Data collection: list, tuple, dict, set

Python has four types of data collections, namely list, tuple, dict, set

List, List

List is an ordered and variable data collection in Python. Its elements can be added or removed. The representation method of List is to use a [] to enclose the elements, and separate the elements with a, sign. For example [2,'hah',True].

Create a List

list = [1,2,3,'apple',true]
List中的元素的数据类型可以不同，除了整数、浮点数、布尔值、字符串等，也可以是list或则其他。

The length of the List

You can use the len() function to get the length of the list.

Get elements of List

You can use list[index] to get an element in the list from front to back.
You can also use list[-n] to get the nth element from the back to the front in the list.

Append elements append()

For example, list.append('haha') can add an element after the list.

Insert element insert()

For example, list.insert(2, "haha") adds an element to position 3 of the list.

Delete elements pop()

list.pop() deletes the last element of the list by default. list.pop(i) deletes the i+1th element.

Replace elements in the list

list[2]='banana'

Tuple, Tuple

Tuple is an ordered but immutable list in Python. Once a Tuple is created, it cannot be modified. The representation method is to use a pair of () to contain the elements and separate them with,.
For example: (1,2,3). But for a tuple that only uses one element, you need to add one after the element, such as (1,) to distinguish it from the operator ().

The acquisition of Tuple elements

is consistent with list, that is, tuple[index].

Dict Dictionary

The dictionary in Python is a data format stored in key-value format. The key in Dict is the only immutable object.

Dict creation method

my_dict ＝ {'name':'Charlie','age':20,'gender':'male'}

Get value based on key

my_dict['name']

But sometimes we are not sure whether the key we want is in the dict. If not, but we obtain the value according to the above method, KeyError will be reported.
We have two ways to solve it

Use in to determine whether the key exists. key in dict

my_dict.get('name'). If the key does not exist, None is returned. You can also know the return value when the key does not exist, that is, my_dict.get('name','value_if_not_existed')

Delete key-value

my_dict.pop('name')

Dict与List相比，Dict查询、插入的速度快，与Dict大小无关。占用内存大。List查询、插入的速度与List大小呈反比，但是占用内存小。

Set

Set是一个有序且不重复的数据集合。Set中的元素都必须是不可变对象。

创建set

s = set([1,2,3,5,4,3])

创建时重复的元素将被自动删除。

添加元素

s.add('9')

删除元素

s.remove('9')

若元素'9'不存在，则会报KeyError错误。