TODO: Add adult logic here
 
print("Done")

# user_system.py
user_type = input("Enter user type (admin/user/guest): ").lower()
 
if user_type == "admin":
    pass  # TODO: Implement admin features
elif user_type == "user":
    pass  # TODO: Implement user features
elif user_type == "guest":
    print("Welcome, guest! Limited access granted.")
else:
    print("Invalid user type.")

# positive_numbers.py
# 양수만 처리합니다
number = int(input("Enter a number: "))
 
if number > 0:
    result = number * 2
    print(f"Double of {number} is {result}")
elif number == 0:
    pass  # 0은 유효하지만 아무 작업도 필요하지 않습니다
else:
    print("Negative numbers are not allowed.")

if temperature > 30:
    print("It's hot outside!")

if age >= 18:
    print("Adult")
else:
    print("Minor")

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

if has_account:
    if password_correct:
        print("Welcome!")
    else:
        print("Wrong password")
else:
    print("Create an account")