[Python] basic data type - list

1. Definition

A list is an ordered collection of objects, surrounded by square brackets [ ], and separated by commas

Lists are dynamic , elements can be added or removed at any time

Lists are heterogeneous and can contain data of different types

Duplicate elements are allowed in the list

Two, create

• Created by square brackets
• Created by the constructor list()
• Created via list comprehension

# 中括号创建
li1 = []
print(li1)            # 输出：[]
li1 = [1, "2", True]
print(li1)            # 输出：[1, "2", True]

# 构造方法list()创建
li2 = list()
print(li2)            # 输出：[]
li2 = list("good")    # 字符串、元组、列表、集合、字典等均可
print(li2)            # 输出：['g', 'o', 'o', 'd']

# 列表推导式创建
li3 = [i for i in range(0,4)]
print(li3)            # 输出：[0, 1, 2, 3]

3. List use

1. Index

Because the list is ordered, you can get the value of the corresponding position in the list through the index.

• Forward indexing, numbering starts from 0.
• Reverse index, numbering starts from -1.

Writing method: list[index]

li = [1, 2, 3, 4, 5]
print(li1[0])        # 输出：1
print(li1[-1])       # 输出：5

2. Slicing

Based on the index, instead of just taking a single value, you can take a section of the list.

Writing method: list[start: stop: step]

• start: Indicates the start index value, the default is 0
• stop: Indicates the end index value (not including the end index value itself), and takes the maximum index value by default.
• step: step value, the default is 1.

Note: Slicing follows the principle of closing first and opening later. The strat value starts from the start value itself, and stop takes the previous value of stop.

li = list("good boy")
print(li[0:-1:1])    # 输出：['g', 'o', 'o', 'd', ' ', 'b', 'o']
print(li[2:4])       # 输出：['o', 'd']，省略step    
print(li[:4])        # 输出：['g', 'o', 'o', 'd']，省略start、step
print(li[2:])        # 输出：['o', 'd', ' ', 'b', 'o', 'y']，省略stop、step
print(li[::-2])      # 输出：['y', 'b', 'd', 'o']，省略start、stop，逆序、步进2打印。

3. Operator

Repeat: use the * operator to repeat the production list elements

Merge: Use the + operator to combine two lists into one

li1 = [1, 'a'] * 5
print(li1)            # 输出：[1, 'a', 1, 'a', 1, 'a', 1, 'a', 1, 'a']

li2 = ["a", "b"]
li3 = list([1, 2, 3])
print(li3 + li2)      # 输出：[1, 2, 3, 'a', 'b']

4. Member detection

in: Checks if an element is in the list. If it returns True, otherwise it returns False.

not in: Checks whether a list does not contain an element. Return True if not, otherwise return False.

li = [i for i in range(1,4)]
print(1 in li)            # 输出：True
print(100 not in li)      # 输出：True

4. Common methods

1、append()

Add a single object to the end of the list

Writing method: list.append(item)

Input parameter: object item

return: None

li = []
li.append("good")
li.append([1, "a"])
li.append({'a': 1})
print(li.append(1))         # 输出：None
print(li)                   # 输出：['good', [1, 'a'], {'a': 1}, 1]
print(len(li))              # 输出：4

2、extend()

Append all elements of an iterable to the end of the list

Writing: list.extend(iterable)

Input parameter: iterable object iterable

return: None

li = []
li.extend("good")
li.extend([1, "a"])
print(li.extend({'a': 1}))    # 输出：None
print(li)                     # 输出：['g', 'o', 'o', 'd', 1, 'a', 'a']
print(len(li))                # 输出：7

3、insert()

Insert an object at the specified index position

Writing method: list.insert(index, item)

Input parameters: index value index, object item

return: None

The original index position and the following elements are shifted one bit backward

li = [1, 2, 3]
print(li.insert(0, "good"))     # 输出：None
print(li)                       # 输出：['good', 1, 2, 3]
li.insert(3, ["a", "b"])
print(li)                       # 输出：['good', 1, 2, ['a', 'b'], 3]

4、pop()

removes and returns the element at the specified index

Writing: list.pop(index) or list.pop()

Input parameter: index value index, optional

Returns: the element at the specified index, or the end element if no index is specified

Raises IndexError if the index value is incorrect or the list is already empty

li = [1, 2, 3, 4, 5]
print(li.pop(1))    # 输出：2
print(li)           # 输出：[1, 3, 4, 5]
print(li.pop())     # 输出：5
print(li)           # 输出：[1, 3, 4]
li.pop(3)           # IndexError: pop index out of range

li1 = []
li1.pop()           # IndexError: pop from empty list

5、remove()

Removes the first element in the list equal to item

Writing method: list.remove(item)

Input parameter: specified element item

return: None

If the target element does not exist, ValueError is raised

li = [1, 2, "a", "b"]
print(li.remove(2))     # 输出：None
print(li)               # 输出：[1, 'a', 'b']
li.remove("c")
print(li)               # ValueError: list.remove(x): x not in list

6、sort()

Sort the list in-place, using only < for comparison between items

Writing method: list.sort(key=None, reverse=False)

Input parameter: supports 2 keyword parameters:

• key: Specifies a function with one argument to extract the comparison key from each list element
• reverse: The default value is False for ascending order, True for descending order

return: None

nums = [2, 4, 3, 1, 5]
print(nums.sort())      # 输出：None
print(nums)             # 输出：[1, 2, 3, 4, 5]
nums.sort(reverse=True)
print(nums)             # 输出：[5, 4, 3, 2, 1]

words = ["Python", "Java", "R", "Go"]
words.sort(key=len)
print(words)            # 输出：['R', 'Go', 'Java', 'Python']

7、reverse()

Reverses the order of elements in a list

Writing method: list.reverse()

Entry parameters: none

return: None

The inversion is only for the index value, and the elements are not compared with each other

nums = [8, 1, 5, 85, 77]
print(nums.reverse())   # 输出：None
print(nums)             # 输出：[77, 85, 5, 1, 8]

Five, list nesting

Nested list refers to storing lists inside lists

Common methods for lists all apply to nested lists

li = [['a', 'b', 'c'], [1, 2, 3]]
print(li[1][2])      # 输出：3
li[0].append('d')
print(li)            # 输出：[['a', 'b', 'c', 'd'], [1, 2, 3]]
li[0][1] = 1
print(li)            # 输出：[['a', 1, 'c', 'd'], [1, 2, 3]]

Six, list comprehension

List comprehension refers to the loop to create a list, which is equivalent to a simplified version of the for loop to create a list.

Writing: [x for x in ... if ...]

# for循环创建列表
li1 = []
for i in range(1, 11):
    if i % 2 == 0:
        li1.append(i ** 2)
print(li1)        # 输出：[4, 16, 36, 64, 100]

# 列表推导式创建列表
li2 = [i ** 2 for i in range(1, 11) if i % 2 == 0]
print(li2)        # 输出：[4, 16, 36, 64, 100]
# 无else时，if在for循环之后

# for循环创建列表
li1 = []
for i in range(1, 11):
    if i % 2 == 0:
        li1.append(i ** 2)
    else:
        li1.append(i)
print(li1)        # 输出：[1, 4, 3, 16, 5, 36, 7, 64, 9, 100]

# 列表推导式创建列表
li2 = [i ** 2 if i % 2 == 0 else i for i in range(1, 11)]
print(li2)        # 输出：[1, 4, 3, 16, 5, 36, 7, 64, 9, 100]
# 有else时，if…else在for循环之前