1:推导式介绍
python3中推导式提供了一种简明快速的方法来创建列表、字典、集合。当需要对他们中的每项执行一个操作时,可以考虑使用推导来完成。
其标准语法为:
[
expression for target1 in interable1 if condition1
for target2 in interable2 if condition2
......
for targetN in interableN if conditionN]
可以编写任意数量的嵌套的for循环,每个for分句都可以带一个if选择
2:列表推导式
2.1创建列表
# 创建列表:[1, 3, 5, 7, 9] #1:for循环创建lst = []for i in range(1,10): if i % 2 == 1: lst.append(i) print(lst) # 2:创建列表: [1, 3, 5, 7, 9]lst = [i for i in range(1, 10, 2)]print(lst) # 3:带if,创建列表[1, 3, 5, 7, 9]lst = [i for i in range(1,10) if i % 2 == 1]print(lst)
2.2:对列表操作
lst = [2,3,4,5,6]print([x*2 for x in lst])# 结果 [4, 6, 8, 10, 12]# 与下面的结果一样,但是推导更经济,速度更快。ret = []for x in lst: ret.append(x*2)print(ret) print([x+100 for x in lst])# 结果 [102, 103, 104, 105, 106]ret = []for x in lst: ret.append(x+100)print(ret)
2.3:嵌套for
# lst = [x + y for x in [1,2,3,4] for y in [100,200,300,400] ]print(lst)# 结果 [101, 201, 301, 401, 102, 202, 302, 402, 103, 203, 303, 403, 104, 204, 304, 404] # 与下面的结果一样ret= []for x in [1,2,3,4]: for y in [100,200,300,400]: ret.append(x+y)print(ret)# 结果 [101, 201, 301, 401, 102, 202, 302, 402, 103, 203, 303, 403, 104, 204, 304, 404]
2.4:带if
print('---1:--' + '*'*60)# 单个iflst = [x + y for x in [1,2,3,4] if x % 2 == 0 for y in [100,200,300,400] ]print(lst)# 结果 [102, 202, 302, 402, 104, 204, 304, 404] # 与下面的结果一样ret= []for x in [1,2,3,4]: if x % 2 == 0: for y in [100,200,300,400]: ret.append(x+y)print(ret)# 结果 [102, 202, 302, 402, 104, 204, 304, 404] print('---2:--' + '*'*60)# 单个iflst = [x + y for x in [1,2,3,4] for y in [100,200,300,400] if x + y > 300 ]print(lst)# 结果 [301, 401, 302, 402, 303, 403, 304, 404] # 与下面的结果一样ret= []for x in [1,2,3,4]: for y in [100,200,300,400]: if x + y > 300: ret.append(x+y)print(ret)# 结果 [301, 401, 302, 402, 303, 403, 304, 404] print('---3:--' + '*'*60)# 多个iflst = [x + y for x in [1,2,3,4] if x % 2 == 0 for y in [100,200,300,400] if y > 200]print(lst)# 结果 [302, 402, 304, 404] # 与下面的结果一样ret= []for x in [1,2,3,4]: if x % 2 == 0: for y in [100,200,300,400]: if y > 200: ret.append(x+y)print(ret)# 结果 [302, 402, 304, 404] print('---4:--' + '*'*60)
3:字典推导式
# 也是可以有嵌套for,可以有if语句dic = {x:x**2 for x in range(1,10) if x % 2 == 0 }print(dic)# 结果 {2: 4, 4: 16, 6: 36, 8: 64}# 与下面结果一样dic = {}for x in range(1,10): if x % 2 == 0: dic[x] = x**2print(dic)
4:集合推导式
print('*'*60)st = {(x,y) for x in range(1,10) if x % 2 == 0 for y in range(1,10) if y % 3 == 1}print(st)# 结果:{(2, 7), (6, 4), (4, 7), (6, 7), (8, 1), (6, 1), (4, 4), (2, 1), (8, 7), (4, 1), (2, 4), (8, 4)}print(type(st)) # print('*'*60)st = { x + y for x in range(1,5) for y in range(1,5)}print(st)# 结果:{2, 3, 4, 5, 6, 7, 8}print(type(st)) #
5:元组没有推导式
元组没有推导式的,(x for x in iterable)这种形式的是生成器表达式(后面会介绍):
tp = (x for x in range(1,5) )print(tp)print(type(tp)) """结果:"""print('*'*60)# 可以使用tuple来生成tp = tuple(x for x in range(1,5) )print(tp)print(type(tp))"""结果:(1, 2, 3, 4)"""
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