Guangdong Ocean University Electronic 1151-hole Yanfei Python language programming third week

Iv. Process Control

     在这块，Python与其它大多数语言有个非常不同的地方，Python语言使用缩进块来表示程序逻辑（其它大多数语言使用大括号等）。例如：

If age < 21:

print("你不能买酒。")print("不过你能买口香糖。")

Print ("This sentence is outside of the IF statement block.) ”)

     这个代码相当于c语言的：

if (age < 21)

{

print("你不能买酒。")print("不过你能买口香糖。")

}

Print ("This sentence is outside of the IF statement block.) ”)

   可以看到，Python语言利用缩进表示语句块的开始和退出（Off-side规则），而非使用花括号或者某种关键字。增加缩进表示语句块的开始（注意前面有个:号），而减少缩进则表示语句块的退出。根据PEP的规定，必须使用4个空格来表示每级缩进（不清楚4个空格的规定如何，在实际编写中可以自定义空格数，但是要满足每级缩进间空格数相等）。使用Tab字符和其它数目的空格虽然都可以编译通过，但不符合编码规范。   为了使我们自己编写的程序能很好的兼容别人的程序，我们最好还是按规范来，用四个空格来缩减（注意，要么都是空格，要是么都制表符，千万别混用）。

1, If-else

     If-else用来判断一些条件，以执行满足某种条件的代码。

[Python] View plain copy
View code slices to my Code slice

################################  ######## procedure control #####  ## if else  if expression: # bool type and do not forget the colon      statement(s) # use four space key   if expression:   statement(s) # error!!!! should use four space key   if 1<2:      print ‘ok, ‘ # use four space key      print ‘yeah‘ # use the same number of space key  if True: # true should be big letter True      print ‘true‘  def fun():      return 1  if fun():      print ‘ok‘  else:      print ‘no‘  con = int(raw_input(‘please input a number:‘))  if con < 2:      print ‘small‘  elif con > 3:      print ‘big‘  else:      print ‘middle‘  if 1 < 2:      if 2 < 3:          print ‘yeah‘      else:          print ‘no‘        print ‘out‘  else:      print ‘bad‘  if 1<2 and 2<3 or 2 < 4 not 0: # and, or, not      print ‘yeah‘  

2. For

     for的作用是循环执行某段代码。还可以用来遍历我们上面所提到的序列类型的变量。

[Python] View plain copy
View code slices to my Code slice

################################ ######## Procedure Control ##### # # for-Iterating_val in sequence:statements (s) # sequence type can is string, tuple or list for I in ' ABCD ': Print I for i in [1, 2, 3, 4]: print I # R Ange (Start, end, step), if not set to step, default is 1, # If does set start, default is 0, should being noted that it is [STA  RT, end), not [start, end] Range (5) # [0, 1, 2, 3, 4] range (1, 5) # [1, 2, 3, 4] range (1,, 2) # [1, 3, 5, 7, 9] for I in range (1, 1): Print I # ergodic for basis sequence fruits = [' apple ', ' banana ', ' mango '] for fruit in R  Ange (len (fruits)): print ' current fruit: ', Fruits[fruit] # ergodic for dictionary dic = {1:111, 2:222, 5:555} For x in Dic:print x, ': ', Dic[x] Dic.items () # return [(1, 111), (2, 222), (5, 555)] for Key,value in Dic.items  (): # because we can:a,b=[1,2] print key, ': ', Value else:print ' ending ' ################################ Import Time # We also CAn use:break, continue-to-control process for x in range (1, one): Print x Time.sleep (1) # sleep 1s if x = =            3:pass # do nothing if x = = 2:continue if x = = 6:break if x = = 7:   Exit () # exit the whole program print ' # ' *50

3. While

     while的用途也是循环。它首先检查在它后边的循环条件，若条件表达式为真，它就执行冒号后面的语句块，然后再次测试循环条件，直至为假。冒号后面的缩近语句块为循环体。

