Python learning (start and end) Chapter 2 lists, tuples, dictionaries and sets

chapter 2

2.1 Lists and tuples (the other two sequence structures in python besides strings)

• The elements in the list are mutable, and the elements in the tuple are immutable

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2.1.1 List

• Use [] or list() to create a list
  • Allow the same elements
  • The list consists of 1 or more elements
  • Suitable for positioning an element using sequence and position

list1 = []
list1 = ['monday','huang','1','1','铭']
empty_list = list() #创建空列表

• Use list() to convert other data types into lists
  • The string is converted to a list of single letters

  >>>list('cat')
  ['c','a','t']
  

  • Convert tuples to lists

  >>>a_tuple = ('huang','jian','ming')
  >>>list(a_tuple)
  ['huang','jian','ming']
  

• Specify range and use slice to extract elements
  • Slice extraction

  >>>marxes = ['huang','jian','ming']
  >>>marxes[0:2]
  ['huang','jian']
  >>>marxes[::2]
  ['huang','ming']
  >>>marxes[::-2]
  ['ming','huang']
  #巧妙实现列表逆序
  >>>marxes[::-1]
  ['ming','jian','huang']
  

• List operation
  • list.append() adds elements to the end

  a_list.append('a_str')
  

  • list.extend() or += merge list

  list1.extend(list2)
  list1 += list2
  

  • list.insert() inserts an element at the specified position

  #insert()可在列表任意指定位置插入元素，其中若偏移量超过了尾部，则会默认插入到列表最后
  >>>a_list = ['huang','jian','ming']
  >>>a_list.insert(1,'huang')
  ['huang','huang','jian','ming']
  

  • del deletes the element at the specified position

  >>>del a_list[-3]
  ['huang','jian','ming']
  

  • list.remove() removes elements with specified values

  a_list.remove('huang')
  

  • list.pop() Get and delete the element at the specified position

  #pop(0)返回列表头元素并删除，pop()或pop(-1)返回列表尾元素并删除
  >>>a_list.pop(1)
  'ming'
  >>>a_list
  ['jian']
  

  • list.index() Query the position of an element with a specific value

  >>>a_list.index('huang')
  0
  

  • Use in to determine whether the value exists

  >>>'huang' in a_list
  True
  #同一个值可能出现在列表多个位置，但至少出现一次，in就会返回True
  

  • list.count() records the number of occurrences of a specific value

  >>>a_list = ['huang','huang','jian','ming']
  >>>a_list.count('huang')
  2
  

  • Rearrange elements
    • list.sort(reverse=True/False) will sort the original list and change the content of the original list
    • The general function sorted() will return a sorted copy of the list, with the original list content unchanged
    • The numeric elements will be sorted in ascending order from smallest to largest by default, and the string will be sorted in alphabetical order (can be converted by reverse=True/False)
  • list2 = list1.copy()
    • Using = assignment is different from list.copy(), the change after assignment will affect the original list, copy will not

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2.1.2 Tuples (tuple tuples are immutable, once defined, they cannot be added, deleted, modified, checked, etc.)

• Create a tuple with ()

#空元组
>>>empty_tuple = ()
()
>>>one_tuple = 'huang',
('huang',)
>>>marx_tuple = 'huang','jian'
('huang','jian')

• Tuple unpacking

>>>a_tuple = 'huang','jian','ming' >>>a,b,c = a_tuple
>>>a
'huang'
>>>b
'jian'
>>>c
'ming'

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2.1.3 Dictionary (dict)

• Similar to a list, but the order of the elements does not matter, access is through the key of the key-value pair
• The key can be a string and any other immutable type in python (boolean, integer, floating point, tuple, string, etc.)
• Create a dictionary

#空字典
>>>empty_dict = {
    
    }
>>>a_dict = {
    
    'name':'hjm','age':24}
#用dict()将包含双值子序列的序列转换成字典
>>>lol = [['name','hjm'],['age','24']]
>>>dict(lol)
{
    
    'name':'hjm','age':'24'}
#包含双值元组的列表
>>>lot = [('a','b'),('c','d')]
>>>dict(lot)
{
    
