Map (), reduce (), filter, and pythonreduce in python

Map (), reduce (), filter, and pythonreduce in python

It is said that it is a function in functional programming (and some tucao py is not suitable for doing this). In my opinion, it is a method of pythonic.

This simplifies our operations. For example, if we want to add 1 to all the numbers in the list, the most basic thing is to write a function:

In [40]: def add_one(i):   ....:     return i+1   ....: In [41]: for i in range(1, 3):   ....:     print add_one(i)   ....:     23

If map is used, it is simpler:

In [42]: map(add_one, range(1, 3))Out[42]: [2, 3]

 

Actually, pythonic is not enough. After all, we forget the lambda anonymous function.

　

In [44]: map(lambda x: x+1, range(1, 3))Out[44]: [2, 3]

 

Reduce is used to perform operations on iteratable objects in sequence, for example, adding or multiplication objects in sequence.

The built-in descriptions are as follows:

Help on built-in function reduce in module __builtin__:reduce(...)    reduce(function, sequence[, initial]) -> value        Apply a function of two arguments cumulatively to the items of a sequence,    from left to right, so as to reduce the sequence to a single value.    For example, reduce(lambda x, y: x+y, [1, 2, 3, 4, 5]) calculates    ((((1+2)+3)+4)+5).  If initial is present, it is placed before the items    of the sequence in the calculation, and serves as a default when the    sequence is empty.(END)

The usage is as follows:

In [18]: reduce(lambda x, y: x*y, [1, 2, 3])Out[18]: 6

In [19]: reduce (lambda x, y: x * y, [1]) Out [19]: 1In [20]: reduce (lambda x, y: x * y, [], 0) # The last one is the initial value Out [20]: 0

　　

　

Filters are used to filter iteratable objects in a way similar to the preceding two methods:

Help on built-in function filter in module __builtin__:filter(...)    filter(function or None, sequence) -> list, tuple, or string        Return those items of sequence for which function(item) is true.  If    function is None, return the items that are true.  If sequence is a tuple    or string, return the same type, else return a list.

In [22]: filter(lambda x: x > 5, range(10))Out[22]: [6, 7, 8, 9]

　

 

The basic usage method is roughly the same as above. I may feel that it is of little use at the beginning. In fact, I will feel very comfortable when I get used to it.

For example, if you want to calculate the sum of 1 to 100, there may be only a few lines of code written by the function, but when you use reduce, there is only one line.

You can use this method to calculate 1! + 2! + .... + 100!

 

(The built-in documentation is awesome ~

 

See:

Http://my.oschina.net/zyzzy/blog/115096

Https://eastlakeside.gitbooks.io/interpy-zh/content/Map%20&%20Filter/index.html