Scala programming (intermediate advanced experimental application)

I talked to you earlier scala Introduction to programming , Let's share something about scala Intermediate application and code skills .
Go straight to the dry goods ~

https://blog.csdn.net/myself_ning/article/details/125945435?spm=1001.2014.3001.5501

Word frequency statistics and sorting （ example ）

import java.util
import scala.collection.mutable
import scala.io.Source

object Test1 {
    
  def main(args:Array[String]): Unit ={
    
    // route 
    var path = "F:\\scala\\article.txt";
    // Read data as string 
    var article = getArticle(path);
    // Count the word frequency of the string content 
    articleCount(article);

  }

  /* Read text , Save as string */
  def getArticle(path:String): String ={
    
    var content = Source.fromFile(path).getLines().mkString
    return content
  }

  /* Count the word frequency of the string */
  def articleCount(article:String): Unit ={
    
    // Split the article into several words 
    var words = article.split(" ").toList   //like : List(There, are, momen ...)

    // Convert words into tuples . list(my, name, is, zhangsan) ——> list((my, 1),(name, 1),(is, 1),(zhangsan, 1))
    var flatMap = words.map((_, 1));

    // Group tuples .  Take the first character of tuple （ key ）, To group .
    var wordGroup = flatMap.groupBy(_._1)

    // Count the   The number of elements is taken as the number of word frequencies . like Map(forgotten -> 1, you,you -> 1, side -> 1, for -> 2)
    var wordCount1 = wordGroup.map(x => (x._1, x._2.size))

    // Sort it out . First convert to list , Then sort 
    var wordCount = wordCount1.toList.sortBy(_._2).reverse;

    wordCount.foreach(println)
  }

}

mysql AIP

//  How to import  jdbc Drive pack 
https://blog.csdn.net/weixin_48185778/article/details/109451759?spm=1001.2101.3001.6650.1&utm_medium=distribute.pc_relevant.none-task-blog-2%7Edefault%7ECTRLIST%7Edefault-1.no_search_link&depth_1-utm_source=distribute.pc_relevant.none-task-blog-2%7Edefault%7ECTRLIST%7Edefault-1.no_search_link

// Add, delete, modify, check and develop 1
https://blog.csdn.net/weixin_48185778/article/details/109451759?spm=1001.2101.3001.6650.1&utm_medium=distribute.pc_relevant.none-task-blog-2%7Edefault%7ECTRLIST%7Edefault-1.no_search_link&depth_1-utm_source=distribute.pc_relevant.none-task-blog-2%7Edefault%7ECTRLIST%7Edefault-1.no_search_link

package com.lining
package com.lining.Hotitems
import java.sql.{
    Connection, DriverManager, PreparedStatement, ResultSet}


object mySql {
    
  class MySqlTool(var ip:String= "" ,var port:String= "",var user:String= "",var passwd:String= "",var DB:String = ""){
    
    private val url = s"jdbc:mysql://${
      ip}:${
      port}/${
      DB}?characterEncoding=utf8&useSSL=false"
    private var connection:Connection=null
    private val driver = "com.mysql.jdbc.Driver"

    //Connect to Mysql
    def getConnection():Unit={
    
      Class.forName(driver)
      connection=DriverManager.getConnection(url,user,passwd)
      println("Connect to the Database successfully!")
    }

    //EXECUTE
    def execute(sql:String): Unit ={
    
      val prStamt = connection.prepareStatement(sql)
      prStamt.executeUpdate()
    }

    //INSERT.
    def insert(sql:String)={
    
      execute(sql)
    }

    //DELETE.

    //UPDATE.

