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This article mainly introduces how python uses list derivation, which is very detailed and has a certain reference value. Friends who are interested must finish reading it!

one。 What is deductive?

Deduction is a method to create data types quickly and succinctly from one or more iterators. it combines loops with conditional judgment to avoid code with lengthy syntax and improve the efficiency of the code. Proficiency in the use of deduction can also indirectly show that you have exceeded the level of beginners in python.

two。 Conditional derivation 1. Syntax''value1: return value1 if the conditional expression condition is true; return value2 if the conditional expression is not valid; condition: conditional expression Value2: return value1 if the conditional expression condition is true; return value2 if the conditional expression is not true;' 'value1 if condition else Value22. Actual combat practice

If there is a need to judge whether a number is odd or even?

#! usr/bin/env python#-*-coding:utf-8 _ *-"" @ Author: why @ Blog (personal blog address): https://www.codersrc.com@WeChat Official Account (official Wechat account): ape says python@Github:www.github.com @ File:python_list.py@Time:2019/9/30 20:45 @ Motto: no accumulation of steps, no accumulation of streams, no success of rivers and seas Program the splendor of life needs to be accumulated unremittingly! # novice code x = 10if x% 2 = 0: print ("novice says: X is even") else: print ("novice says x is odd") # veteran driver x = 15print ("veteran driver says x is even") if x% 2 = 0 else print ("veteran driver says x is odd")

Output result:

The novice said: X is an even number

The old driver said: X is an odd number.

Looking at the above code, we can see that contestant 1 (novice) takes up a total of 5 lines of code, while contestant 2 (veteran driver) only needs two lines to complete the same function, which is the ratio of performance to price. Often this is the details of identifying work ability / promotion and salary in the workplace.

three。 List derivation

List deduction is a conditional derivation that works with a loop and returns a list, and the entire expression needs to be within [], because the return value is also a list.

1. Syntax 1: exp1: in the for loop, if the value of x satisfies the conditional expression condition (that is, the conditional expression holds), exp1 is returned If the conditional expression is not valid, the variable data in the x:for loop is not returned: a sequence (for example, list / tuple / string, etc.) condition: conditional expression''[exp1 for x in data if condition]''Syntax 2: exp1: in the for loop, if the value of x satisfies the conditional expression condition (that is, the conditional expression holds), return exp1 If the conditional expression is not valid, return exp2 condition: conditional expression exp2: in the for loop, if the value of x satisfies the conditional expression condition (that is, the conditional expression holds), and if the conditional expression returns exp1;, the variable data: sequence (e.g. list / tuple / string, etc.)''[exp1 if condition else exp2 for x in data] 2 is returned in the exp2 x:for loop. Actual combat practice

Requirement 1: get all the even numbers of 020 and multiply it by 10, and return all the calculated results. The sample code is as follows: (using list-derived syntax-implementation)

'' 1. Because it is to get exp13.x%2 20, including 20, we use exp13.x%2 = = 0 in range (0Magne21) 2.x*10 equivalent grammar 1, conditional expression condition4.range (0Magazine 21) data (sequence) in equivalent grammar 1 'list1 = [Xuan 10 for x in range (0Magne21) if x% 2 = = 0] print (list1) print (type (list1))

Output result:

[0, 20, 40, 60, 80, 100, 120, 140, 160, 180, 200]

Requirement 2: multiply the even number of 020 by 10, the odd number by 100, and return all the calculated results. The sample code is as follows: (implemented using list-derived syntax 2)

'' 1. Because it is to get 0 else 20, including 20, we use exp24.x%2 = = 0 in exp13.x*100 equivalent grammar 2 of range (0Magne21) 2.x*10 equivalent grammar 2, the conditional expression condition5.range in equivalent grammar 2 (0Magne21) data (sequence)''in equivalent grammar 1, list2 = [x = 0 else x% 2 = 0 else x% 2)] print (list2) print (type (list2))

Output result:

[0, 100, 20, 300, 40, 500, 60, 700, 80, 900, 100, 1100, 120, 1300, 140, 1500, 160, 1700, 180, 1900, 200]

3. Efficiency comparison

There may be children's shoes wonder, I clearly for cycle can achieve the function, why do you want to use this bird-driven type?

The efficiency of using list derivation is much higher than that of for loops. It may only take 0.0002 seconds to execute a print ("helloworld") for cpu. You may not feel the difference. What if you need to output 100 million helloworld times? Often the details feel success or failure!

If there is a requirement to store all integers less than 10000000 in the list, compare the list derivation with the time consuming of the for loop:

Import time # add time module Used to count the running time of the code # add data to the list for a total of 10000000 times total_num = 10000000 # use list inference start_time = time.time () list1 = [x for x in range (0) Total_num)] # list-derived end_time = time.time () print ("using list-derived time: {} seconds" .format (end_time-start_time)) # uses a normal for loop start_time = time.time () list2 = list () for x in range (0 Total_num): # for loop list2.append (x) end_time = time.time () print ("use normal for loop time: {} seconds" .format (end_time-start_time))

Output result:

Time to use list deduction: 0.5455152988433838 seconds

Time to use a normal for loop: 1.2068836688995361 seconds

The output result is obvious, to achieve general functions, the efficiency of list derivation is exactly twice as high as that of ordinary for loops.

The above is all the content of the article "how to use list-driven python". Thank you for reading! Hope to share the content to help you, more related knowledge, welcome to follow the industry information channel!

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