老齐python-基础6(循环 if while for)

Posted 2020-10-06

1、条件语句if

      依据某个条件，满足这个条件后执行下面的内容

>>> if bool(conj):     #语法
    do something
    

>>> a = 8
>>> if a == 8:
    print(a)    
8

    1.1 if...elif...else

       

 基本结构:

if 条件 1:
语句块1
elif 条件2:
语句块2
elif 条件3:
语句块3
...
else
语句块4

       if使用示例程序

#!/usr/bin/env python     #调用系统环境的python
#coding:utf-8             #支持中文 python3可省略

print("清输入任意一个整数数字:")
number = int(input())   #用户交互输入函数

if number == 10:
    print("您输入的数字是:{}".format(number))
    print("You are SMART")
elif number > 10:
    print("您输入的数字是:{}".format(number))
    print("This number is more than 10.")
else:
    print("您输入的数字是:{}".format(number))
    print("This number is less than 10.")

    1.2三元操作符

        三元操作，是条件语句中比较简练的一种赋值方式:

>>> name = "qiwsir" if 29 > 21 else "github"
>>> name
‘qiwsir‘

        如果抽象成为一个公式，三元操作符就是这样的: A = Y if X else Z

               如果X为真，那么就执行 A = Y

               如果X为假，就执行A = Z

>>> x = 2
>>> y = 8
>>> a = "python" if x > y else "qiwsir"
>>> a
‘qiwsir‘
>>> b = "python" if x < y else "tajzhang"
>>> b
‘python‘

 

2、for循环

for 循环规则:
操作语句

    2.1实例:

>>> hello = "world"   #赋值语句
>>> for i in hello:    #必须是可迭代的类型
    print(i)    
w
o
r
l
d

>>> for i in range(len(hello)):  #得到hello引用字符串长度 为5
    print(hello[i])     #对应每个索引输出值，直到最后一个 i=4为止
w
o
r
l
d

    2.2多种序列类型for循环:

        1)循环列表:

>>> names = ["Newton","Einstein","Hertz","Maxwell","Bohr","Cavendish","Feynman"]
>>> for name in names:
    print(name,end="-*-")   #end方法help(print)
Newton-*-Einstein-*-Hertz-*-Maxwell-*-Bohr-*-Cavendish-*-Feynman-*-
>>> for i in range(len(names)):
    print(names[i])
Newton
Einstein
Hertz
Maxwell
Bohr
Cavendish
Feynman
>>> 

     2)循环字典:

>>> d = dict([("website","www.itdiffer.com"),("lang","python"),("author","laoqi")])
>>> d
{‘website‘: ‘www.itdiffer.com‘, ‘lang‘: ‘python‘, ‘author‘: ‘laoqi‘}
>>> for k in d:       #获取key
    print(k)
website
lang
author
>>> d.keys()
dict_keys([‘website‘, ‘lang‘, ‘author‘])
>>> for k in d.keys():   #相同方法
    print(k)
website
lang
author
>>> for k,v in d.items():    #获取键值
    print (k + "-->" + v)
website-->www.itdiffer.com
lang-->python
author-->laoqi

    2.3判断对象是否是可迭代的

>>> import collections
>>> isinstance(321,collections.Iterable)
False
>>> isinstance([1,2,3],collections.Iterable)
True
>>> isinstance({"name":"canglaoshi","work":"php"},collections.Iterable)
True

    2.4 range(start,stop[,step])

        start:开始数值，默认为0，也就是如果不写这项，则认为start=0

        stop:结束的数值，这是必须要写的

        step:变化的步长，默认是1，坚决不能为0

>>> range(0,9,2)
range(0, 9, 2)
>>> type(range(0,9,2))   #是一个Range类型的对象
<class ‘range‘>
>>> list(range(0,9,2))   
[0, 2, 4, 6, 8

        如果是从0开始，步长为1，可以写成range(9)

        start=0,step=2,stop=9，值从0开始 结束是start + (n-1)step

>>> range(0,-9,-1)
range(0, -9, -1)
>>> list(range(0,-9,-1))
[0, -1, -2, -3, -4, -5, -6, -7, -8]

    2.5并行迭代

     可迭代(iterable):在python中的表现就是用for循环，从对象中获得一定数量的元素.