    //SELECT.
    def select(sql:String):Unit={
    
      println("SQL："+sql)
      val rs: ResultSet=connection.createStatement().executeQuery(sql)
      while (rs.next()){
    
        var id = rs.getString("id")
        var age = rs.getInt("cnt")
        println(id+"\t"+age)
      }
    }

  }

  def main(args: Array[String]): Unit = {
    
    val msT = new MySqlTool("139.196.237.231","3306","root","123456","test")
    msT.getConnection()

    val sql = "insert into tab1(id,cnt) values('a22',22);"
    msT.insert(sql)

    val sql2 = "select * from tab1;"
    msT.select(sql2)
  }

}

pgSql API

// Incomplete , Write it yourself , The general framework process is written 

private var connection: Connection = null
private var ps: PreparedStatement = null

/*-------------  Connect -------------- */
val driver = "org.postgresql.Driver"
val url = "jdbc:postgresql://139.196.237.231:5432/jobanalysis"
val username = "ln"
val password = "ning0852"
//1. The load driver 
Class.forName(driver)
//2. Create connection 
connection = DriverManager.getConnection(url, username, password)

/*-------------  perform -------------- */
val sql = s"insert into job_data(name,work_link,price,city) values('${
      value.name}','${
      value.workLink}','${
      value.price}','${
      value.city}'');"
ps = connection.prepareStatement(sql)
ps.executeUpdate()

/*-------------  close -------------- */
ps.close()
connection.close()

class PgTool(var ip:String= "139.196.237.231" ,
             var port:String= "5432",
             var user:String= "ln",
             var passwd:String= "ning0852",
             var DB:String = "jobanalysis"){
    

  private var connection: Connection = null
  private var ps: PreparedStatement = null
  private val driver = "org.postgresql.Driver"
  private val url = s"jdbc:postgresql://${
      ip}:${
      port}/${
      DB}"

  // Connect 
  def getConnection():Unit={
    
    //1. The load driver 
    Class.forName(driver)
    //2. Create connection 
    connection=DriverManager.getConnection(url,user,passwd)

    println("Connect to the Database successfully!")
  }

  // disconnect 
  def close():Unit={
    
    connection.close()
    println(" Database connection closed ...")
  }

  //SELECT
  def select(sql:String):List[String]={
    
    // Exhibition sql
    println("SQL："+sql)

    val rs: ResultSet=connection.createStatement().executeQuery(sql)
    val workNameList = ListBuffer[String]()

    while (rs.next()){
    
      workNameList += rs.getString("class")
      workNameList += rs.getString("name")
    }

    workNameList.toList
  }
}

scattered

scala Read the file

Read csv

import scala.io.Source

val sourceFile = Source.fromFile(" File path "," code ")
for(i <- sourceFile.getLines){
    
    println(i);
}

String.split() function

https://www.cnblogs.com/davidhaslanda/p/4050471.html

 Commonly used ：
S.split(" Separator ",-1)

Input

object compute {
    
  def main(args: Array[String]): Unit = {
    
    import  scala.io.StdIn
    // Scala Read data from the console 
    println("name")
    val name = StdIn.readLine()
    println("age")
    val age = StdIn.readInt()
    println("salary")
    val salary = StdIn.readDouble()
      
      
      
      
    //Java  from   The console reads data 
    System.out.println("name")
    val scanner= new Scanner(System.in)
    val nameJ= scanner.next()
    System.out.println("age")
    val ageJ = scanner.next()
    System.out.println("salary")
    val salaryJ = scanner.next()
  }
}

There is judgment 【.contains（） .exists（）】

// Study 
https://blog.csdn.net/power0405hf/article/details/50371438

val m=Map(1 -> 1, 2 -> 2, 3 -> 3, 4 -> 4)
// m: scala.collection.immutable.Map[Int,Int]= Map(1 -> 1, 2 -> 2, 3 -> 3, 4-> 4)

val l = List(1,2,3,4) 
// l:List[Int] = List(1,2,3,4)

val v = Vector(1,2,3,4)
// v: scala.collection.immutable.Vector[Int] = Vector(1,2,3,4)

val S = "12345"

m.exists(_._1 == 3)                       //> res0: Boolean = true
m.contains(3)                             //> res1: Boolean = true

l.exists(_ == 3)                          //> res2: Boolean = true
l.contains(3)                             //> res3: Boolean = true

v.exists(_ == 3)                          //> res4: Boolean = true
v.contains(3)                             //> res5: Boolean = true

