Python built map()and reduce()function.
If you read Google's famous essay papers " MapReduce: Simplified ON the Data Processing Large Clusters ", you can probably understand the concept for map / reduce.
We look at the map. map()Function takes two parameters, a function, a is Iterable, mapto pass successively to the function of each element of the sequence, and the result as a new Iteratorreturn.
Illustration, we have such a function f (x) = x2, should this function on a list [1, 2, 3, 4, 5, 6, 7, 8, 9]on, can be used map()to achieve the following:
f(x) = x * x
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[ 1 2 3 4 5 6 7 8 9 ]
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[ 1 4 9 16 25 36 49 64 81 ]
Now, we use the Python code to achieve:
>>> def f(x): ... return x * x ... >>> r = map(f, [1, 2, 3, 4, 5, 6, 7, 8, 9]) >>> list(r) [1, 4, 9, 16, 25, 36, 49, 64, 81] map()The first argument is fthat the function object itself. Since the result ris a Iterator, Iteratoris an inert sequence, so by list()the function it calculated the entire sequences and returns a list.
You might be thinking, no map()function, write a loop, you can calculate the results:
L = []
for n in [1, 2, 3, 4, 5, 6, 7, 8, 9]: L.append(f(n)) print(L) Indeed, however, from the top of the loop code, we can understand at a glance "the f (x) action and the results to generate a new list in each element list of" do?
Therefore, map()as a higher-order functions, in fact it is the abstract operation rules, therefore, we can not only computationally simple f (x) = x2, also in arbitrary complex functions, for example, this list all numbers into a string :
>>> list(map(str, [1, 2, 3, 4, 5, 6, 7, 8, 9]))
['1', '2', '3', '4', '5', '6', '7', '8', '9'] Only one line of code.
Look reduceusage. reduceA role in the function of a sequence [x1, x2, x3, ...], the function must receive two parameters, reducethe results and continues to the next element in the sequence do cumulative basis, the effect is:
reduce(f, [x1, x2, x3, x4]) = f(f(f(x1, x2), x3), x4)
Let's say a sequence of summation, it can be used reduceto achieve:
>>> from functools import reduce
>>> def add(x, y): ... return x + y ... >>> reduce(add, [1, 3, 5, 7, 9]) 25 Of course, you can use Python's built-summation function directly sum(), no need to use reduce.
But if we want the sequence [1, 3, 5, 7, 9]into an integer 13579, reduceyou can come in handy:
>>> from functools import reduce
>>> def fn(x, y): ... return x * 10 + y ... >>> reduce(fn, [1, 3, 5, 7, 9]) 13579 This example itself is not much use, however, considering the string stris a sequence, for example above a little change, with map(), we can write the strconvert intfunction:
>>> from functools import reduce
>>> def fn(x, y): ... return x * 10 + y ... >>> def char2num(s): ... digits = {'0': 0, '1': 1, '2': 2, '3': 3, '4': 4, '5': 5, '6': 6, '7': 7, '8': 8, '9': 9} ... return digits[s] ... >>> reduce(fn, map(char2num, '13579')) 13579 Organized into a str2intfunction is:
from functools import reduce
DIGITS = {'0': 0, '1': 1, '2': 2, '3': 3, '4': 4, '5': 5, '6': 6, '7': 7, '8': 8, '9': 9} def str2int(s): def fn(x, y): return x * 10 + y def char2num(s): return DIGITS[s] return reduce(fn, map(char2num, s)) It can be further simplified using lambda functions to:
from functools import reduce
DIGITS = {'0': 0, '1': 1, '2': 2, '3': 3, '4': 4, '5': 5, '6': 6, '7': 7, '8': 8, '9': 9} def char2num(s): return DIGITS[s] def str2int(s): return reduce(lambda x, y: x * 10 + y, map(char2num, s)) That is, assuming Python does not provide int()a function, you can write your own function to convert a string to an integer, and requires only a few lines of code!
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