欢迎访问www.showerlee.com, 您的支持就是我前进的动力.

[PYTHON] 核心编程笔记之八-Python条件和循环

showerlee 2013-11-22 22:04 LAN, PYTHON 阅读 (1,780) 抢沙发

8.1 if语句

if expression:

   expr_true_suite

8.1.1 多重条件表达式

单个if 语句可以通过使用and,or和not实现多重判断条件或是否定判断条件

if not warn and (system_load>=10):

   print "WARNING: losing resources"

   warn +=1

8.1.2 单一语句的代码块

if make_hard_copy: send_data_to_printer()

8.2 else 语句

if expression:

   expr_true_suite

else:

   expr_false_suite

8.2.1 避免"悬挂else"

8.3 elif(即else-if)语句

if expression1:

   expr1_true_suite

elif expression2:

   expr2_true_suite

elif expressionN:

   expeN_true_suite

else:

   none_of_the_above_suite

8.4 条件表达式(即"三元操作符")

>>> x, y=4, 3    

>>> if x < y:    

...   smaller = x

... else:        

...   smaller = y

...

>>> smaller

3

>>> smaller = (x < y and [x] or [y])[0]

>>> smaller

3

>>> smaller = x if x < y else y

>>> smaller

3

8.5 while 语句

8.5.1 一般语法

while 循环的语法如下:

while expression:

   suite_to_repeat

while循环的suite_to_repeat子句会一直循环下去,直到expression值为假

8.5.2 技术循环

>>> count = 0        

>>> while (count < 9):

...   print 'the index is:',count

...   count +=1

...

the index is: 0

the index is: 1

the index is: 2

the index is: 3

the index is: 4

the index is: 5

the index is: 6

the index is: 7

the index is: 8

8.5.3 无限循环

while True:

   handle,indata = wait_for_client_connect()

   outdata = process_request(indata)

   ack_result_to_client(handle,outdata)

注: 以上代码为无限循环,因为服务器代码是用来等待客户端(可能通过网络)来连接的,这些客户端向服务器发送请求,服务器处理请求,请求被处理,服务器向客户端返回数据

8.6 for 语句

8.6.1 一般语法:

for iter_var in iterable:

   suite_to_repeat

每次循环,lter_var迭代变量被设置为可迭代对象的当前元素,提供给suite_to_repeat语句块使用

8.6.2 用于序列类型

>>> for eachLetter in 'Names':

...     print 'current letters',eachLetter

...

current letters N

current letters a

current letters m

current letters e

current letters s

通过序列项迭代

>>> nameList = ['Walter',"Nicole",'Steven','Henry']

>>> for eachName in nameList:

...     print eachName,"Lim"

...

Walter Lim

Nicole Lim

Steven Lim

Henry Lim

通过序列索引迭代

>>> nameList = ['Cathy',"Terry","Joe",'heather','Lucy']

>>> for nameIndex in range(len(nameList)):            

...   print "Liu",nameList[nameIndex]                  

...

Liu Cathy

Liu Terry

Liu Joe

Liu heather

Liu Lucy

>>> len(nameList)

5

>>> range(len(nameList))

[0, 1, 2, 3, 4]

使用range()我们可以得到用来迭代nameList的索引数列表:使用切片/下标操作符([]),就可以访问对应的序列对象

使用项和索引迭代

使用enumerate()函数

>>> nameList = ['Donn','Shirley','Ben','Janice','David','Yen','Wendy']

>>> for i,eachLee in enumerate(nameList):

...   print "%d %s Lee" %(i+1,eachLee)

...

1 Donn Lee

2 Shirley Lee

3 Ben Lee

4 Janice Lee

5 David Lee

6 Yen Lee

7 Wendy Lee

8.6.3 用于迭代器类型

用for循环访问迭代器和访问序列的方法差不多,区别是for语句会做一些额外的事情

迭代器对象有一个next()方法,调用后返回下一条目,所有条目迭代完,迭代器引发一个StopIteration一场告诉程序循环结束,for语句在内部调用next()病捕获异常

8.6.4 range()内建函数:

range(start, end, step = 1)

range()会返回一个包含所有k的列表,这里start<=k<end, 从start到end, k每次递增step, step不可以为零,否则将发生错误

>>> range(2,19,3)

[2, 5, 8, 11, 14, 17]

只给定两个参数,省略step,step就使用默认1

>>> range(3,7)

[3, 4, 5, 6]

例:

>>> for eachVal in range(2,19,3):

...    print "value is: ",eachVal

...

value is:  2

value is:  5

value is:  8

value is:  11

value is:  14

value is:  17

range()简略语法:

range(end)

range(start,end)

>>> range(5)

[0, 1, 2, 3, 4]

>>> for count in range(2,5):

...   print count

...