    'a':'b','c':'d'}
#包含双值列表的元组
>>>tol = (['name','hjm'],['age','24'])
>>>dict(tol)
{
    
    'name':'hjm','age':'24'}
#双字符的字符串组成的列表
>>>los = ['ab','cd','ef']
>>>dict(los)
{
    
    'a':'b','c':'d','e':'f'}

• Use [key] to add or modify elements

#若字典中没有对应的key，则向字典中添加该键值对
>>>a_dict = {
    
    'name':'hjm'}
>>>a_dict['age'] = '24'
{
    
    'name':'hjm','age':'24'}

• Use dict1.update(dict2) to merge dictionaries

>>>pythons = {
    
    
    'name1':'h1',
    'name2':'h2',
    'name3':'h3'
}
>>>others = {
    
    
    'age1':'m1',
    'age2':'m2'
    'name1':'m1'
}
>>>pythons.update(others)
>>>pythons
# 注意此处相同的键，对应的值会覆盖掉pyhtons中对应的值
{
    
    'name1':'m1','name2':'h2','name3':'h3','age1':'m1','age2':'m2'}

• del deletes the element with the specified key

>>>del pythons['age2']
{
    
    'name1':'m1','name2':'h2','name3':'h3','age1':'m1'}

• dict.clear() clear all elements
• Use in to determine whether a key exists in a dictionary

>>>'name' in a_dict  
True/False

• Use dict.keys() to get all keys

>>>customers = {
    
    'h':12,'j':23,'m':45}
>>>customers.keys()
dict_keys(['h','j','m'])
>>>list(customers.keys())  
['h','j','m']

• Use dict.values() to get all values

>>>customers.values()
dict_values([12, 23, 45])

• Use dict.items() to get all key-value pairs

>>>customers.items()  
dict_items([('h', 12), ('j', 23), ('m', 45)])

• Also use = assignment, modification will affect the original dictionary, using dict.copy() will not affect the original dictionary

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2.1.4 Collection

• Use set() to create a collection

#创建空集
>>>empty_set = set()  
set() #空集，{}表示空字典
>>>a_set = {
    
    0,2,4,6,8}
{
    
    0,4,2,6,8}
#与字典的键一样，集合是无序的

• Use set() to convert other types to sets

#重复的值将被丢弃

#包含重复字母的字符串
>>>set('letters')
{
    
    'l','e','t','r','s'}
#转换列表为集合  
>>>set(['huang','jian','ming'])  
{
    
    'huang','jian','ming'}  
#元组转换为集合
>>>set(('huang','jian','ming'))  
{
    
    'huang','jian','ming'}  
#当字典作为参数进行转换时，只有键会被使用
>>>set({
    
    'name':'hjm','sex':'man','stature':174})
{
    
    'name','sex','stature'}

• Use in to test whether the value exists

>>>drinks = {
    
    
    'martini':{
    
    'vodka','vermouth'},
    'black russian':{
    
    'vodka','kahlua'},
    'white russian':{
    
    'cream','kahlua','vodka'},
    'manhattan':{
    
    'rye','vermouth','bitters'},
    'screwdriver':{
    
    'orange juice','vodka'}
}
#分别提取字典中的键和值
>>>for name,contents in drinks.items():
    if 'vodka' in contents:
        print(name)

screwdriver
martini
black russian
white russian

>>>for name,content in drinks.items():
    if 'vodka' in content and not ('vermouth' in content or 'cream' in content):
        print(name)

screwdriver
black russian

• Common set operations
  • Use & or set1.intersection(set2) to get the intersection of sets
  • Use | or set1.union(set2) to get the union of the set (at least one element in the set appears)
  • Use-or set1.difference(set2) to get the difference of two sets (appears in the first set but not in the second set)
• Very use of set set operations
  • Use ^ or set1.symmetric_difference(set2) to get the exclusive OR set of two sets (only once in two sets)
  • Use <= or set1.issubset(set2) to determine whether a set is a subset of another set (all elements of the first set appear in the second set)
  • Use >= or issuperset() to determine whether it is a superset
  • Use <or> to judge the true subset

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