python内置函数filter,map,reduce
Python list function build in three useful funtion(1):filter(funtion,sequence)
This function will return the item in the sequence invoke the function and return true
such as:
def f(x):
if x > 0:
return True
else:
return False
filter(f,range(-3,4))
will return(1,2,3)
(2):map(function,sequence[,sequence...])
This funcion will map all the sequence one by one, so the sequence list's size must be all the same.
for example:
def m(x,y,z):
return x+y+z
map(f,range(3),range(3),range(3))
will return
so, how many params the function has. how many sequence are needed
(3):reduce(function,sequence)returns a single value constructed by calling the binary function function on the first two items of the sequence, then on the result and the next item, and so on.
for example:
>>> def r(x,y):
print 'x is %s, y is %s' % (x,y)
>>> reduce(r,range(5))
x is 0, y is 1
x is None, y is 2
x is None, y is 3
x is None, y is 4
So the function must take two params,
and there is a third param, use it to set the first param
for example:
>>> reduce(r,range(5),1)
x is 1, y is 0
x is None, y is 1
x is None, y is 2
x is None, y is 3
x is None, y is 4
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