Python学习笔记(三)——迭代
- 知识点
- 迭代
- 迭代
什么是迭代呢?和递归又有什么区别呢?
根据维基百科和网上的资料:
迭代:迭代是重复反馈过程的活动,其目的通常是为了接近并到达所需的目标或结果。每一次对过程的重复被称为一次“迭代”,
而每一次迭代得到的结果会被用来作为下一次迭代的初始值。
如求1+2+3+4+5的和:
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递归呢?指在函数的定义中使用函数自身的方法。也就是函数不停的调用自己直到满足一定条件。
而用递归是怎样求1--5的和呢?
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斐波那契数列的递归和迭代实现:
def fib(n):
if n>0:
if (n==1 or n==2):
return 1
else:
return fib(n-1)+fib(n-2)
else:
return -1
def fib_2(n):
if n<=0:
return -1
else:
a=1
b=1
s=1
for i in range(n-2):
s=a+b
a=b
b=s
return s
print fib(int(raw_input('Please input a number you want to calculate the fib number:')))
print fib_2(int(raw_input('Please input a number you want to calculate the fib number:')))
- 那Python中哪些是可迭代的呢?
从左往右扫描对象的方式都是可迭代的。
如何判断一个对象是不是可迭代的呢?
stackoverflow是这么说的。http://stackoverflow.com/questions/1952464/in-python-how-do-i-determine-if-an-object-is-iterable
- 使用Python的内置函数hasattr ,hasattr(object,name)当object具有name属性时返回真。迭代属性就是__iter__.
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2. 使用collections模块的Iterable类型来判断
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" alt="" />
aaarticlea/png;base64,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" alt="" />
- 对于一般的可迭代类型用for in就可以对它们进行迭代操作
- 但是对于有键值对的字典呢?对于值value则需要itervalues()函数来对值进行操作
capitals={'China':'beijing','America':'Washington','Russia':'Moscow'}
print 'countries:'
for country in capitals:
print country
print 'capitals:'
for capital in capitals.itervalues():
print capital
aaarticlea/png;base64,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" alt="" /> 可以看出字典进行迭代操作时,得出的值不一定按顺序。
我们以前写Java和c++的程序员都习惯下标循环,那在Python中怎样实现呢?通过enumerate()函数即可实现。
根据Python官方文档,enumerate(sequence, start=0) 返回一个有默认下标从0开始的object。
seasons = ['Spring', 'Summer', 'Fall', 'Winter']
print list(enumerate(seasons))
print list(enumerate(seasons, start=1))
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