• JS foundation — regular expression explanation


    Regular expressions are very useful to find, match, process strings, replace and transform strings, input and output, etc. Here are some common regular expressions. Common regular characters Regular character description \: marks the next character as a special character, a literal character, a backward reference, or an octal escape character. For example, ‘n’ matches the […]

  • JavaScript serialization 24 regular expressions


    1、 Regular expression 1. Basic concepts (brief) Three functions: matching, extracting and replacing Composition of regular expressions: ordinary characters and metacharacters Metacharacter Metacharacter describe \d Match a numeric character, equivalent to [0-9] \D Matches a non numeric character, equivalent to [^ 0-9] \w Match any word character including underline, equivalent to [a-za-z0-9] \W Matches any […]

  • React: asynchronous update of usestate


    introduce function App() { const [n, setN] = useState(0) const onClick = ()=>{ setN(n+1) Set n (n + 1) // at this time, we find that n can only + 1, not + 2 // setN(i=>i+1) // setN(i=>i+1) } return ( <div className=”App”> <h1>n: {n}</h1> <button onClick={onClick}>+2</button> </div> ); } ReactDOM.render(<App />, document.getElementById(“root”)); The react […]

  • Relational algebra


    Relational algebra Abstract query language, using relational operations to express queries Operands: relationships Operation result: relation       Traditional relational operation (set relational operation) 1. Union operation (ν) R ν s: put the two relations together and eliminate the repetition   2. Intersection (∩) R ∩ s: the repetitive part of two relations   […]

  • Analysis of five join strategies of spark


    Join operation is a very common data processing operation. As a unified big data processing engine, spark provides very rich join scenarios. This article will introduce five join strategies provided by spark, hoping to help you. This paper mainly includes the following contents Factors affecting join operation Five strategies of join execution in spark How […]

  • Regular expressions commonly used in JS


    A common regularization method   1   \  :Marks the next character as a special character, an original character, a backward reference, or an octal escape character. For example, “d” matches a numeric character, not the letter D.   2  ^  : matches the start position of the input. If the multiline flag is set to true, […]

  • cmder tips(qbit)


    Releases:https://github.com/cmderdev/cmder/releases/ Right click a directory to open the cmder cdmer /register user or cmder /register all If the cmder prompts that api-ms-win-crt-runtime-l1-1-0.dll is missing, installVc2015 runtimeThat’s it. Chinese characters overlap: Settings > main, cancel “monospace”. Add alias: edit the config / aliases file. For example: ll=ls -alF –show-control-chars 1.3. X: Settings > startup > environment. set LANG=zh_CN.UTF-8 CHCP UTF-8 # is […]

  • Regular expression metacharacter


    Common metacharacter table Match character Metacharacter explain \ Marks the next character as a special character, or a literal character, or a backward reference, or an octal escape character. For example, ‘n’ matches the character ‘n’. ‘\ n’ matches a newline character. The sequence ‘\ \’ matches’ \ ‘and’ \ (‘matches’ (‘ \cx Matches the […]

  • Python text processing and regular expressions


    [unfinished draft]This week, I encountered a lot of work that needed to add, delete, check and modify a character. For the first time, I used my own script to do it. As a result, the reusability was not high, and the efficiency was not very good. I also used the regular expression part re in […]

  • MySQL learning notes: index failure


    Series articles: MySQL learning notes: combination index – leftmost principle MySQL learning notes: Composite Index – index push down MySQL learning notes: index failure When type in explain is range and rows is too large: For the meaning of rows and the enumeration value of type, please refer to:EXPLAIN Output Format Query threshold example: General […]

  • How to find the intersection, union, difference and symmetric difference set of the set in Python.


    In fact, intersection, union, difference and symmetric difference sets are also special operations of sets. A & B: the common element of two sets of intersection table, equivalent to A. intersection (b) A | B: Union table all elements of two sets, equivalent to A. Union (b) A-B: elements of difference table belonging to a […]

  • Unveiling tidb new optimizer: analysis of cascade planner principle


    Author: Mingcong Han In “ten minutes as a contributor series | adding optimization rules for cascades planner”, we briefly introduced the background knowledge of cascades. This paper will introduce the framework and principle of cascades planner, a new optimizer of tidb. Brief introduction of tidb current optimizer The function of query optimizer in relational database […]