8. if 文による判断

if condition:
    # condition が True のときに実行するコード
    statement1
    statement2
    # ... さらに文が続く

# temperature_check.py
temperature = 30
 
if temperature > 25:
    print("It's a warm day!")
    print("Consider wearing light clothing.")
 
print("Have a great day!")  # これは常に実行されます

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!")  # これは常に実行されます

Have a great day!

# voting_eligibility.py
age = 16
 
if age >= 18:
    print("You are eligible to vote.")
# 何も表示されません。16 >= 18 は False だからです
 
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:  # 空文字列は偽 (falsy)、空でない文字列は真 (truthy)
    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  # balance から withdrawal を差し引く
    print(f"Withdrawal successful!")
    print(f"Your new balance is: ${balance}")
    print("Thank you for your transaction.")
 
print("Transaction complete.")  # 常に実行されます

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.")  # 常に実行されます

Transaction complete.

if condition:
    # condition が True のときに実行するコード
    statement1
    statement2
else:
    # condition が False のときに実行するコード
    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:
    # condition1 が True のときに実行
    block1
elif condition2:
    # condition1 が False で condition2 が True のときに実行
    block2
elif condition3:
    # condition1 と condition2 が False で、condition3 が True のときに実行
    block3
else:
    # すべての条件が False のときに実行
    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:  # ここが最初に一致してしまいます!
    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:  # もっとも限定的
    grade = "A"
elif score >= 80:
    grade = "B"
elif score >= 70:
    grade = "C"
elif score >= 60:
    grade = "D"
else:  # もっとも一般的
    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 節はなし - どれにも一致しなければ何も起こりません
 
print("Have a great day!")

Have a great day!

if outer_condition:
    # outer_condition が True のときに実行されるコード
    if inner_condition:
        # outer_condition と inner_condition が両方 True のときに実行されるコード
        inner_block
    else:
        # outer_condition は True だが inner_condition が False のときに実行
        inner_else_block
else:
    # outer_condition が False のときに実行
    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.")

# ネストが深すぎて追いにくい例
if condition1:
    if condition2:
        if condition3:
            if condition4:
                if condition5:
                    print("All conditions met!")

# より良い書き方: 'and' を使って条件をまとめる (第 9 章で詳しく学びます)
# 'and' は複数の条件がすべて True かどうかをチェックします
if condition1 and condition2 and condition3 and condition4 and condition5:
    print("All conditions met!")

# 関数の中では、途中で return して抜けることができます
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"

# 範囲チェックのためにネストした if を使う代わりに
if score >= 90:
    grade = "A"
elif score >= 80:
    grade = "B"
elif score >= 70:
    grade = "C"
# ... というように続けます

# INCORRECT - IndentationError
age = 20
 
if age >= 18:
print("You are an adult.")  # インデントされていない!

  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.")  # 正しくインデントされている

# INCORRECT - スペースとタブの混在、またはスペース数の不一致
if temperature > 30:
    print("It's hot!")
      print("Stay hydrated!")  # スペースが多すぎる - インデントが不一致

# CORRECT
if temperature > 30:
    print("It's hot!")
    print("Stay hydrated!")  # 同じインデント幅

# INCORRECT - インデントのレベルが間違っている
if has_account:
    print("Welcome!")
    if password == "correct":
    print("Login successful!")  # もっと深くインデントする必要があります

# CORRECT
if has_account:
    print("Welcome!")
    if password == "correct":
        print("Login successful!")  # ネストされたブロックとして正しくインデントされている

# INCORRECT - SyntaxError: コロンの欠如
if age >= 18
    print("Adult")

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

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

# CORRECT - すべての行にコロンが付いている
if score >= 90:
    grade = "A"
elif score >= 80:
    grade = "B"
else:
    grade = "F"

# INCORRECT - これは age に 18 を代入しており、比較していません!
age = 20
 
if age = 18:  # SyntaxError
    print("You are 18")

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

# INCORRECT - この elif は決して実行されない
score = 85
 
if score >= 60:
    print("You passed!")
elif score >= 80:  # ここは決して実行されません!
    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:
    # 後でここにコードを追加する予定
 
print("Done")

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

# CORRECT - プレースホルダーとして pass を使用
if temperature > 30:
    pass  # TODO: 後でここにコードを追加する
 
print("Done")

if condition:
    pass  # 今のところは何もしない

# INCORRECT - これはエラーになります
if age >= 18:
    # TODO: 成人向けの処理をここに追加
 
print("Done")

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

# CORRECT - プレースホルダーとして pass を使用
if age >= 18:
    pass  # TODO: 成人向けの処理をここに追加
 
print("Done")

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

# positive_numbers.py
# 正の数だけを処理する
number = int(input("Enter a number: "))
 
if number > 0:
    result = number * 2
    print(f"Double of {number} is {result}")
elif number == 0:
    pass  # 0 は有効だが特に何もする必要がない
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")