Victor Alfonso Pérez Espino's questions

I am beginning to see DB, I am trying to understand how relationships work, the example I am trying to do consists of having a teacher table and a student table;

I want to query the students table, but with teacher data, that is, tell me to select all the students who have "teacherX" as their teacher.

My tables and relationship are like this:

Here I am doing select, but with the id that already contains the student table, what I want to know is how to indicate that, instead of putting the id, put the teacher's name something like ***select * from alumno where id_profesor = (select id from profesor where nombre = Gabriel)***(I know that I don't know how to write it that way, but I don't I know how to do it).

Here are the records for my 2 tables.

I'm doing a school project, I have to make the animation of a "ball", which is nothing more than a label with a CSS style that turns the label into a red circle.

I have to make the ball move in a certain direction, I already have that. But it occurred to me to try moving 2 balls at the same time, each ball with its own thread...

Create a class, called "movement", which inherits from Thread , in which I receive a Label, an AnchorPane (to know the coordinates of the edges, which are limiting) and a boolean, to know whether to move left or right, if I execute only one thread, the program runs fine, but if I launch the threads, that is, 2 instances of the movement class , one works perfectly, but the other stops.

How could I make the two "balls" move?

The first string is ball V, and the second string is ball A.

This is my code:

main class

public class U3HilosAnimar0002 extends Application {


@Override
public void start(Stage stage) throws Exception {
    Parent root = FXMLLoader.load(getClass().getResource("pelot.fxml"));

    Scene scene = new Scene(root);

    stage.setScene(scene);
    stage.show();
}

public static void main(String[] args) {
    launch(args);
}

}

The FXML Controller

Here I receive the click of the Start button, to start the movement

package u3hilosanimar0002;

import java.net.URL;
import java.util.ResourceBundle;
import javafx.event.ActionEvent;
import javafx.fxml.FXML;
import javafx.fxml.Initializable;
import javafx.scene.control.Button;
import javafx.scene.control.Label;
import javafx.scene.layout.AnchorPane;



public class PelotController implements Initializable {
@FXML
private Label lbl;
@FXML
private Button btnStart;
@FXML
private AnchorPane APfondo;
@FXML
private Label lbl2;
@FXML
private AnchorPane AP2;

/**
 * Initializes the controller class.
 */
@Override
public void initialize(URL url, ResourceBundle rb) {
    // TODO



}    

@FXML
private void startHilo(ActionEvent event) throws InterruptedException {

    Movimiento m1 = new Movimiento(lbl,APfondo,true);
    Movimiento m2 = new Movimiento(lbl2,APfondo,false);


  m1.start();
          System.out.println("Segundo Hilo");

    m2.start();


}


}

Movement class that inherits from Thread

    package u3hilosanimar0002;

import java.util.logging.Level;
import java.util.logging.Logger;
import javafx.scene.control.Label;
import javafx.scene.layout.AnchorPane;

/**
 *
 * @author ESPINO
 */

public class Movimiento extends Thread {


int c;
Label pelota = new Label();
 AnchorPane AP;
private boolean direccion;


Movimiento(Label lbl, AnchorPane APfondo,boolean direccion) {
    c=5;
    this.pelota = lbl;
    this.AP = APfondo;
    this.direccion = direccion;//flse left , true derech
}


@Override
public  void  run(){

    while(true){

        if(this.pelota.getLayoutX()+this.pelota.getWidth()>AP.getWidth()||this.pelota.getLayoutX()<0){

            //hará que rebote a la direccion opuesta, cuando llegué a la derecha del todo
         c*=(-1);   
           //Thread.interrupted();
            //break;
        }            

        try {


            //if(direccion){
              this.pelota.setLayoutX(this.pelota.getLayoutX()+c);
              //direccion = false;
            //}
            //else{
              //  this.pelota.setLayoutX(this.pelota.getLayoutX()-c);
                //direccion =false;
            //}


            sleep(35);
        } catch (InterruptedException ex) {
            Logger.getLogger(Movimiento.class.getName()).log(Level.SEVERE, null, ex);
        }  


    }

}


}

FXML

<?xml version="1.0" encoding="UTF-8"?>

<?import java.net.URL?>
<?import javafx.scene.control.Button?>
<?import javafx.scene.control.Label?>
<?import javafx.scene.layout.AnchorPane?>
<?import javafx.scene.text.Font?>

