Python High-order function--map/reduce

This content I refer to Liaoche's blog, excerpt from some of the content, with a solution to his final problem code.

2. This is the grammar I have never seen in C + + (maybe I didn't know it) and it took me ten or 20 minutes to understand it. It provides a link to Google's paper: "Mapreduce:simplified Data processing on Large Clusters", is said to be a very good article. When I understood the concept, I felt it was really convenient.
3. Look at map first.map()The function receives two parameters, one is a function and the other isIterable，mapFunctions the passed-in function sequentially to each element of the sequence and takes the result as a newIteratorReturn.

   For example, we have a function f (x) =x2, to function on a list [1, 2, 3, 4, 5, 6, 7, 8, 9] , it can be map() implemented as follows:
   
   

               f(x) = x * x                  │                  │  ┌────────────--------───┐  │   │   │   │   │   │   │   │   │  ▼   ▼   ▼   ▼   ▼   ▼   ▼   ▼   ▼[ 1    2    3    4    5    6    7    8    9 ]  │   │   │   │   │   │   │   │   │  │   │   │   │   │   │   │   │   │  ▼   ▼   ▼   ▼   ▼   ▼   ▼   ▼   ▼[ 1    4    9   16    25   36   49   64   81 ]

   Now, we use Python code to implement:

   1 def f (x): 2 ...     return x * x3 ... 4 >>> r = map (f, [1, 2, 3, 4, 5, 6, 7, 8, 9])5 >>> list (r)
        
         6 [1, 4, 9, 16, 25, 36, 49, 64, 81]
        

   map()The first parameter passed in is the f function object itself. Since the result r is one Iterator , it is an Iterator inert sequence, so the list() function allows it to calculate the entire sequence and return a list.

4. Look againreduceThe usage.reduceA function that acts on a sequence[x1, x2, x3, ...], this function must receive two parameters,reduceThe result continues and the next element of the sequence is accumulated, and the effect is:
   

   reduce(f, [x1, x2, x3, x4]) = f(f(f(x1, x2), x3), x4)
   
   For example, to sum a sequence, it can be implemented with reduce:

    from Import Reduce def Add (x, y): ...      return x + y ... >>> reduce (add, [1, 3, 5, 7, 9])25

   This example is not very useful in itself, but if we consider that the string str is also a sequence, with a slight change to the above example, map() we can write the str converted int function:

   >>> fromFunctoolsImportReduce>>>deffn (x, y): ...returnX * 10 +y ...>>>defChar2num (s): ... digits= {'0': 0,'1': 1,'2': 2,'3': 3,'4': 4,'5': 5,'6': 6,'7': 7,'8': 8,'9'8 {}...     returnDigits[s] ...>>> reduce (FN, map (Char2num,'13579'))13579

5. Practice
   1. Usemap()function, the user entered an irregular English name, into the first letter uppercase, other lowercase canonical name. Input:[‘adam‘, ‘LISA‘, ‘barT‘]Output:[‘Adam‘, ‘Lisa‘, ‘Bart‘]：
      

      # -*-coding:utf-8-*- def Normalize (name):     return name.capitalize () # Test: L1 = ['Adam'LISA'BarT '  = List (map (normalize, L1))print(L2)

   2. Python provides thesum()function can accept a list and sum, write aprod()function, you can accept a list and use thereduce()Seeking Product
      

      #-*-coding:utf-8-*- fromFunctoolsImportReducedefprod (L):deffn (x, y):returnx*yreturnreduce (fn,l)Print('3 * 5 * 7 * 9 =', prod ([3, 5, 7, 9]))ifProd ([3, 5, 7, 9]) = = 945:    Print('Test Success!')Else:    Print('Test failed!')

   3. UsemapAndreduceWrite astr2floatfunction, put the string‘123.456‘Convert to floating point123.456：
      

      #-*-coding:utf-8-*- fromFunctoolsImportReducedefStr2float (s): DIGITS= {'0': 0,'1': 1,'2': 2,'3': 3,'4': 4,'5': 5,'6': 6,'7': 7,'8': 8,'9'8 {} POS= Len (s)-s.index (".")-1#looking for decimal digits    defChar2num (MY_STR):if(My_str! ="."):            returnDigits[my_str]deffn (x, y):ify==None:returnxElse:            return10*x+yreturnReduce (Fn,map (char2num,s))/(10**POS)Print('str2float (\ ' 123.456\ ') =', Str2float ('123.456'))ifABS (Str2float ('123.456')-123.456) < 0.00001:    Print('Test Success!')Else:    Print('Test failed!')

Python High-order function--map/reduce