     求两个可迭代对象每个元素的和zip():

>>> c = [1,2,3]
>>> d = [9,8,7,6]
>>> zip(c,d)
<zip object at 0x105912b88>
>>> list(zip(c,d))
[(1, 9), (2, 8), (3, 7)]

>>> list(zip(d,c))
[(9, 1), (8, 2), (7, 3)]
>>> m = {"name","lang"}
>>> n = {"tajzhang","python"}
>>> list(zip(m,n))
[(‘name‘, ‘tajzhang‘), (‘lang‘, ‘python‘)]
>>> s = {"name":"tajzhang"}
>>> t = {"lang":"python"}
>>> list(zip(s,t))
[(‘name‘, ‘lang‘)]
>>> a = ‘tajzhang‘
>>> c = [1,2,3]
>>> list(zip(c))
[(1,), (2,), (3,)]
>>> list(zip(a))
[(‘t‘,), (‘a‘,), (‘j‘,), (‘z‘,), (‘h‘,), (‘a‘,), (‘n‘,), (‘g‘,)]

>>> a = [1,2,3,4,5]
>>> b = [9,8,7,6,5]
>>> d = []
>>> for x,y in zip(a,b):   #使用zip使列表相加
    d.append(x+y)

>>> d
[10, 10, 10, 10, 10]

        zip扩展用法

>>> for x,y in zip(a,b):
    d.append(str(x) + ":" +y)
>>> d
[‘1:python‘, ‘2:www.itdiffer.com‘, ‘3:tajzhang‘]
>>> result = [(2,11),(4,13),(6,15),(8,17)]
>>> list(zip(*result))
[(2, 4, 6, 8), (11, 13, 15, 17)]

        使用zip解决字典key value调换

        方法1:使用for循环

>>> myinfor = {"name":"tajzhang","stie":"www.bokey.io","lang":"python"}
>>> infor = {}
>>> for k,v in myinfor.items():
    infor[v] =k
>>> infor
{‘tajzhang‘: ‘name‘, ‘www.bokey.io‘: ‘stie‘, ‘python‘: ‘lang‘}

       方法2:使用zip()

>>> dict(zip(myinfor.values(),myinfor.keys()))
{‘tajzhang‘: ‘name‘, ‘www.bokey.io‘: ‘stie‘, ‘python‘: ‘lang‘}

        解析

>>> myinfor.values()
dict_values([‘tajzhang‘, ‘www.bokey.io‘, ‘python‘])
>>> myinfor.keys()
dict_keys([‘name‘, ‘stie‘, ‘lang‘])
>>> temp = zip(myinfor.values(),myinfor.keys())  #压缩成一个列表，每个元素是一个元祖，元祖中第一个是值，第二个是键
>>> temp
<zip object at 0x10239ee08>
>>> dict(temp)   #这是函数dict()的功能
{‘tajzhang‘: ‘name‘, ‘www.bokey.io‘: ‘stie‘, ‘python‘: ‘lang‘}

    2.6 enumerate()

         功能:类似同事得到元素索引和元素

>>> for i in range(len(week)):
    print(week[i] + ‘is‘ + str(i) )

mondayis0
sundayis1
fridayis2
>>> for (i,day) in enumerate(week):
    print(day + ‘is‘ + str(i))
    
mondayis0
sundayis1
fridayis2

         使用举例:

>>> seasons = [‘Spring‘,‘Summer‘,‘Fall‘,‘Winter‘]
>>> list(enumerate(seasons))
[(0, ‘Spring‘), (1, ‘Summer‘), (2, ‘Fall‘), (3, ‘Winter‘)]
>>> list(enumerate(seasons,start=1))
[(1, ‘Spring‘), (2, ‘Summer‘), (3, ‘Fall‘), (4, ‘Winter‘)]
>>> mylist = ["tajzhang",703,"python"]
>>> enumerate(mylist)
<enumerate object at 0x1023b6288>
>>> list(enumerate(mylist))
[(0, ‘tajzhang‘), (1, 703), (2, ‘python‘)]

        使用练习：

    2.7列表解析

        先得到1-9每个整数的平方

>>> power2 = []
>>> for i in range(1,10):
    power2.append(i*i)

>>> power2
[1, 4, 9, 16, 25, 36, 49, 64, 81]

 

>>> squares = [x**2 for x in range(1,10)]  #更pythonic的用法
>>> squares
[1, 4, 9, 16, 25, 36, 49, 64, 81]
>>> mybag = [‘ glass‘,‘ apple‘,‘ green leaf‘]    
>>> [one.strip() for one in mybag]
[‘glass‘, ‘apple‘, ‘green leaf‘]

 

3、while循环

原理图