<AnchorPane id="AnchorPane" fx:id="APfondo" prefHeight="400.0" prefWidth="645.0" styleClass="mainFxmlClass" xmlns="http://javafx.com/javafx/10.0.1" xmlns:fx="http://javafx.com/fxml/1" fx:controller="u3hilosanimar0002.PelotController">
    <stylesheets>
        <URL value="@pelot.css" />
    </stylesheets>
   <children>
      <AnchorPane fx:id="AP2" prefHeight="400.0" prefWidth="645.0" AnchorPane.bottomAnchor="0.0" AnchorPane.leftAnchor="0.0" AnchorPane.rightAnchor="0.0" AnchorPane.topAnchor="0.0" />
      <Label fx:id="lbl" alignment="CENTER" contentDisplay="CENTER" layoutX="278.0" layoutY="177.0" prefHeight="46.0" prefWidth="44.0" text="V" textFill="WHITE">
         <font>
            <Font size="28.0" />
         </font></Label>
      <Button fx:id="btnStart" layoutX="253.0" layoutY="345.0" mnemonicParsing="false" onAction="#startHilo" prefHeight="25.0" prefWidth="95.0" text="Start" />
      <Label fx:id="lbl2" alignment="CENTER" contentDisplay="CENTER" layoutX="278.0" layoutY="98.0" prefHeight="46.0" prefWidth="44.0" text="A" textFill="WHITE">
         <font>
            <Font size="28.0" />
         </font>
      </Label>
   </children>
</AnchorPane>

I am creating a login, it consists of entering a username and password, then when I click on validate, the Login closes and a new window opens (where I will include functions that I have not yet implemented)... but this new window has a logout button, how can I make clicking that button return me to Login?

I've already been looking for solutions, but everything seems too confusing, I've seen that I can load everything in the same Stage, just changing the Scene, but I see it as complex, I don't understand how it does it..., this is my code.

Main class

    package mainPack;

import javafx.application.Application;
import javafx.event.ActionEvent;
import javafx.fxml.FXMLLoader;
import javafx.scene.Parent;
import javafx.scene.Scene;
import javafx.stage.Stage;

public class mainC extends Application {

    @Override
    public void start(Stage primaryStage) throws Exception {

        Parent root = FXMLLoader.load(getClass().getResource("/GUIs/Login.fxml"));



        Scene scene = new Scene(root);
        primaryStage.setScene(scene);
        primaryStage.show();


    }

}

LoginController

package GUIs;

import java.io.IOException;
import java.net.URL;
import java.util.ResourceBundle;
import javafx.fxml.FXML;
import javafx.fxml.Initializable;
import javafx.scene.control.PasswordField;
import javafx.scene.control.TextField;
import javafx.event.ActionEvent;
import javafx.fxml.FXMLLoader;
import javafx.scene.Node;
import javafx.scene.Parent;
import javafx.scene.Scene;
import javafx.scene.control.Label;
import javafx.stage.Stage;

public class LoginController implements Initializable {

    /**
     * Initializes the controller class.
     */

    @FXML PasswordField txtPass;
    @FXML TextField txtUser;
    @FXML Label lblError;
    @FXML Label lblStatus;


    @FXML
    public void btnHandle(ActionEvent e) throws Exception{

        if(txtUser.getText().equals("Sistemas")  && txtPass.getText().equals("2019")){

            lblError.setOpacity(0);
            lblStatus.setText("Conectado.");

            Stage stage = new Stage();
            Parent root = FXMLLoader.load(getClass().getResource("/GUIs/Principal.fxml"));

            Scene escena = new Scene(root);
            stage.setScene(escena);
            stage.show();
//en esta linea, esconde el Stage del Login y carga el nuevo stage
            ( (Node) (e.getSource() ) ).getScene().getWindow().hide();

        }else{
            lblStatus.setText("Desconectado.");
            lblError.setOpacity(1);

        }


    }

    @Override
    public void initialize(URL url, ResourceBundle rb) {
        // TODO
    }    

}

WindowPrincipalController (after login)

package GUIs;

import java.net.URL;
import java.util.ResourceBundle;
import javafx.event.ActionEvent;
import javafx.fxml.FXML;
import javafx.fxml.Initializable;

/**
 * FXML Controller class
 *
 * @author VAPESIN
 */
public class PrincipalController implements Initializable {

    /**
     * Initializes the controller class.
     */

    @FXML
    public void cerrarSesHandle(ActionEvent e){

    }


    @Override
    public void initialize(URL url, ResourceBundle rb) {
        // TODO
    }    

}

Login.fxml

<?xml version="1.0" encoding="UTF-8"?>

<?import java.net.URL?>
<?import javafx.scene.control.Button?>
<?import javafx.scene.control.Label?>
<?import javafx.scene.control.PasswordField?>
<?import javafx.scene.control.TextField?>
<?import javafx.scene.layout.AnchorPane?>
<?import javafx.scene.text.Font?>

<AnchorPane id="AnchorPane" prefHeight="400.0" prefWidth="493.0" styleClass="mainFxmlClass" xmlns="http://javafx.com/javafx/10.0.1" xmlns:fx="http://javafx.com/fxml/1" fx:controller="GUIs.LoginController">
    <stylesheets>
        <URL value="@login.css" />
    </stylesheets>
   <children>
      <Label layoutX="49.0" layoutY="71.0" prefHeight="46.0" prefWidth="52.0" text="Usuario" />
      <Label layoutX="49.0" layoutY="148.0" prefHeight="46.0" prefWidth="66.0" text="Contraseña" />
      <Button layoutX="145.0" layoutY="252.0" mnemonicParsing="false" onAction="#btnHandle" text="Validar" />
      <TextField fx:id="txtUser" layoutX="145.0" layoutY="82.0" />
      <PasswordField fx:id="txtPass" layoutX="145.0" layoutY="159.0" />
      <Label fx:id="lblError" layoutX="74.0" layoutY="301.0" opacity="0.0" prefHeight="46.0" prefWidth="345.0" text="Usuario o Contraseña incorrecta." textFill="RED">
         <font>
            <Font size="17.0" />
         </font>
      </Label>
      <Label fx:id="lblStatus" layoutX="101.0" layoutY="25.0" prefHeight="46.0" prefWidth="330.0" text="Desconectado" />
   </children>
</AnchorPane>