2

3

4

注:

>>> range(start=0,end,step=1)  

 File "<stdin>", line 1

SyntaxError: non-keyword arg after keyword arg

这个语法不可以使用两个参数调用,因为step要求给定start

8.6.5 xrange()内建函数:

8.6.6 与序列相关的内建函数:

sorted(),reversed(),enumerate(),zip()

>>> albums = ('Poe','Gaudi','Freud','Poe2')

>>> years = (1976,1987,1990,2003)

>>> for album in sorted(albums):

...     print album,

...

Freud Gaudi Poe Poe2

>>> for album in reversed(albums):

...     print album,              

...

Poe2 Freud Gaudi Poe

>>> for i,album in enumerate(albums):

...     print i,album

...

0 Poe

1 Gaudi

2 Freud

3 Poe2

>>> for album,yr in zip(albums,years):

...     print yr, album

...

1976 Poe

1987 Gaudi

1990 Freud

2003 Poe2

8.7 break语句

break语句可以结束当前循环然后跳转到下条语句,常用于while和for循环

count = num / 2

while count > 0:

   if num % count == 0:

print count, 'is the largest factor of', num

break

   count -= 1

8.8 continue 语句

continue在开始下一次循环前需要满足一些先决条件,否则循环会正常结束,常用在while和for循环里

#!/usr/bin/env python

valid = False

count = 3

while count > 0:

   input = raw_input("enter password")

   for eachPasswd in passwdList:

       if input == eachPasswd:

           valid = True

           break

       if not valid:

           print "invalid input"

           count -= 1

           continue

       else:

           break

8.9 pass语句

def foo_func():

   pass

或是

if user_choice = 'do_calc':

   pass else:

   pass

这样的代码结构在开发和调试时很有用,因为编写代码的时候你可能要先把结构定下来,但你不希望它干扰其他已经完成的代码,在不需要它做任何事情的地方,放一个pass将是一个很好的主意

8.10 再谈else语句

Python可以在while和for循环中使用else语句,在循环使用时,else子句只在循环完成后执行,也就是break会跳过else块

例:

-------------------------

#!/usr/bin/env python

def showMaxFactor(num):

   count = num / 2

   while count > 1:

       if num % count ==0:

           print 'largest factor of %d is %d' %(num,count)

           break

       count -= 1

   else:

       print num, "is prime"

for eachNum in range(10,21):

   showMaxFactor(eachNum)

--------------------------

showMaxFactor()函数中第3行的循环从amount的一般开始计数(这样就可以检查这个数是否能被2整除,如果可以,那就找到了最大约数),然后循环每次递减1,直到发现约数,如果循环递减到1还没有找到约束,那么这个数一定是素数,11-12行的else子句负责处理这样的情况

largest factor of 10 is 5

11 is prime

largest factor of 12 is 6

13 is prime

largest factor of 14 is 7

largest factor of 15 is 5

largest factor of 16 is 8

17 is prime

largest factor of 18 is 9

19 is prime

largest factor of 20 is 10

for与while处理方式相同,只要for循环为正常结束,else子句就会执行

条件与循环语句对照表

ifwhilefor

elif*

else***

break**

continue**

pass***

8.11 迭代器和iter()函数:

8.11.1 什么是迭代器?

8.11.2 为什么要迭代器?

