Python map and reduce function usage example, pythonreduce

Python map and reduce function usage example, pythonreduce

First look at map. The map () function receives two parameters. One is a function, and the other is a sequence. map sequentially applies the input function to each element of the sequence and returns the result as a new list.

For example, if we have a function a (x) = x * 2, we need to apply this function to a list [1, 2, 3, 4, 5, map () can be implemented as follows:
Copy codeThe Code is as follows:
>>> Def a (x ):
... Return x * 2
...
>>> Map (a, [1, 2, 3, 4, 5])
[2, 4, 6, 8, 10]

The first parameter a passed in by map is function a. You can also implement this function without the map function:

Copy codeThe Code is as follows:
>>> List = []
>>> For I in [1, 2, 3, 4, 5]:
... List. append (a (I ))
...
>>> Print list
[2, 4, 6, 8, 10]

In terms of code quantity, map is much simpler. Therefore, map (), as a high-order function, actually abstracts the calculation rules. Therefore, we can not only calculate a (x) = x * 2. You can also compute any complex functions. For example, convert all the numbers in this list into strings:

Copy codeThe Code is as follows:
>>> Map (str, [1, 2, 3, 4, 5])
['1', '2', '3', '4', '5']
>>>

You only need a line of code. Let's take a look at the exercises from Gu Xuefeng's python Tutorial: Use the map () function to change the nonstandard English names entered by users into uppercase letters and other standard names in lower case. Input: ['Adam ', 'lisa', 'bart'], and output: ['Adam ', 'lisa', 'bart']. As an individual, I may first convert all nonstandard English names to lowercase letters, and then use the capitalize () function to convert the first letter to write. The Code is as follows:

Copy codeThe Code is as follows:
>>> Def caps (name ):
... Return name. capitalize ()
...
>>> Def lowers (name ):
... Return name. lower ()
...
>>> Map (caps, map (lowers, ['Adam ', 'lisa', 'bart'])
['Adam ', 'lisa', 'bart']

Let's look at the reduce usage. Reduce (function, sequence, starting_value): calls the function sequentially for items in sequence. If starting_value exists, it can also be called as an initial value. For example, it can be used to sum the List:

Copy codeThe Code is as follows:
>>> Def add (x, y ):
... Return x + y
...
>>> Reduce (add, [1, 3, 5, 7, 9])
25
>>> Reduce (add, range (1, 11 ))
55
>>> Reduce (add, range (1, 11), 20)
75

Of course, the sum operation can directly use the Python built-in function sum (), and there is no need to use reduce. However, to convert the sequence [1, 2, 3, 4, 5, 6, 7] to an integer of 1234567, reduce can be used in the following ways:

Copy codeThe Code is as follows:
>>> Def fn (x, y ):
... Return x * 10 + y
...
>>> Reduce (fn, [1, 3, 4, 5, 6, 7])
134567