理解列表理解 [英] Understanding list comprehension

问题描述

我对编程有点新鲜感。我创建了一个在其初始化程序中使用列表理解的类。如下所示：

$ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $'其中存储
字典，其中键是单词，值是它们的出现'''

def __init __（self：'Collection_of_words'，file_name：str） - > None：
'''这个初始化器会读取文件中的单词
，并将它们存储在self.counts中'
l_words = open（file_name）.read（）。 （$）
s_words = set（l_words）

self.counts = dict（[[word，l_words.count（word）]
for word
in s_words] ）

我觉得我对新手来说没问题。有用！但我不完全明白这是如何在for循环中表示的。我的猜测是错的：

  self.counts = [] 
 for s_words中的单词：
 self .counts = [word，l_words.count（word）] 
 dict（self.counts）
  

解决方案

你的猜测根本没有错;你只是忘记追加并分配回 self.counts ：

 计数= [] 
在s_words中的单词：
 counts.append（[word，l_words.count（word）]）
 self.counts = dict（counts）
  

这就是列表理解所要做的事情。

您也可以将其翻译为字典理解：

<$ p $
$ self.counts = {word：l_words.count（word）for word in s_words}


或更好，请使用 collections.Counter（） object 并保存所有这些工作：

  from collections import Counter 
 
 def __init __（self：'Collection_of_words'，file_name：str） - >无：
'''这个初始化程序会读取文件
中的单词，并将它们存储在self.counts的
中，并以open（file_name）作为infile：
 self.counts = Counter（infile.read（）.split（））
  

Counter（）对象可以更有效地计算您的单词，并为您提供其他有用的功能，例如列出最前N个计数和合并计数的能力。

I'm sort of new to programming. I have created a class that uses list comprehension in its initializer. It is as follows:

class Collection_of_word_counts():
 '''this class has one instance variable, called counts which stores a 
dictionary where the keys are words and the values are their occurences'''

def __init__(self:'Collection_of_words', file_name: str) -> None:
    '''  this initializer will read in the words from the file,
    and store them in self.counts'''
    l_words = open(file_name).read().split()
    s_words = set(l_words)

    self.counts = dict([ [word, l_words.count(word)] 
                        for word 
                        in s_words])

I think I did alright for a novice. It works! But I don't exactly understand how this would be represented in a for-loop. My guess was terribly wrong:

self.counts =[] 
for word in s_words:
    self.counts = [word, l_words.count(word)]
dict(self.counts)

解决方案

Your guess was not wrong at all; you just forgot to append and assign back to self.counts:

counts = [] 
for word in s_words:
    counts.append([word, l_words.count(word)])
self.counts = dict(counts)

That's what a list comprehension does, essentially; build a list from the loop expression.

You could also translate that to a dictionary comprehension instead:

self.counts = {word: l_words.count(word) for word in s_words}

or better still, use a collections.Counter() object and save yourself all that work:

from collections import Counter

def __init__(self:'Collection_of_words', file_name: str) -> None:
    '''  this initializer will read in the words from the file,
    and store them in self.counts'''
    with open(file_name) as infile:
        self.counts = Counter(infile.read().split())

The Counter() object goes about counting your words a little more efficiently, and gives you additional helpful functionality such as listing the top N counts and the ability to merge counts.

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