推导式

一、列表推导式

        作用：用一个表达式创建一个有规律的列表或控制规律列表

                列表推导式又叫列表生成式

        1、 for列表推导式

                list3 = [i for i in range(10)]

                print(list3)  #[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

        2、带if的列表推导式

                list4 = [i for i in range(10) if i % 2 ==0]

                print(list4)  # [0, 2, 4, 6, 8]

        3、多个for循环实现的列表推导

                list5 = [(i,j) for i in range(1,3) for j in range(3)]

                print(list5) # [(1, 0), (1, 1), (1, 2), (2, 0), (2, 1), (2, 2)]

二、字典推导式

        作用：快速合并列表为字典或提取字典中的目标数据

        1、创建一个字典：字典的key是1-5，value是key的平方

                dict1 = {i:i**2 for i in range(5)}

                print(dict1)#{0: 0, 1: 1, 2: 4, 3: 9, 4: 16}

        2、将两个列表合并为一个字典

                list1 = ['name','age','gender']

                list2 = ['Tom',10,'男']

                dict2 = {list1[i]: list2[i] for i in range(len(list1))}

                print(dict2)#{'name': 'Tom', 'age': 10, 'gender': '男'}

        3、提取字典中目标数据

                counts = {'MBP':268,'HP':125,'DELL':201,'Lenovo':199,'acger':99}

                count1 = {key :value for key,value in counts.items() if value >= 200}

                print(count1)#{'MBP': 268, 'DELL': 201}

三、集合推导式：注意集合有去重功能

                list1 = [1,1,2]

                set1 = {i**2 for i in list1}#**代表平方

                print(set1) #{1, 4}

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