[Python] basic data type - collection

1. Definition

A collection is an unordered collection of objects, surrounded by braces { }, and separated by commas

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

Collections are heterogeneous and can contain different types of data

The only element in the set

Two, create

• Created by curly braces
• Created by the constructor set()
• Created via set comprehension

# 大括号创建
st1 = {1, "2", True}
print(st1)            # 输出：{1, "2"} 因为元素唯一，1和True重复

# 构造方法set()创建
st2 = set()
print(st2)            # 输出：set()
st2 = set("good")     # 字符串、元组、列表、集合、字典等均可
print(st2)            # 输出：{'d', 'o', 'g'}

# 集合推导式创建
st3 = {i for i in range(0,4)}
print(st3)            # 输出：{0, 1, 2, 3}

# 注意：不要单独使用{}创建空集合
st4 = {}
print(st4)            # 输出：{}
print(type(st4))      # 输出：<class 'dict'>

3. Collective use

member detection

in: Checks whether an element is in the set. If it returns True, otherwise it returns False.

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

st = {i for i in range(1,4)}
print(1 in st)            # 输出：True
print(100 not in st)      # 输出：True

4. Common methods

1、add()

Add a single object to a collection

Writing method: set.add(item)

Input parameter: object item

return: None

st = {0, 1, 2, 3}
st.add(False)
st.add('good')
print(st.add(99))    # 输出：None
print(st)            # 输出：{0, 1, 2, 3, 99, 'good'}

2、update()

Adds all elements of an iterable to the collection

Writing: set.update(iterable)

Input parameter: iterable object iterable

return: None

st = {1}
st.update(('a', 'b', 'c'))
print(st.update("good"))    # 输出：None
print(st)                   # 输出：{1, 'a', 'b', 'o', 'c', 'd', 'g'}

3、remove()

Remove the specified element item from the collection

Writing: set.remove(item)

Input parameter: specified element item

return: None

The target element must already exist, otherwise a KeyError will be reported

st = {1, 2, 3}
print(st.remove(2))     # 输出：None
print(st)               # 输出：{1, 3}
st.remove(99)
print(st)               # KeyError: 99

4、discard()

Remove the specified element item from the collection

Writing: set.discard(item)

Input parameter: specified element item

return: None

No error will be reported if the target element does not exist

st = {1, 2, 3}
print(st.discard(2))    # 输出：None          
print(st)               # 输出：{1, 3}
st.discard(99)
print(st)               # 输出：{1, 3}

5、pop()

Randomly removes an element from a collection

Writing method: set.pop()

Entry parameters: none

Returns: the removed element

Raises a KeyError if the collection is empty

st = {1, 2}
st.pop()
print(st)            # 输出：{2} 或者 {1}
print(st.pop())      # 输出：被移除的 2 或者 1
print(st)            # 输出：set()
st.pop()             # KeyError: 'pop from an empty set'

6、clear()

Empty the collection, removing all elements

Entry parameters: none

return: None

st = {1, 2, 3, 'a', 'b'}
print(st.clear())    # 输出：None
print(st)            # 输出：set()

5. Set operations

1. Intersection operation

Method: intersection()

Operator: &

Writing: set1.intersection(set2), set1 & set2

st1 = {1, 3, 2}
st2 = {5, 1, 4}
print(st1.intersection(st2))    # 输出：{1}
print(st1 & st2)                # 输出：{1}

2. Union operation

Method: union()

Operator: |

Writing: set1.union(set2), set1 | set2

st1 = {1, 3, 2}
st2 = {5, 1, 4}
print(st1.union(st2))           # 输出：{1, 2, 3, 4, 5}
print(st1 | st2)                # 输出：{1, 2, 3, 4, 5}

3. Difference operation

Method: difference()

Operator: -

Writing: set1.difference(set2), set1 - set2

st1 = {1, 3, 2}
st2 = {5, 1, 4}
print(st1.difference(st2))      # 输出：{2, 3}
print(st1 - st2)                # 输出：{2, 3}

Six, collection derivation 

Set comprehension refers to creating collections in a loop, similar to list comprehension

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

# for循环创建集合
st1 = set()
for i in 'hello world':
    if i in 'good':
        st1.add(i)
print(st1)            # 输出：{'d', 'o'}

# 集合推导式创建集合
st2 = {i for i in 'hello world' if i in 'good'}
print(st2)            # 输出：{'d', 'o'}