Python Special Syntax: filter, map, reduce, lambda [go]

Python Special Syntax: filter, map, reduce, lambda [go]

Python has built in some very interesting but very useful functions, fully embodies the language charm of Python!

filter (function, sequence): Executes function (item) on item in sequence, and the item that executes the result of true is composed of a list/string/ The Tuple (depending on the type of sequence) returns:
>>> def f (x): return x% 2! = 0 and x% 3! = 0
>>> filter (F, Range (2, 25))
[5, 7, 11, 13, 17, 19, 23]
>>> def f (x): return x! = ' a '
>>> filter (F, "abcdef")
' Bcdef '

map (function, sequence) : Executes function (item) on item in sequence, and the result of execution consists of a list return:
>>> def Cube (x): Return x*x*x
>>> Map (cube, range (1, 11))
[1, 8, 27, 64, 125, 216, 343, 512, 729, 1000]
>>> def Cube (x): return x + x
...
>>> map (Cube, "ABCDE")
[' AA ', ' BB ', ' cc ', ' dd ', ' EE ']
In addition, the map supports multiple sequence, which requires that the function also supports the corresponding number of parameter inputs:
>>> def add (x, y): Return x+y
>>> Map (Add, Range (8), range (8))
[0, 2, 4, 6, 8, 10, 12, 14]

reduce (function, sequence, starting_value): Calls the function in the order of the item in the sequence, and if there is a starting_value, it can also be called as an initial value. For example, you can use the sum of a list:
>>> def add (x, y): return x + y
>>> reduce (Add, range (1, 11))
55 (Note: 1+2+3+4+5+6+7+8+9+10)
>>> reduce (Add, range (1, 11), 20)
75 (Note: 1+2+3+4+5+6+7+8+9+10+20)

Lambda: This is a funny syntax for Python, which allows you to quickly define the smallest function of a single line, similar to a macro in C, these functions, called Lambda, are borrowed from Lisp and can be used wherever functions are needed:
>>> g = Lambda x:x * 2
>>> g (3)
6
>>> (Lambda x:x * 2) (3)
6


We can also use the filter map reduce and lambda together, the function can be simply written in one line.
For example
Kmpathes = Filter (lambda Kmpath:kmpath,
Map (Lambda Kmpath:string.strip (Kmpath),
String.Split (L, ': ')))
Looks like trouble, in fact, as in the language to describe the problem, very elegant.
Make a list of all the elements in L that are split with ': '. Each element of this list is made into a string strip, forming a list. Do a direct return operation on each element of this list (this place can be added to the filter limit) and eventually get a string that is ': ' In the split list, each string in the list is strip and can be filtered on a special string.

[Turn] http://hi.baidu.com/black/item/307001d18715fc322a35c747

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lambda expressionReturns an example of a function object: func = lambda X,y:x+yfunc equivalent to the following function def func (x, y): return x+y Note that DEF is a statement and lambda is an expression under which the lambda is the only way to use the def[( Lambda x:x*x) (x) for x in range (1,11)] The function in Map,reduce,filter can be generated with a lambda expression! map (function,sequence)Pass the worth parameter in the sequence to function, and return a list containing the result of the function execution. If function has two parameters, that is, map (FUNCTION,SEQUENCE1,SEQUENCE2). Example: Seek 1*1,2*2,3*3,4*4map (lambda x:x*x,range (1,5)) return value is [1,4,9,16] reduce (function,sequence)function receives only 2 parameters to function, the first value in sequence and the second worth parameter, then the return value of function and the third worth parameter to function, and then returns only one result. Example: 1 to 10 of the cumulative reduce (lambda x,y:x+y,range (1,11)) return value is 55. filter (function,sequence)The return value of a function can only be true or false to pass the value in the sequence to function individually, and if the return value of function (x) is true, the X is added to the return value of the filter. In general, the return value of filter is list, special cases such as sequence is a string or tuple, then the return value is the type of sequence. Example: Find the odd filter (Lambda X:x%2!=0,range (1,11)) return value between 1 and 10 [1,3,5,7,9] If sequence is a stringfilter (lambda X:len (x)!=0, ' Hello ') returns ' Hello ' filter (lambda x:len (x) ==0, ' hello ') return '

[Turn] http://blog.csdn.net/myzhan/article/details/7269471

Python Special Syntax: filter, map, reduce, lambda [go]