How to count the number of occurrences of letters in Python

This time I will show you how to count the number of occurrences of letters in Python. What are the precautions for counting the number of occurrences of letters in Python? The following is a practical case, let's take a look.

Title:

Count the number of occurrences of each word in a file and list the 5 most frequently occurring words.

Foreword:

This question is widely used in practical application scenarios, such as the statistics of high-level students who have appeared in the CET-4 and CET-6 exams over the years. Frequency vocabulary, I remember that Li Xiaolai used his programming skills to publish a best-selling book on word memorization. He memorized words based on word frequency, which was very popular among students. This is a typical scenario where programming skills are used to solve real problems. In addition, during data analysis, those word cloud effects are essentially based on word frequency statistics to adjust the font size. If you can skillfully use the knowledge in Python to solve problems, it means that you are really getting started with Python.

Analysis

This question mainly examines the following knowledge points:

1. How to read correctly Write files

To read and write files in python, you can use the

built-in functionopen(), and the open function has certain differences between python2 and python3. For example, in Python, you can specify the read The encoding format for writing files, but Python cannot. In order to be compatible with both 2 and 3, we usually use the open function under the io module. You can check the documentation to find out the difference between them, and cultivate the ability to actively learn and look up information. Habit.

Another point is that the file descriptor needs to be closed after reading and writing the file. In addition to using the try...except...finally syntax, we can also use the more elegant with... as syntax. to automatically close the file.

2. How to sort data

The sorted function is a frequently used built-in function, and its usage is also very powerful because it can specify parameters key to perform custom sorting, which means that you can not only sort numbers, letters, but also lists, dictionaries, and custom objects. You only need to tell the sorted function what the sorting rules are, such as For a people object, I can sort it by age or height and weight, so this function is very flexible. In addition, there is a sort method for list objects. If you can clearly distinguish the difference between list.sort and sorted That means you can already use it flexibly.

3. The use of dictionary data type

To do word frequency statistics, using a dictionary is undoubtedly the most appropriate data type, with words as the keys of the dictionary , the number of times a word appears is used as the value of the dictionary, and the frequency of each word is easily recorded. The dictionary is much like our phone book, with each name associated with a phone number. In addition, the biggest feature of the dictionary is that its query speed is very fast. Under ideal circumstances, the time complexity is O(1). I mean ideal circumstances. If you want to learn more about dictionaries, it is recommended to read this article

https://www.laurentluce.com/posts/ python-dictionary-implementation/

4, Use of regular expressions

For text and string processing, regular expressions are simply an artifact. , it is very widely used whether it is doing data crawling or data cleaning. Of course, regular expressions are not unique to Python. They are supported by all

programming languages. What we have to do is not only learn regular expressions but also other things. API, only if we are familiar with the API can we apply it to actual scenarios. I recommend an article about regular expressions: http://www.cnblogs.com/huxi/archive/2010/07/04/1771073.html. In addition, I also found that some students introduced the jieba word segmentation library. This library is doing Chinese word segmentation is very useful. If you are interested, you can learn about it.

Implementation

After the analysis, we can actually implement it very quickly. So when we get a requirement, we must first clarify the requirement and think about what technologies can be used to achieve it, and then start writing code. In fact, at work, we actually spend less than half of the time writing code. .

# -*- coding:utf-8 -*-
import io
import re
class Counter:
 def init(self, path):
 """
 :param path: 文件路径
 """
 self.mapping = dict()
 with io.open(path, encoding="utf-8") as f:
  data = f.read()
  words = [s.lower() for s in re.findall("\w+", data)]
  for word in words:
  self.mapping[word] = self.mapping.get(word, 0) + 1
 def most_common(self, n):
 assert n > 0, "n should be large than 0"
 return sorted(self.mapping.items(), key=lambda item: item[1], reverse=True)[:n]
if name == 'main':
 most_common_5 = Counter("importthis.txt").most_common(5)
 for item in most_common_5:
 print(item)

Print results:

('is', 10)
('better', 8)
('than', 8)
('the', 6)
('to' , 5)

Summary

When I look at your code, many codes still have irregular naming (recommended to read PEP8), the code layout is confusing (it is difficult to read, it is recommended to use Pycharm for formatting). There are also many codes whose implementation methods look very complicated (the more complex the code, the more bugs it usually has). Of course, the implementation method is not the only one.

For example, the Python module itself provides a collections.Counter class, which inherits from the dict class and is used for statistics. I found that some students use this class to implement it. Please be careful. You may have discovered that the Counter I implemented is very similar to the Counter under collections. In fact, this is making wheels. Making wheels can exercise our programming thinking. Of course, if you have ready-made things at work, there is no need to make your own wheels, unless you Have the confidence to do better. You can also think about what you would do if Python did not provide the Counter tool.

In addition, this module also provides an ordered dictionary object OrderedDict, which can save us from manual sorting operations. Finally, I recommend that you study and summarize all the content I mentioned above. If you can persist for 100 days, I believe you will have a good grasp of Python.

I believe you have mastered the method after reading the case in this article. For more exciting information, please pay attention to other related articles on the php Chinese website!

Recommended reading:

How to convert a matrix into a list in Python

##Lists, arrays, and matrices are mutually exclusive in python Conversion method

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