python的几个内建函数:apply(),filter(),map(),reduce()（转）

这四个函数用得非常爽，特别在对字符串切割方面，一句话搞定，Nice

＝＝＝＝＝＝＝＝＝＝华丽丽的分割线，转载如下＝＝＝＝＝＝＝＝＝＝＝＝＝＝

apply()函数: apply(func[,nkw][,kw]) 它的返回值就是func函数的返回值

filter()函数:  filter(func,seq) 调用bool函数func，遍历处理序列中seq的每个元素。它的返回值是 一个序列，其元素都是让func函数返回true值的原seq序列中的元素

map()函数:

def map(func,seq):
    mapped_seq = []
    for eachItem in seq:
        mapped_seq.append(apply(func,eachItem))
    return mapped_seq

reduce()函数:reduce(func,seq[,init]),用二元函数func对序列seq中的元素进行处理，每次处理两个 数据项(一个是前次处理的结果，一个是序列中的下一个元素)，如此反复的递归处理，最后对整个序列求出一个单一的返回值。

1.apply()函数

 1.1 apply()函数对于调用那些参数是动态生成的函数是非常方便的，一般牵扯到拼凑一个参数清单这种问题

matheasy.py

#!/usr/bin/env/ python
from string import lower
from operator import add,sub,mul
from random import randint,choice

ops = {'+':add,'-':sub,'*':mul}
MAXTRIES = 2

def doprob():
    op = choice('+-*')
    nums = [randint(1,10),randint(1,10)]
    nums.sort();nums.reverse()
    ans = apply(ops[op],nums)
    pr = '%d %s %d = ' %( nums[0],op,nums[1])
    oops = 0
    while 1:
        try:
            if int(raw_input(pr)) == ans:
                print "correct"
                break
            if oops == MAXTRIES:
                print 'answer\n%s%d' %(pr,ans)
                break
            else:
                print 'incorrect ... try again'
                oops = oops + 1
        except(KeyboardInterrupt,EOFError,ValueError):
            print 'invalid input data ... try again'
           
def main():
    print 'welcome to play this game:'
    while 1:
        doprob()
        try:
            opt = lower(raw_input('Try again? ([y]/n)'))
        except(KeyboardInterrupt,EOFError):
            print ;break
        if opt and opt[0] =='n':
            print 'quit the game,byebye'
            break

if __name__ == '__main__':
    main()
           
       
       
       
       
       
 1.2 apply()函数对于应用程序的调试纠错和性能测试方面也是很有用的。一般是编写一个诊断性函数来建立测试环境，然后调用准备对它进行测试的函数，考虑 到系统的灵活性和适应性，被测试函数作为一个参数传递进去

testit.py

#!/usr/bin/env/ python
def testit(funcname,*nkwargs,**kwargs):
    try:
        retval = apply(funcname,nkwargs,kwargs)
        result = (1,retval)
    except Exception,diag:
        result = (0,str(diag))
    return result

def test():
    funcs = (int,long,float)
    vals = (1234,12.34,'1234','12.34')
   
    for eachFunc in funcs:
        print '-'*40
        for eachVal in vals:
            retval = testit(eachFunc,eachVal)
           
            if retval[0]:
                print '%s(%s) = ' %(eachFunc.__name__,eachVal),retval[1]
            else:
                print '%s(%s) = FAILED ' %(eachFunc.__name__,eachVal),retval[1]
               
if __name__ == '__main__':
    test()
       

2.filter()函数

oddnumgen.py

from random import randint
def odd():
    allNums = []
    for eachNum in range(100):
        allNums.append(randint(1,1000))
    oddNums = filter(lambda n:n%2,allNums)
    print "length of sequence = %d\n" %(len(oddNums)),oddNums

3.map()函数

>>>map(lambda x,y:(x+y,x-y),[1,3,5],[2,4,6])
>>> [(3, -1), (7, -1), (11, -1)]

from string import strip,upper
def strupper():
    f = open('map.txt')
    lines = f.readlines()
    f.close()
    print ' ... BEFORE processing ...'
    for eachLine in lines:
        print '<%s>' %eachLine[:-1]#去掉换行符
    print "\n"
    print ' ... AFTER processing ...'   
    for eachLine in map(upper,map(strip,lines)):#strip不清理字符串之间的空格
        print '<%s>' %eachLine

def rounder():
    f = open('round.txt')             
    values = map(float,f.readlines())
    f.close()
   
    print 'original\trounded'
    for eachVal in map(None,values,map(round,values)):#函数为None,那么参数列表中的n个序列(此时为values和 map(round,values) )组成了返回值的列表
        print '%6.02f\t\t%6.02f' %eachVal

4.reduce()函数

 >>> print 'the total is:',total = reduce((lambda x,y:x+y),range(1000))