广东海洋大学 电子1151 孔yanfei python语言程序设计 第三周
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四、流程控制
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(“这句话处于if语句块的外面。”)
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(“这句话处于if语句块的外面。”)
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。
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################################
######## 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。
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################################
######## procedure control #####
## for
for iterating_val in sequence:
statements(s)
# sequence type can be string, tuple or list
for i in "abcd":
print i
for i in [1, 2, 3, 4]:
print i
# range(start, end, step), if not set step, default is 1,
# if not set start, default is 0, should be noted that it is [start, end), not [start, end]
range(5) # [0, 1, 2, 3, 4]
range(1, 5) # [1, 2, 3, 4]
range(1, 10, 2) # [1, 3, 5, 7, 9]
for i in range(1, 100, 1):
print i
# ergodic for basis sequence
fruits = [‘apple‘, ‘banana‘, ‘mango‘]
for fruit in range(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, 11):
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。
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################################
######## 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：
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#############################
## 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
五、函数
1、自定义函数
在Python中，使用def语句来创建函数：
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################################
######## function #####
def functionName(parameters): # no parameters is ok
bodyOfFunction
def add(a, b):
return a+b # if we do not use a return, any defined function will return default None
a = 100
b = 200
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 are 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, and can only be used
## in its own function
## the local value will cover the global if they have the same name
val = 100 # global value
def fun():
print val # here will access the val = 100
print val # here will access the val = 100, too
def fun():
a = 100 # local value
print a
print a # here can not access the a = 100
def fun():
global a = 100 # declare as a global value
print a
print a # here can not access the a = 100, because fun() not be called yet
fun()
print a # here can access the a = 100
############################
## other types of parameters
def fun(x):
print x
# the follows are all ok
fun(10) # int
fun(‘hello‘) # string
fun((‘x‘, 2, 3)) # tuple
fun([1, 2, 3]) # list
fun({1: 1, 2: 2}) # dictionary
## tuple
def fun(x, y):
print "%s : %s" % (x,y) # %s stands for string
fun(‘Zou‘, ‘xiaoyi‘)
tu = (‘Zou‘, ‘xiaoyi‘)
fun(*tu) # can transfer 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 dictionary should be same as fun()
fun(**dic) # can transfer dictionary parameter like this
fun(age = 25, 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 will stored in args as tuple type
print x
print args
# the follows are ok
fun(10)
fun(10, 12, 24) # x = 10, args = (12, 24)
## the dictionary
def fun(x, **args): # the extra parameters will stored in args as dictionary type
print x
print args
# the follows are ok
fun(10)
fun(x = 10, y = 12, z = 15) # x = 10, args = {‘y‘: 12, ‘z‘: 15}
# mix of tuple and dictionary
def fun(x, *args, **kwargs):
print x
print args
print kwargs
fun(1, 2, 3, 4, y = 10, z = 12) # x = 1, args = (2, 3, 4), kwargs = {‘y‘: 10, ‘z‘: 12}
2、Lambda函数
Lambda函数用来定义一个单行的函数，其便利在于：
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#############################
## 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内置函数
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：
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###################################
## built-in function of python ####
## if do not how to use, please use help()
abs, max, min, len, divmod, pow, round, callable,
isinstance, cmp, range, xrange, type, id, int()
list(), tuple(), hex(), oct(), chr(), ord(), long()
callable # test a function whether can be called or not, if can, return true
# or test a function is exit or not
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) # will create a 10000 list, and cost memory
for i in xrange(1, 10001)# do not create such a list, no memory is 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, numList)
# zip()
name = ["me", "you"]
age = [25, 26]
tel = ["123", "234"]
zip(name, age, tel) # return a list: [(‘me‘, 25, ‘123‘), (‘you‘, 26, ‘234‘)]
# map()
map(None, name, age, tel) # also return a list: [(‘me‘, 25, ‘123‘), (‘you‘, 26, ‘234‘)]
test = ["hello1", "hello2", "hello3"]
zip(name, age, tel, test) # return [(‘me‘, 25, ‘123‘, ‘hello1‘), (‘you‘, 26, ‘234‘, ‘hello2‘)]
map(None, name, age, tel, test) # return [(‘me‘, 25, ‘123‘, ‘hello1‘), (‘you‘, 26, ‘234‘, ‘hello2‘), (None, 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, 30]
# reduce()
reduce(lambda x, y: x+y, [1, 2, 3, 4, 5]) # return ((((1+2)+3)+4)+5)
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