如果if語句為true，為什么else語句運行？

Question

這是我的代碼。 我應該問用戶他們最喜歡的書是什么，如果他們的書與我告訴他們的我的其中一本書匹配，否則我說這不是我的最愛之一。 我遇到的問題是當我在中放入true語句時，打印出else語句。 因此，例如，如果我輸入“ Jane Eyre”，它將打印if語句“我們倆都喜歡Jane Eyre！” 在else語句下面，“這不是我的前5個最愛，但是不錯的選擇！”

這可能是一個簡單的解決方法，我只是在考慮一下，但是請您提供一些幫助！

（由於復制和粘貼，縮進可能已關閉）

def main():

    bookList = ["Jane Eyre", "To Kill a Mockingbird", "My Antonia","Pride and Prejudice", "The Bible"]

    book = input("What is your favorite book?")

    for x in range(0,len(bookList)):
        if (book == bookList[x]):
            print("We both like " + book + "!")

    else:
        print("That is not one of my top 5 favorites, but great choice!")

    print("          ")
    print("Here are my top 5 favorite books!")
    print("         ")

    for  n in range(0, len(bookList)):
        print(str(n + 1) + " " + bookList[n])

main()

Answer 1

您需要一個break語句來盡早退出loop ，以避免for循環的else子句：

for x in range(0,len(bookList)):
    if (book == bookList[x]):
        print("We both like " + book + "!")
        break
else:
    # This only executes if break is never encountered, i.e. if the
    # loop simply "runs out".
    print("That is not one of my top 5 favorites, but great choice!")

即使用戶輸入了The Bible ，break語句仍會 “提前”退出循環，因為該循環直到實際嘗試將x設置為下一個（不存在）值時才知道它在最后一次迭代中。

也就是說，您實際上不需要循環，僅需要in運算符：

if book in bookList:
    print("We both like {}!".format(book)
else:
    print("That is not one of my top 5 favorites, but great choice!")

---

順便說一句，使用enumerate可以使您的第二個循環更加慣用：

for n, book in enumerate(book, start=1):
    print("{} {}".format(n, book))

Answer 2

邏輯的else部分與if無關，並且無論何時運行程序都將執行。 我相信您正在尋找一個將return匹配值的函數。 這具有增加的好處，即如果找到匹配項，則減少執行的迭代次數。 它看起來像：

def iterate_books(user_book, book_list):
    for x in range(0, len(book_list)):
        if user_book == book_list[x]:
            return "We both like {}!".format(user_book)
    return "That is not one of my top 5 favorites, but great choice!"

然后，您需要像這樣調用已創建的函數：

book = input("What is your favorite book?")
iterate_books()

放在一起看起來像：

def iterate_books(user_book, book_list):
    for x in range(0, len(book_list)):
        if user_book == book_list[x]:
            return "We both like {}!".format(user_book)
    return "That is not one of my top 5 favorites, but great choice!"

def main():
    bookList = ["Jane Eyre", "To Kill a Mockingbird", "My Antonia", "Pride and Prejudice", "The Bible"]

    book = input("What is your favorite book?\n>>>")
    match = iterate_books(book, bookList)
    print(match)

    print("          ")
    print("Here are my top 5 favorite books!")
    print("         ")

    for n in range(0, len(bookList)):
        print(str(n + 1) + " " + bookList[n])

main()

輸出示例如下：

What is your favorite book?
>>>Jane Eyre
We both like Jane Eyre!

Here are my top 5 favorite books!

1 Jane Eyre
2 To Kill a Mockingbird
3 My Antonia
4 Pride and Prejudice
5 The Bible

Process finished with exit code 0

Answer 3

邏輯 ：您將要檢查用戶的書是否在您的列表中，然后根據該書來打印相關的消息。

建議 ：

• 不知道這是否是您想要的，但是如果用戶的書不同於列表，我只會在bookList中打印5本書。 下面的代碼反映了這一點。
• 我會說favBooks而不是命名變量bookList，因此很明顯它代表什么。

代碼 ：這是您要查找的內容：

#favorite books
bookList = ["Jane Eyre", "To Kill a Mockingbird", "My Antonia","Pride and Prejudice", "The Bible"]

#get book from user
book = input("What is your favorite book? ")

#check if user's book matches book in bookList
favBook = False
for x in range(0,len(bookList)):
    if (book == bookList[x]):
        favBook = True

#display when user's book matches book in bookList
if (favBook == True):
    print("We both like " + book + "!")
#display when user's book does not match a book in bookList
else:
    print("That is not one of my top 5 favorites, but great choice!")
    print("          ")
    print("Here are my top 5 favorite books!")
    print("         ")

    #display bookList since user's book is different from bookList
    for  n in range(0, len(bookList)):
        print(str(n + 1) + " " + bookList[n])

輸出 ：

What is your favorite book? Jane Eyre
We both like Jane Eyre!

What is your favorite book? random book
That is not one of my top 5 favorites, but great choice!

Here are my top 5 favorite books!

1 Jane Eyre
2 To Kill a Mockingbird
3 My Antonia
4 Pride and Prejudice
5 The Bible

Answer 4

添加中斷：

def main():

    bookList = ["Jane Eyre", "To Kill a Mockingbird", "My Antonia","Pride and Prejudice", "The Bible"]

    book = input("What is your favorite book?")

    for x in range(0,len(bookList)):
        if (book == bookList[x]):
            print("We both like " + book + "!")
            break    # <---- here

    else:
        print("That is not one of my top 5 favorites, but great choice!")

    print("          ")
    print("Here are my top 5 favorite books!")
    print("         ")

    for  n in range(0, len(bookList)):
        print(str(n + 1) + " " + bookList[n])

main()