列表理解中的双重 for 语句

Question

wordlist = ['cat','dog','rabbit']
letterlist = [ ]

要列出所有单词中的所有字符，我们可以这样做：

letterlist = [word[i] for word in wordlist for i in range(len(word))]

['c', 'a', 't', 'd', 'o', 'g', 'r', 'a', 'b', 'b', 'i', 't']

但是，当我尝试以这种方式进行时：

letterlist = [character for character in word for word in wordlist]

我得到错误：

NameError: name 'word' is not defined on line 9

有人可以解释我在理解列表理解如何工作方面的错误吗？

谢谢。

Answer 1

写作

wordlist   = ["cat", "dog", "rabbit"]
letterlist = [character for character in word for word in wordlist]

相当于下面的嵌套循环：

wordlist   = ["cat", "dog", "rabbit"]
letterlist = []

for character in word:

    for word in wordlist:

        letterlist.append(character)

此循环将引发与列表理解相同的错误，因为您在将 word 定义为 wordlist 的元素之前尝试引用 character in word。你只是把顺序倒过来。请尝试以下操作：

letterlist = [character for word in wordlist for character in word]