[Python] View plain copy
View code slices to my Code slice

################################  ######## procedure control #####  ## while  while expression:      statement(s)  while True:      print ‘hello‘      x = raw_input(‘please input something, q for quit:‘)      if x == ‘q‘:          break  else:      print ‘ending‘  

4. Switch

     其实Python并没有提供switch结构，但我们可以通过字典和函数轻松的进行构造。例如：

[Python] View plain copy
View code slices to my Code slice

#############################  ## switch ####  ## this structure do not support by python  ## but we can implement it by using dictionary and function  ## cal.py ##  #!/usr/local/python  from __future__ import division  # if used this, 5/2=2.5, 6/2=3.0  def add(x, y):      return x + y  def sub(x, y):      return x - y  def mul(x, y):      return x * y  def div(x, y):      return x / y  operator = {"+": add, "-": sub, "*": mul, "/": div}  operator["+"](1, 2) # the same as add(1, 2)  operator["%"](1, 2) # error, not have key "%", but the below will not  operator.get("+")(1, 2) # the same as add(1, 2)  def cal(x, o, y):      print operator.get(o)(x, y)  cal(2, "+", 3)  # this method will effect than if-else  

Five, function

1. Custom Functions

     在Python中，使用def语句来创建函数：

[Python] View plain copy
View code slices to my Code slice

################################ ######## function ##### def functionname (parameters): # No parameters is OK bodyo  Ffunction def add (A, B): Return A+b # If we do not use a return, any defined function would return default None a = + b = sum = Add (A, b) ##### function.py ##### #!/usr/bin/python #coding: UTF8 # support Chinese def add (a = 1  , B = 2): # Default parameters return A+b # can return any type of data # The followings is all OK Add () Add (2)  Add (y = 1) Add (3, 4) ###### the global and local value ##### # # Global value:defined outside any function, and can be Used # in-anywhere, even in functions, this should be noted # # local value:defined inside a function, an D can only be used # in its own function # # The local value would cover the global if they have the same NA  Me val = # global Value def fun (): Print Val # Here's access the Val = + Print Val # Here's access the val = too def fun(): A = # local value print a print a # here can not access the A = + def fun (): Global a = # D  Eclare as a global value print a print a # here can not access the A = +, because fun () not being called yet fun () Print a # here can access the A = ############################ # # Other types of Parameters def fun (x): Print X # The follows is all OK Fun () # int fun (' Hello ') # string Fun ((' X ', 2, 3)) # Tuple Fun ([1, 2, 3]) # list Fu N ({1:1, 2:2}) # dictionary # tuple def fun (x, y): print "%s:%s"% (x, y) #%s stands-string fun (' Zou ', ' XI Aoyi ') Tu = (' Zou ', ' Xiaoyi ') Fun (*tu) # can transfer a tuple parameter like this # # dictionary def fun (name = "Name") , age = 0): print ' name:%s '% name print ' Age: '% age dic = {name: ' Xiaoyi ', age:25} # The keys of Dictionar Y should be same as fun (**dic) # can transfer dictionary parameter like this fun (age = +, name = ' Xiaoyi ') # the result is the same # # THe advantage of dictionary is can specify value name ############################# # # redundancy Parameters # # # # The Tuple def fun (x, *args): # The extra parameters would stored in args as tuple type print x print args # the F Ollows is OK fun (ten, a) # x = ten, args = (k, x) # # The Dictionary def Fun (**args): # The Extra para Meters'll stored in args as dictionary type print x print args # The follows is OK fun (x = ten, Y = x, z = +) # × = ten, args = {' Y ': +, ' Z ': $} # Mix of tuples and dictionary def Fun (x, *args, **kwargs): Print x print args Print Kwargs fun (1, 2, 3, 4, y = ten, z = +) # x = 1, args = (2, 3, 4), Kwargs = {' Y ': ten, ' Z ': 12   }

