[开发工具]python学习5-判断、循环语句

?# 单行注释

""" """或''' ''' 多行注释

#这是单行注释
"""
这是
多行
注释
"""

\ 连接符，连接两行代码

>>> a = "aaaaa\
... 1111"
>>> a
'aaaaa1111'

if elif else 判断，要在语句末尾加上：

str1 = True
if str1:
    print("yes!")
else:
    print("no")
#结果：yes!

判断语句嵌套

str2 = True
str3 = False
if str2:
    if str3:
        print("aaa")
    else:
        print("bbb")
else:
    if str3:
        print(111)
    else:
        print(2222)
#结果：bbb

判断多个条件

str4 = 1
if str4 == 9:
    print(111)
elif str4 > 3:
    print(222)
elif str4 < 2:
    print(3333)
else:
    print("over")
#结果：3333

比较操作符

?==? !=? ?<? ?<=? ?>? ?>=? in...?

str5 = "apple"
if "a" in str5:
    print(1)
else:
    print(2)
#结果 1

布尔操作符

and、or、and not

（布尔操作符优先级低于比较操作符）

a = 10
if a >= 5 and not a < 9:
    print(1)
else:
    print(0)
if (a == 10 or a == 1) and a ==2:
    print(1)
else:
    print(0)
if 5 < a < 11:
    print(1)
else:
    print(0)
#结果 1 0 1

?被认为False的情况：

False? None? 0? 0.0? ""? []? ()? {}? set()

set1 = set()
if set1:
    print("not empty")
else:
    print("empty")
#结果：empty

while 循环

count = 1
while count < 6:
    print(count)
    count += 1
'''
结果：
1
2
3
4
5
'''

break 跳出循环

while 1:
    str6 = input()
    if str6 == "quit":
        break
    print(str6,"hhh")
'''
a
a hhh
b
b hhh
c
c hhh
quit
'''

continue 跳到循环开始

while 1:
    print("please input")
    str6 = input()
    if str6 == "quit":
        break
    if str6 == "continue":
        continue
    print(str6,"hhh")
'''
please input
1
1 hhh
please input
continue
please input
2
2 hhh
please input
quit
'''

循环外使用else（适用于while循环正常结束时）

while count < 1:
    print(count)
    count += 1
else:
    print("while over")
#while over

for 循环

break 跳出循环，continue 跳到循环开始，循环外使用else

str7 = "abcdef"
for strs in str7:
    print(strs)
list1 = [1,2,3]
for list in list1:
    print(list)
tuple1 = ("aa","bb","c cc")
for tuple in tuple1:
    print(tuple)
dict1 = {1:"aaaa",2:"bbbb"}
for keys in dict1.keys():
    print(keys)
for values in dict1.values():
    print(values)
for items in dict1.items():
    print(items)
'''
a
b
c
d
e
f
1
2
3
aa
bb
c cc
1
2
aaaa
bbbb
(1, 'aaaa')
(2, 'bbbb')
'''

zip() 并行迭代

在最短序列循环结束时就会停止

list1 = [1,2,3,4]
list2 = ["a","b","c"]
for l1,l2 in zip(list1,list2):
    print(l1,l2)
'''
1 a
2 b
3 c
'''

可将zip()函数返回值变成列表、元组、字典、集合

list1 = [1,2,3,4]
list2 = ["a","b","c"]
print(list(zip(list1,list2)))
print(tuple(zip(list1,list2)))
print(dict(zip(list1,list2)))
print(set(zip(list1,list2)))
'''
[(1, 'a'), (2, 'b'), (3, 'c')]
((1, 'a'), (2, 'b'), (3, 'c'))
{1: 'a', 2: 'b', 3: 'c'}
{(3, 'c'), (1, 'a'), (2, 'b')}

'''

range() 生成自然数序列。

用法类似切片：

range(start,end,step)，start默认为0，step默认为1，产生的最后一个数值是end的前一个；

正向创建自然数序列时，start＜end，step为正整数；

反向创建自然数序列时，start＞end，step为负整数；

for i in range(0,10,2):
    print(i)
print("------")
for i in range(5,1,-1):
    print(i)
'''
0
2
4
6
8
------
5
4
3
2
'''

可转换为 列表、元组、集合

print(list(range(1,9)))
print(tuple(range(1,9)))
print(set(range(1,9)))
'''
[1, 2, 3, 4, 5, 6, 7, 8]
(1, 2, 3, 4, 5, 6, 7, 8)
{1, 2, 3, 4, 5, 6, 7, 8}
'''