8.11.3 如何迭代?

next() reversed() enumerate() any() all()方法

8.11.4 使用迭代器

例:

>>> myTuple = (123, 'xyz', 45,67)

>>> i = iter(myTuple)

>>> i.next()

123

>>> i.next()

'xyz'

>>> i.next()

45

>>> i.next()

67

>>> i.next()

Traceback (most recent call last):

 File "<stdin>", line 1, in <module>

StopIteration

如果这是一个实际应用程序,那么我们需要把代码放在一个try-except块中,序列现在会自动的产生他们自己的迭代器,所以一个for循环:

for i in seq:

   do_something_to(i)

------------------------------

fetch = iter(seq)

while True:

   try:

i = fetch.next()

   except StopIteration:

break

do_something_to(i)

字典

语句for eachKey in myDict.keys()可以缩写为for eachKey in myDict

>>> legends = {('Poe', 'author'):(1809, 1849, 1976),('Gaudi','architect'):(1852,1906,1987),('Freud','psychoanalyst'):(1856,1939,1990)}

>>> for eachLegend in legends:

...     print 'Name: %s \t Occupation: %s' % eachLegend

...     print 'Birth:%s\tDeath: %s\tAlbum: %s\n' %legends[eachLegend]

...

Name: Poe        Occupation: author

Birth:1809      Death: 1849     Album: 1976

Name: Gaudi      Occupation: architect

Birth:1852      Death: 1906     Album: 1987

Name: Freud      Occupation: psychoanalyst

Birth:1856      Death: 1939     Album: 1990

Name: Poe        Occupation: author

Birth:1809      Death: 1849     Album: 1976

Name: Gaudi      Occupation: architect

Birth:1852      Death: 1906     Album: 1987

Name: Freud      Occupation: psychoanalyst

Birth:1856      Death: 1939     Album: 1990

另外:python还引进了三个新的内建字典方法来定义迭代:myDict.iterkeys() (通过keys迭代),myDict.itervalues() (通过values迭代),以及myDict.iteritems() (通过key/value对来迭代)

文件

文件对象生成的迭代器会自动调用readline()方法

for eachLine in myFile.readlines(): 可以简化为 for eachLine in myFile:

例:

--------------------------------

>>> myFile = open('test20.py')

>>> for eachLine in myFile:

...     print eachLine,

...

#!/usr/bin/env python

while True:

   try:

       usr_input_1 = int(raw_input("please input the first word: ").strip())

       usr_input_2 = int(raw_input("please input the second word: ").strip())

       usr_input_3 = int(raw_input("please input the third word: ").strip())

       break

   except:

       print "please input the number!"

list = [usr_input_1,usr_input_2,usr_input_3]

average = sum(list)/len(list)

print "The list is %s" %list

print "The average of the list is %i" %average

>>> myFile.close()

--------------------------------------------------

8.11.5 可变对象和迭代器

循环列表的时候删除满足(或不满足)特定条件的项:

for eachURL in allURLs:

   if not eachURL startswith('http://'):

allURLs.remove(eachURL)

在迭代字典key时,绝不能改变字典,否则不会继续执行下去:

>>> myFile.close()

>>> myDict = {'a':1,'b':2,'c':3,'d':4}

>>> for eachKey in myDict:

...     print eachKey,myDict[eachKey]

...     del myDict[eachKey]

...

a 1

Traceback (most recent call last):

 File "<stdin>", line 1, in <module>

RuntimeError: dictionary changed size during iteration

8.11.6 如何创建迭代器

iter(obj)检查传递的是不是序列,如果是,根据索引从0一直迭代到结束

iter(fuc,sentinel)重复调用func,直到迭代器的下个值等于sentinel

8.12 列表解析

语法:

[expr for iter_var in iterable]

for循环迭代iterable对象的所有条目,前边的expr应用于序列的每个成员,最后的结果值是该表达式产生的列表

例:

计算序列成员的平方的lambda函数表达式

>>> map(lambda x: x**2,range(6))

[0, 1, 4, 9, 16, 25]

列表解析方法:

>>> [x ** 2 for x in range(6)]

[0, 1, 4, 9, 16, 25]

结合if语句,列表解析还提供一个扩展版本的语法:

[expr for iter_var in iterable if cond_expr]

使用filter()和lambda挑出序列的奇数:

>>> seq = [11,10,9,9,10,10,9,8,23,9,7,18,12,11,12]

>>> filter(lambda x: x % 2,seq)

