Python, map / reduce usage

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 the sequence, mapthe function is passed sequentially applied to each element of the sequence, and to return the result as a new list.

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:

Now, we use the Python code to achieve:

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

map()The first argument is fthat the function object itself.

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 :

>>> 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 reduce the usage. reduce to a function on a sequence [x1, x2, x3 ...], this function must receive two parameters, and continue to reduce the result of 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 reduce implementation:

>>> 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 integer 13579, reduce can come in handy:

>>> 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:

>>> 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]
...
>>> reduce(fn, map(char2num, '13579'))
13579

Organized into a str2intfunction is:

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))

It can be further simplified using lambda functions to:

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]

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!

Python, map / reduce usage

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