如果if语句为true，为什么else语句运行？

Question

This is my code. I am supposed to ask the user what their favorite book is, if their book matches one of mine I tell them, if not I say it isn't one of my favorites. The problem I'm having is when I put a true statement in , my else statement prints. So for example if I input "Jane Eyre" it would print the if statement "We both like Jane Eyre!" And below the else statement "That is not one of my top 5 favorites, but great choice!"

This might be a easy fix and I am just over thinking but I would appreciate some help please!

(The indenting might be off because of copy and paste)

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

You need a break statement to exit the loop early in order to avoid the else clause of the for loop:

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!")

Even if the user inputs The Bible , the break statement still exits the loop "early", in the sense that the loop doesn't know it's on the last iteration until it actually tries to set x to the next (non-existent) value.

That said, you don't actually need a loop, just the in operator:

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

---

As an aside, your second loop would be more idiomatic using enumerate :

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

Answer 2

The else part of your logic is in no way associated with your if , and will execute every time your program is run regardless. I believe you are looking to make a function that will return the matching values. This has the added benefit of reducing the number of iterations performed should a match be found. It would look something like:

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!"

You would then need to call the function that you have made like:

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

Putting it together would look like:

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()

A sample output looks like:

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

Logic : You will want to check if the user's book is in your list and then based on that, you will want to print the relevant message.

Recommendation :

• Not sure if this is what you intended or not, but I would only print the 5 books in bookList if the user's book is different from the list. Code below reflects this.
• Rather than naming your variable bookList, I would say favBooks so it is obvious what it represents.

Code : This is what you're looking for:

#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])

Output :

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

Add break:

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()