tupples的妙用
def smallest(alist):
smallest = 1000000
smallestPos = -1
for pos, value in enumerate(alist):
if value < smallest:
smallest = value
smallestPos = pos
return smallest, smallestPos
- 在这个函数中，
pos,value是一个tupple(pos,value),返回值smallest,smallestPos也是一个tupple(smallest,smallestPos)。所以用tupple可以返回多个函数值。 - enumerate([1,2,3]) = [(0,1),(1,2),(2,3)]
- 用代码
x,y = y,x可以互换x,y的值，这点和其他语言不同。
python中function的特性
- 在python中，function可以像变量一样，可以赋值给其他变量，作为参数和返回值。
- First class citizen
Types one can assign to a variable, one can pass as arguments to functions and that can be return types of functions are called First class citizen of a language.
lambda
lambda expression是你不想想函数名字，而且函数比较简单时可以使用。lambda函数也是函数，所以它也是First class citizen。
f = lambda x: 2*x
def test(f,x,y):
return f(x,y)
print(test(lambda x,y:x+y,1,2))
def ff():
return lambda x: x*2
print(ff()(1))
这里的ff()(1)是把1作为参数，传到ff函数里。
print((lambda x,y: x+y)(11,12))
这里也是同理，把11，12作为参数传到lambda函数里。
List Comprehension
List comprehensions provide a nice syntax for transforming a list into some new list.
- 表达式
[oprate for x in list] - 几种类型
print([abs(x) for x in [1,-1,3,-5]])
print([x for x in [1,-1,3,-5] if x>0])
print([-1 if x==0 else x for x in [1,0,3,0]])
print([(x,y) for x in ["apple","pie"] for y in [6,7,8,9,10]])
Map
Map is a function that applies a function to each element of the list and stores the results in a new list.
- 表达式
map(function, list)
print(map(x>2), [0,1,2,3])
注意：这条语句的运行结果是
[False, False, False,True]
Filter
Filter is a function that applies a function to choose special element of the list and stores the results in a new list.
def filt2(function,alist):
result = []
for x in alist:
if function(x):
result.append(x)
return result
print(filt2(lambda x:x>0,[-1,2,-3,5,-5,7]))
flit2是一个Filter函数，lambda x:x>0是条件函数，[-1,2,-3,5,-5,7]是被过滤的list。