S.contains(3)							  //> res6: Boolean = true

[ Failed to transfer the external chain picture , The origin station may have anti-theft chain mechanism , It is suggested to save the pictures and upload them directly (img-Ov836vQ8-1658547643165)(scala.assets/image-20211228150811358.png)]

doubt — answer

1. Why? main The method must be written in object in ？

	/* The program must take a function main As an entrance , But because there is no global function , So it must be written in the class , And it is not entered through object calls , So if it's static , but scala No, static This keyword and this semantics , So again obejct Companion objects, this kind of thing . Semantically, it is the instance method of the associated singleton object , It feels more object-oriented , Follow java/C#（ Are not purely object-oriented ） That's different .C++ Multiple paradigm , You can define global functions , therefore main Just direct the global function , There is no need to go around this bend .  The official answer is ： scala and Java equally , If you want to run a program , There has to be one main Method . And in the Java in main The method is static , And in the scala No static methods in . stay scala in , This main Methods must be placed in a singleton object .（ and scala Single column objects in object  Is similar to java in object.） 

2. Immutable list 、 Set 、 Array … etc. . Don't you mean immutable ？ Why are there some operations ？ Such as splicing …

/*  answer ： Yes . When doing some operations on immutable types , It is not operated on the original variable , Instead, generate a new variable . */


// List List together , Make a new one List
scala> val b = List(1, 2, 3) ++ List(1, 2, 3)
b: List[Int] = List(1, 2, 3, 1, 2, 3)

scala> b
res8: List[Int] = List(1, 2, 3, 1, 2, 3)



//Set Set addition operation , Generate a new set 
scala> Set(1,2,3,4,5,6) ++ Set(6,7,8)
res13: scala.collection.immutable.Set[Int] = Set(5, 1, 6, 2, 7, 3, 8, 4)



/* Sum up , It can be seen that , This immutable type   Even if there is operation , That is also generating a new variable , The content of the original variable will not be modified */

3. Variable type references

// Immutable types are immutable In the library , By default, it has been referenced . If you want to reference variable types , You need to quote mutable library .
	import scala.collection.immutable.xxxxx
	import scala.collection.mutable.xxxx

// as follows   Examples show   Immutable set   And   Operation of variable sets .【 Pay attention to the following 】
	//1. The default reference is an immutable set 
	//2. Variable sets are created in the same way as immutable sets , Just need to import in advance mutable.set Variable set class 
	//3. Immutable types cannot be modified during operation , Variable types can .


scala> val a = Set(1,2,3,4)
a: scala.collection.immutable.Set[Int] = Set(1, 2, 3, 4)

scala> a += 1
<console>:13: error: value += is not a member of scala.collection.immutable.Set[Int]
       a += 1
         ^




scala> import scala.collection.mutable.Set
import scala.collection.mutable.Set

scala> val b = Set(1,2,3,4)
b: scala.collection.mutable.Set[Int] = Set(1, 2, 3, 4)

scala> b += 1
res21: b.type = Set(1, 2, 3, 4)





scala> import scala.collection.immutable.Set
import scala.collection.immutable.Set

scala> val c = Set(1,2,3,4)
<console>:13: error: reference to Set is ambiguous;
it is imported twice in the same scope by
import scala.collection.immutable.Set
and import scala.collection.mutable.Set
       val c = Set(1,2,3,4)
              

4. Instance members 、 Static members

/*  answer 1：  Static members categorize all , Instance members belong to the object ！  answer 2： JAVA Can include 2 Species ： Instance members and class members （ Static members ）  Instance member belongs to the object , Instance members include member variables and instance member methods . Only after the object is created , To access instance member variables and call instance member methods through objects .  Class member belongs to class , Class members need keywords static Mark , Also known as static members , Class members include class member variables and class member methods . Through the class name, you can directly access class member variables and call class member methods , Even if no objects are created , You can also reference class members . Class members can also be referenced by objects .  answer 3： */

thus ,scala This is the end of intermediate simple learning .

.
.
.
Blood stained paintings of rivers and mountains ,
How can I defeat you with a little cinnabar ,
Lose the world ,
It's always just a bustling .