如何在 Python 中更新字典中键的值？
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How to update the value of a key in a dictionary in Python?
提问by Animeartistfromhell7
I have a dictionary which represents a book shop. The keys represent the book title and values represent the number of copies of the book present. When books are sold from the shop,the number of copies of the book must decrease.
我有一本代表书店的字典。键代表书名，值代表现有书的副本数。从商店出售书籍时，书籍的副本数量必须减少。
I have written a code for decreasing the number of copies of the sold book, but on printing the dictionary after the update, I get the initial dictionary and not the updated one.
我已经编写了一个代码来减少已售书的副本数量，但是在更新后打印字典时，我得到的是初始字典而不是更新的字典。
n=input("Enter number of books in shop:")
book_shop={} #Creating a dictionary book_shop
#Entering elements into the dictionary
for i in range(n):
book_title=raw_input("Enter book title")
book_no=input("Enter no of copies")
book_shop[book_title]=book_no
ch=raw_input("Do you want to sell")
if (ch in 'yesYES'):
for i in range(n):
print"which book you want to sell??",book_shop
ch1=raw_input("choice")
if(book_shop.keys()[i]==ch1):
book_shop.keys()[i]=(book_shop.values()[i]-1)
break
print book_shop
I would like to solve the problem in the simplest way possible. Have I missed any logic or any line in the code?
我想以最简单的方式解决问题。我是否错过了代码中的任何逻辑或任何行？
回答by Christian W.
Well you could directly substract from the value by just referencing the key. Which in my opinion is simpler.
好吧，您可以通过引用键直接从值中减去。在我看来哪个更简单。
>>> books = {}
>>> books['book'] = 3
>>> books['book'] -= 1
>>> books
{'book': 2}
In your case:
在你的情况下：
book_shop[ch1] -= 1
回答by Nikhil Sagotiya
You are modifying the list book_shop.values()[i], which is not getting updated in the dictionary. Whenever you call the values()method, it will give you the values available in dictionary, and here you are not modifying the data of the dictionary.
您正在修改列表book_shop.values()[i]，该列表未在字典中更新。每当您调用该values()方法时，它都会为您提供字典中可用的值，并且在这里您没有修改字典的数据。
回答by vic
n = eval(input('Num books: '))
books = {}
for i in range(n):
titlez = input("Enter Title: ")
copy = eval(input("Num of copies: "))
books[titlez] = copy
prob = input('Sell a book; enter YES or NO: ')
if prob == 'YES' or 'yes':
choice = input('Enter book title: ')
if choice in books:
init_num = books[choice]
init_num -= 1
books[choice] = init_num
print(books)
