Python---map/reduce

Map () and reduce () functions are built in # map/reducefrom functools import reduce# python # map () The function receives two parameters, one is the function, the other is Iterable,map the passed function to each element of the sequence sequentially, and returns the result as a new iterator def f (x):     Return x * xr = map (f, [1, 2, 3, 4, 5, 6, 7,  8, 9]) Print (list (r)) print (R) # map () the first parameter passed in is F, which is the function object itself #  because the result R is a iterator,iterator is an inert sequence, so through list ( function allows it to calculate the entire sequence and return a list# map as a higher order function, in fact it abstracts the arithmetic rules, it can calculate any complex function #  the list all the numbers into the string l = list (map ( STR, [1, 2, 3, 4, 5, 6, 7, 8, 9]) print (l) #  Reduce takes a function in a sequence [x1, x2, x3,  ...], this function must receive two parameters # reduce the result continues and the next element of the sequence is calculated #  The effect is   reduce (f, [x1, x2, x3, x4])  = f (f (f (x1, x2),  x3),  x4) #  uses reduce to sum a sequence of Def add (x, y):     return x + ys =  reduce (ADD, [1, 3, 5, 7, 9]) print (s) #  of course, the sum operation can be directly in Python built function sum (), no need to use reduce#  but if you want to put the sequence [1, 2, 5, 7,  9] transform into an integer 13579,reduce can come in handy Def fn (x, y):     return x * 10  + ys = reduce (Fn, [1, 3, 5, 7, 9]) print (s) #  consider the string str is also a sequence , with map (), we can write the function def fn (x, y) that converts str to int:    return x * 10  + ydef char2num (s):     return {' 0 ': 0,  ' 1 ': 1,  ' 2 ':  2,  ' 3 ': 3,  ' 4 ': 4,  ' 5 ': 5,  ' 6 ': 6,  ' 7 ': 7,  ' 8 ':  8,  ' 9 ':  9}[s]s = reduce (Fn, map (char2num,  ' 13579 ')) print (s) #  Organize into a str2int function def str2int (s):     def fn (x, y):         return x * 10 + y    def char2num (s) :         return {' 0 ': 0,  ' 1 ': 1,  ' 2 ': 2,  ' 3 ': 3,  ' 4 ':  4,  ' 5 ': 5,  ' 6 ': 6,  ' 7 ': 7,  ' 8 ': 8,  ' 9 ': 9}[s]     return reduce (Fn, map (char2num, s)) s = str2int (' 112233 ') print (s) #   Further simplifies def char2num (s) with the Lambada function:     return {' 0 ': 0,  ' 1 ':  1,   ' 2 ': 2,  ' 3 ': 3,  ' 4 ': 4,  ' 5 ': 5,  ' 6 ': 6,  ' 7 ':  7,   ' 8 ': 8,  ' 9 ':  9}[s]def str2int (s):     return reduce (lambda  x, y: x * 10 + y, map (char2num, s)) print (Str2Int (' 9999 ')) #  The use of lambda functions is not described in detail here

Python---map/reduce