2. Lambda function

     Lambda函数用来定义一个单行的函数，其便利在于：

[Python] View plain copy
View code slices to my Code slice

#############################  ## lambda function ####  ## define a fast single line function  fun = lambda x,y : x*y # fun is a object of function class  fun(2, 3)  # like  def fun(x, y):      return x*y  ## recursion  # 5=5*4*3*2*1, n!  def recursion(n):      if n > 0:          return n * recursion(n-1) ## wrong  def mul(x, y):      return x * y  numList = range(1, 5)  reduce(mul, numList) # 5! = 120  reduce(lambda x,y : x*y, numList) # 5! = 120, the advantage of lambda function avoid defining a function  ### list expression  numList = [1, 2, 6, 7]  filter(lambda x : x % 2 == 0, numList)  print [x for x in numList if x % 2 == 0] # the same as above  map(lambda x : x * 2 + 10, numList)  print [x * 2 + 10 for x in numList] # the same as above  

3. Python built-in functions

   Python内置了很多函数，他们都是一个个的.py文件，在python的安装目录可以找到。弄清它有那些函数，对我们的高效编程非常有用。这样就可以避免重复的劳动了。下面也只是列出一些常用的：

[Python] View plain copy
View code slices to my Code slice

################################### # # built-in function of Python # # # # # If do not ' how to ' use, ' use ' Help () ABS, Max, Min, Len, Divmod, POW, round, callable, Isinstance, CMP, range, xrange, type, id, int () list (), tuple (), Hex (), OC T (), Chr (), Ord (), Long () callable # Test A function whether can is called or not, if can, return true # or test a funct Ion is exit or isn't isinstance # test Type numlist = [1, 2] if type (numlist) = = Type ([]): print "It is a list" if Isinstance (Numlist, list): # The same as above, return true print ' It is a list ' for I in range (1, 10001) # would CRE Ate a 10000 list, and cost memory for I in xrange (1, 10001) # does not create such a list, no memory was cost # # some basic Functions about string str = ' Hello World ' str.capitalize () # ' Hello World ', first letter transfer to big Str.replace ("  Hello "," good ") # ' good world ' IP = ' 192.168.1.123 ' ip.split ('. ') # return [' 192 ', ' 168 ', ' 1 ', ' 123 '] help (Str.split) Import string str = ' HeLlo World ' string.replace (str, "Hello", "good") # ' Good World ' # # Some basic functions about sequence len, Max, Min # Filter (function or none, Sequence) def fun (x): if x > 5:return True numlist = [1, 2, 6, 7] filter (fun , numlist) # get [6, 7], if fun return True, retain the element, otherwise delete it filter (lambda x:x% 2 = = 0, Numlis T) # Zip () name = ["Me", "you"] = ["123", "234"] Zip (name, age, tel) # return a list: [(' Me ', 25, ' 123 '), (' You ', ' + ', ' 234 ')] # map (None, name, age, tel) # also return a list: [(' Me ', ' + ', ' 123 '), (' You ', 26, ' 234 ')] test = ["Hello1", "Hello2", "Hello3"] Zip (name, age, Tel, test) # return [(' Me ', ' + ', ' 123 ', ' Hello1 '), (' You ', 26, ' 234 ', ' Hello2 ')] Map (None, name, age, Tel, test) # return [(' Me ', ' + ', ' 123 ', ' Hello1 '), (' You ', ' + ', ' 234 ', ' Hello2 '), (No NE, none, none, ' Hello3 ')] a = [1, 3, 5] b = [2, 4, 6] def mul (x, y): Return x*y map (Mul, A, b) # return [2, 12, # Reduce () reduce (Lambda x, Y:x+y, [1, 2, 3, 4, 5]) # Return ((((1+2) +3) +4) +5)   

Guangdong Ocean University electronic 1151-hole Yanfei Python language programming third week