[11, 9, 9, 9, 23, 9, 7, 11]

使用列表解析:

>>> [x for x in seq if x % 2]

[11, 9, 9, 9, 23, 9, 7, 11]

矩阵样例:

迭代一个有三行五列的矩阵

>>> [(x+1,y+1) for x in range(3) for y in range(5)]

[(1, 1), (1, 2), (1, 3), (1, 4), (1, 5), (2, 1), (2, 2), (2, 3), (2, 4), (2, 5), (3, 1), (3, 2), (3, 3), (3, 4), (3, 5)]

磁盘文件样例:

计算出一个数据文件所有非空白字符的数目:

>>> f = open('test20.py','r')

>>> len([word for line in f for word in line.split()])

57

快速计算文件大小

>>> import os

>>> os.stat('test20.py').st_size

497

>>> f.seek(0)

>>> sum([len(word) for line in f for word in line.split()])

391

8.13 生成器表达式

列表解析:

[expr for iter_var in iterable if cond_expr]

生成器表达式:

(expr for iter_var in iterable if cond_expr)

>>> f = open('test20.py','r')                                  

>>> sum(len(word) for line in f for word in line.split())      

391

交叉配对例子:

>>> rows = [1, 2, 3, 17]                                  

>>> def cols():

...    yield 56

...    yield 2

...    yield 1

...

>>> x_product_pairs = ((i,j) for i in rows for j in cols())

>>> for pair in x_product_pairs:

...   print pair          

...

(1, 56)

(1, 2)

(1, 1)

(2, 56)

(2, 2)

(2, 1)

(3, 56)

(3, 2)

(3, 1)

(17, 56)

(17, 2)

(17, 1)

重构样例:

我们通过一个寻找最长行的例子来看看如何改进代码

例:

---------------------------

#!/usr/bin/env python

f = open('/etc/motd','r')

longest = 0

while True:

   linelen = len(f.readline().strip())

   if not linelen:

       break

   if linelen > longest:

       longest = linelen

f.close()

print longest

-----------------------------

版本1

--------------------------------

#!/usr/bin/env python

f = open('/etc/motd','r')

longest = 0

allLines = f.readlines()

f.close()

for line in allLines:

  linelen = len(line.strip())

  if linelen > longest:

       longest = linelen

print longest

--------------------------------

版本2

--------------------------------

#!/usr/bin/env python

f = open('/etc/motd','r')

longest = 0

allLines = [x.strip() for x in f.readlines()]

f.close()

for line in allLines:

  linelen = len(line)

  if linelen > longest:

       longest = linelen

print longest

--------------------------------

版本3

----------------------------------

#!/usr/bin/env python

f = open('/etc/motd', 'r')

allLine = [x.strip() for x in f ]

allLineList = []

for Line in allLine:

   allLineList.append(len(Line))

print max(allLineList)

----------------------------------

版本4

--------------------------------

#!/usr/bin/env python

f = open('/etc/motd', 'r')

allLineLens = [len(x.strip()) for x in f ]

print allLineLens

f.close()

print max(allLineLens)

--------------------------------

版本5

---------------------------------

#!/usr/bin/env python

f = open('/etc/motd','r')

longest = max (len(x.strip()) for x in f)

f.close()

print longest

---------------------------------

版本6 (最终版)

----------------------------------

>>> print max(len(x.strip()) for x in open('/etc/motd','r'))      

74

-----------------------------------

8.14 R相关模块

正文部分到此结束
版权声明:除非注明,本文由(showerlee)原创,转载请保留文章出处!
本文链接:http://www.showerlee.com/archives/992

继续浏览:PYTHON

还没有评论,快来抢沙发!

发表评论

icon_wink.gif icon_neutral.gif icon_mad.gif icon_twisted.gif icon_smile.gif icon_eek.gif icon_sad.gif icon_rolleyes.gif icon_razz.gif icon_redface.gif icon_surprised.gif icon_mrgreen.gif icon_lol.gif icon_idea.gif icon_biggrin.gif icon_evil.gif icon_cry.gif icon_cool.gif icon_arrow.gif icon_confused.gif icon_question.gif icon_exclaim.gif