python - 循环列表的两种方式 - 区别

Question

我必须反转列表中长度大于 4 的每个单词。所以我尝试了:

for word in words:
    if len(word) >= 5:
        word = word[::-1]

但它没有用。但是这个:

 for i in range(len(words)):
        if len(words[i]) >= 5:
            words[i] = words[i][::-1]

工作正常。有什么区别？

Answer 1

当您遍历列表时，Python 会创建对您的变量的引用(具有相同的 ID)。但是，这些不能就地编辑。例如检查这个:Can't modify list elements in a loop Python

考虑这个例子，希望它能帮助你:

words = ['abcdef','abc']

for ind,i in enumerate(words):
    print('Loop {}'.format(ind))
    i = i[::-1]
    print('words equal {}'.format(words))
    words[ind] = words[ind][::-1] 
    print('words equal {}'.format(words))    
    print()

返回:

Loop 0
words equal ['abcdef', 'abc']   # <--- after changing i (nothing changed)
words equal ['fedcba', 'abc']   # <--- after changing words[ind]

Loop 1
words equal ['fedcba', 'abc']   # <--- after changing i (nothing changed)
words equal ['fedcba', 'cba']   # <--- after changing words[ind]

---

在你的情况下

最简单的解决方案是使用列表理解。考虑一下:

rWords = [word[::-1] if len(word) >=5 else word for word in words]