Python "Map, reduce, filter" built-in function usage instructions

A: Map

Map (...)    Map (function, sequence[, sequence, ...]) list

Description

Execute function (item) on item in sequence, and the result output is list.

Example:

 >>> Map (str, range (5)) #对range (5) for STR operations [' 0 ', ' 1 ', ' 2 ', ' 3 ', ' 4 '] #返回列表 >>> def A DD (n): Return n+n ... >>> map (add, Range (5)) #对range (5) for the add operation [0, 2, 4, 6, 8]>>> map (lambda x: X+x,range (5)) #lambda function, each + itself [0, 2, 4, 6, 8] 
 >>> map (lambda x:x+1,range) #lambda function, each +1 
 [1, 2, 3, 4, 5, 6, 7, 8, 9, [] 
 >>> map (Add, ' zhoujy ') [' zz ', ' hh ', ' oo ', ' uu ', ' JJ ', ' yy '] #想要输入  multiple sequences 
    , need to support multiple parameters of the function, note that the length of each sequence must be the same, otherwise error:>>> def add (x, y): return x+y ... >>> map (add, ' zhoujy ', ' Python ') [' ZP ', ' hy ', ' ot ', ' uh ', ' Jo ', ' yn ']>>> def add (x, Y, z): Return x+y+z ... >>> map (add, ' zhoujy ', ' Python ', ' Test ') # ' test ' is longer than the other 2 small traceback (most recent call last): File "<stdin>", line 1, in <module>typ Eerror:add () takes exactly 2 arguments (3 given) >>> map (Add, ' zhoujy ', ' Python ', ' testop ') [' zPt ', ' Hye ', ' ots ', ' u HT ', ' Joo ', ' YNP '] 

Two: Reduce

Reduce (...)    Reduce (function, sequence[, initial]), value

Description

For the item order in sequence, the function must be called with 2 parameters. If there is a 3rd argument, the initial value can continue to be called, returning a value.

Example:

>>> def add (x, y): return x+y ... >>> reduce (add,range)        #1 +2+3+...+945>>> Reduce (add , Range (one))        #1 +2+3+...+1055>>> reduce (lambda x,y:x*y,range (1,3), 5)           #lambda function, 5 is the initial value, 1*2*510> >> Reduce (lambda x,y:x*y,range (1,6))             #阶乘,1*2*3*4*5120>>> reduce (lambda x,y:x*y,range (1,6), 3)           #初始值3, and the results *3360 again.
>>> reduce (lambda x,y:x+y,[1,2,3,4,5,6])          #1 +2+3+4+5+6

Three: Filter

Filter (...)    Filter (function or None, sequence), list, tuple, or string

Description

Execute function (item) on item in sequence and execute the result to True (! =0) item consists of a list/string/tuple (depending on the type of sequence) returned, false exits (0) for filtering.

Example:

>>> def div (n): Return n%2 ... >>> Filter (Div,range (5))                    #返回div输出的不等于0的真值 [1, 3]>>> Filter (Div,range) [1, 3, 5, 7, 9]>> > Filter (lambda x:x%2,range)        #lambda function returns an odd number, returning the list [1, 3, 5, 7, 9]>>> filter (lambda x:not x%2,range (1 0)) [0, 2, 4, 6, 8]>>> def fin (n): return n!= ' z '                #过滤 ' z ' function, Z returns false ... >>> filter (FIN, ' zhoujy ') c4/># ' z ' is filtered ' houjy ' >>> filter (lambda x:x! = ' Z ', ' zhoujy ')     #labmda返回True值 ' houjy ' >>> filter ( Lambda x:not x== ' z ', ' zhoujy ')  #返回: String ' houjy '

A lambda expression is used in all of the examples above, for example, more information here.

>>> A=lambda x:x+3>>> A (2) 5>>> A=lambda x,y:x+y>>> A (2,3) 5

Iv. Application of Map,reduce,filter

1): Achieve 5!+4!+3!+2!+1!

#!/usr/bin/env python#-*-coding:utf-8-*-def add_factorial (n):    empty_list=[]           #声明一个空列表, save the results of each factorial to facilitate the addition of these results For    i in map (lambda X:x+1,range (n)):    #用传进来的变量 (n) to generate a list with the map +1,eg:range (5) = [1,2,3,4,5]        a= Reduce (lambda x,y:x*y,map (i))   #生成阶乘, remove 0        empty_list.append (a) from the list by using map            Append the factorial result to the empty list    return empty_listif __name__ = = ' __main__ ':    import sys#2 Select # (a)    try:        n = input (" Enter a number (int): ")        result=add_factorial (n)   #传入变量        print reduce (lambda x,y:x+y,result)      # Factorial result addition    except (nameerror,typeerror):        print "That's not a number!" # (ii) #    result = add_factorial (int (sys.argv[1]))   #传入变量 #    print reduce (lambda x,y:x+y,result)      # Factorial result addition

Results:

2): Select the prime number within 100~200

Ideas:

Prime numbers are: only 1 and its own two factors, such as 2, 3, 5, 7 are prime numbers, which can be divisible by 1 and itself, 1 is not prime.
For example, a number N, to see if it is prime, it is necessary to see: There is no divisible "2,n" between the number of X (does not contain itself), that is, whether the n%x is 0, if not for prime numbers.
So the algorithm we're going to implement is: Take a number N, remove the number x between "2,n" to get prime numbers, namely: N/2,n/3,......,n/n-2,n/n-1 ===> N/range (2,n)

#!/usr/bin/env python#-*-coding:utf-8-*-def is_prime (start,stop):    Stop  = stop+1     #包含列表右边的值    prime =  Filter (lambda X:not [x%i for I in range (2,x) if x%i = = 0],range (start,stop))   #取出质数, x number of print from range (start,stop)    Primeif __name__ = = ' __main__ ':    try:        start = input ("Enter A start Number:")    except:        start = 2   #开 Start value default 2    try:        Stop  = input ("Enter a Stop number  :")    except:        stop  = 0   #停止数, default 0, That is, no value is returned    is_prime (start,stop)

Results:

[Email protected]:~/desktop/python/4$ Python prime.py Enter A start number:enter a stop  number:10[2, 3, 5, 7][email Prote cted]:~/Desktop/python/4$ Python prime.py Enter A start number:10enter a stop  number:20[11, +, 19][email protected]: ~/Desktop/python/4$ Python prime.py Enter A start number:enter a stop number  : []

Python "Map, reduce, filter" built-in function usage instructions