Python Programming

5.1 Strings

• A string is a sequence of characters. The elements of a string are characters. Empty string ''.(not' ') You can access the characters one at a time with the bracket operator.

>>> fruit = 'banana'
>>> fruit[1]
'a'
>>> len(fruit)
6

>>> fruit = 'banana'
>>> fruit[1:3]
'an'
>>> fruit[3:]
'ana'

• Iteration

fruit = 'banana'
for char in fruit:
    print(char)

• Strings are immutable (similar to Tuples)

>>> greeting = 'Hello, world!'
>>> greeting[0] = 'J'

TypeError: 'str' object does not 
support item assignment

>>> greeting = 'Hello, world!'
>>> new_greeting = 'J' + greeting[1:]
>>> print(new_greeting)

Jello, world!

• Comparison operations are useful for putting words in alphabetical order.

>>> print('apple'>'banana')
False
>>> print('ba' > 'banana')
False
>>> a_list = ["orange", "apple", "banana"]
>>> sorted(a_list)
['apple', 'banana', 'orange']

Methods of Strings

Converting Characters: .title(), .lower(), .upper()

• String's methods do not change the original variable but return values.

>>> name = "ada lovelace"
>>> print(name.title())
Ada Lovelace

>>> print(name)
ada lovelace

>>> name = "Ada Lovelace"
>>> print(name.upper())
ADA LOVELACE

>>> print(name.lower())
ada lovelace

Stripping Whitespace: .lstrip(), .rstrip(), .strip()

• To programmers 'python' and 'python ' look pretty much the same. But to a program, they are two different strings. To ensure that no whitespace exists at the right end of a string, use the rstrip() method.
• You can also strip whitespace from the left side of a string using the lstrip() method or strip whitespace from both sides at once using strip().

>>> favorite_language = ' python '
>>> favorite_language.rstrip()
' python'
>>> favorite_language.lstrip()
'python '
>>> favorite_language.strip()
'python'

• However, it is only removed temporarily. To remove the whitespace from the string permanently, you have to store the stripped value back into the variable.

>>> favorite_language = 'python '
>>> favorite_language = favorite_language.rstrip()
>>> favorite_language
'python'

Parsing Strings: .find(), split(), .isalpha(), isspace()

• .find() searches for the position of a string in another string

>>> word = 'banana'
>>> index = word.find('a')
>>> print(index)
1
>>> word.find('na')
2
>>> word.find('na', 3)
4

>>> data = 'From stephen.marquard@uct.ac.za Sat Jan 5 09:14:16 2008'
>>> atpos = data.find('@')
>>> print(atpos)
21
>>> sppos = data.find(' ',atpos)
>>> print(sppos)
31
>>> host = data[atpos+1 : sppos]
>>> print(host)
uct.ac.za

• .split() breaks a sentence into words and make a list

>>> s = 'break a sentence into words'
>>> t = s.split()
>>> print(t)
['break', 'a', 'sentence', 'into', 'words']

• .isalpha() returns True if all characters in the string are alphabetic (A–Z, a–z), otherwise False.

  .isspace() returns True if the string only contains whitespace characters (spaces, tabs, newlines), otherwise False.

>>> print("Hello".isalpha())   # True（全是字母）
>>> print("Hello123".isalpha()) # False（包含数字）
>>> print("你好".isalpha())     # True（支持中文等非拉丁字符）

>>> print("   ".isspace())  # True（全是空格）
>>> print("\t\n".isspace()) # True（制表符和换行符也算空白）
>>> print("Hello".isspace()) # False（包含字母）

Formatted String Literals

>>> number = 42
>>> print('I have spotted number camels.') #error.
>>> print('I have spotted '+str(number)+' camels.') #not simple

>>> number = 42
>>> print(f'I have spotted {number} camels.')
I have spotted 42 camels.

>>> animal = 'camels'
>>> number = 42.12345678
>>> print(f'''I spotted {number:.2f} {animal}.''') 
I spotted 42.12 camels.
>>> print(f'''I spotted {number:.0f} {animal}.''') 
I spotted 42 camels.

>>> print(f'''I spotted {number:.2} {animal}.''') 
I spotted 4.2e+01 camels.
>>> print(f'''I spotted {number:.5} {animal}.''') 
I spotted 42.123 camels.

>>> print(f'''I spotted {number:.2%} {animal}.''') 
I spotted 4212.35% camels.

String: Summary

• The elements of a string are characters. Empty string ''

• Features: Ordered, Repeatable, Immutable

• Index and slice are the same with that of tuples.

• You can compare two strings in Alphabetical order.

• String Methods

  • .upper(), lower(), .title()

  • rstrip(), .lstrip(), .strip()

  • .find(), .split(), .isalpha(), isspace()

• Formatted String Literals

Exercise

写一个程序来操作下面这句话：

My email, wang.lu@mail.shufe.edu.cn, was created on 1st Sept. 2017.

任务要求如下：
1. 使用 find() 方法，提取电子邮件地址中的主机部分（即 mail.shufe.edu.cn）。
2. 使用 find() 和 split() 方法，提取时间信息形成一个列表： ['1st', 'Sept.', '2017']

5.2 Sentiment Analysis (情感分析）

情感分析(简化版)

• 社交媒体分析：检测品牌口碑，舆情监测
• 客户反馈分析：产品评论分析，客服聊天分析
• 电影音乐书籍评鉴分析：推荐算法改进，自动总结评论
• 竞争对手分析：市场调研，行业趋势分析
• 股票市场预测：金融新闻，社交媒体

1. 文本清洗：去除标点符号，转换为小写，分割单词，去除停用词。
2. 情感分析：统计正面和负面词汇的数量，以判断文本的情感倾向。

text = "This movie was absolutely amazing! The story was engaging\
 and the characters were great. However, some scenes felt unnecessary\
 and a bit boring. Overall, I loved it!"

text = text.lower()
# 去除标点（只保留字母和空格）
cleaned_text = ""
for char in text:
    if char.isalpha() or char.isspace():
        cleaned_text += char

words = cleaned_text.split()

# 去除简单的停用词（自己定义）
stop_words = ["the", "was", "and", "a", "it", "i", "some"]
filtered_words = []  # 创建一个空列表来存放保留的单词
for word in words:
    if word not in stop_words:  # 仅当单词不在停用词中时才添加
        filtered_words.append(word)

print("关键词：", filtered_words)

5.3 Chinese Idiom Relay (成语接龙)

• 1. 加载成语词典。
• 2. 给定一个成语，找到可以接上的所有成语。
• 3. 从一个给定的成语开始，一直接下去，到不能接下去为止。

# 1. 加载成语词典
from pathlib import Path
path = Path('idiom_dictionary.txt')
contents = path.read_text()
lines = contents.splitlines()

d_game={}
for line in lines:
    if line!="":
        endpoint=line.find("拼音")
        idiom = line[:endpoint].strip()
        pinyin_start = line.find("：", endpoint)
        pinyin_end =line.find("释义")
        each= line[pinyin_start+1: pinyin_end]
        pinyin_list = each.split()
        d_game[idiom] = pinyin_list
print(len(d_game))

Summary

• Strings
• Reading: Python for Everybody
• Strings Chapter 6