MainWindow.fxml

<?xml version="1.0" encoding="UTF-8"?>

<?import java.net.URL?>
<?import javafx.scene.control.Button?>
<?import javafx.scene.control.Label?>
<?import javafx.scene.layout.AnchorPane?>
<?import javafx.scene.text.Font?>

<AnchorPane id="AnchorPane" prefHeight="400.0" prefWidth="600.0" styleClass="mainFxmlClass" xmlns:fx="http://javafx.com/fxml/1" xmlns="http://javafx.com/javafx/10.0.1" fx:controller="GUIs.PrincipalController">
    <stylesheets>
        <URL value="@principal.css" />
    </stylesheets>
   <children>
      <Label layoutX="167.0" layoutY="77.0" prefHeight="103.0" prefWidth="271.0" text="Bienvenido">
         <font>
            <Font size="53.0" />
         </font>
      </Label>
      <Button layoutX="438.0" layoutY="345.0" mnemonicParsing="false" onAction="#cerrarSesHandle" prefHeight="25.0" prefWidth="142.0" text="Cerrar sesión" />
   </children>
</AnchorPane>

When clicking on close, I would like it to return me to the Login.

How could it be done to approximate a number by default and by excess? Understanding that we say, we have the number 2.534823

A default approximation to 4 decimal places would be 2.5348

and for approximation by excess to 4 decimal places, it would be 2.5349

It's like rounding but for a specific decimal place, is there a way to do it in C++?

I have the following example, but it would only be to round to the nearest whole number, whether greater or less:

#include<iostream>
#include<math.h>
using namespace std;


int main(){

    float num;
    int decim;

    cout<<"\tCalcular aproximaciones por defecto y exceso"<<endl<<endl;

    cout<<"Ingrese un numero: " ;
    cin>>num;

    cout<<"Ingresa la cantidad de dedcimales a dejar: ";
    cin>>decim;

    /*
        con ceil y floor me acerca al entero,
        ceil al entero más cercano mayor
        floor al  entero más cercano menor
    */

    cout<<"Aproximación por exceso: "<<ceil(num)<<endl;

    cout<<"Aproximación por defecto: "<<floor(num)<<endl;

    /*
       la aproximación debería quedar como:
       numero ingresado = 1.432986

       para por defecto  con 4 decimales, quedaria 1.4329
       para exceso con 4 decimales quedaría 1.4323
       pero con ceil, subiría a 2
       y con floor bajaría a 1


    */

    return 0;
}

In C you could truncate a number, to a specified number of decimal places when printing it (i.e. the default approximation):

printf("%.3f", x);

EITHER

printf("%3f", x);

I don't remember which of the 2 ways, what I want to know is how the same could be done but in C++ and also obtain the approximation by excess

I'm reviewing Multithreading and I'm (redundancy) testing ways to use tasks, I made a static class for testing,

static class PruebaTask
{
    public static void usoTasks()
    {
        DateTime Inicio = DateTime.Now;

        Task task;
        for (int i = 0; i < 5000000; i++)
        {
            task = new Task(() => metodoPrueba());

            //task = Task.Run(() => metodoPrueba());//así no requiere instancia
        }

        MessageBox.Show("El task Tardó" + (DateTime.Now - Inicio)+ " thread: " + Thread.CurrentThread.ManagedThreadId);


    }

    static void metodoPrueba()
    {
        double d = 0;
        for (int j = 0; j < 10000; j++)
        {
            d = 12445 / 434 * 495 / 3.09;
        }
    }

}

And I call from here

 private void button1_Click(object sender, EventArgs e)
    {

        //PruebaThread.usoThreads();
        PruebaTask.usoTasks();
        //PruebaBox.probarMsg();
    }

My question is:

What is the difference between these 2 sentences:

task = new Task(() => metodoPrueba());

task = Task.Run(() => metodoPrueba());           

the second can also be left alone as:

Task.Run(() => metodoPrueba());

It returns a Task so I can assign it to a Task variable, but in the end the result is the same

what I see is that when using it task = new Task(() => metodoPrueba());it takes a wonderful amount of o.3 seconds, instead Task.Run(() => metodoPrueba());it takes practically 1 minute, and it is doing it 5 million times.

The difference is obvious, but does anyone know why this happens?

(I'm just getting started in C#, I haven't started with await or async yet, I've also tried with new instances of Threads directly, and it still takes a long time, there, I understand that it hits performance when creating new threads, and instead, the Tasks are optimizations to use already existing Threads, if I'm wrong correct me)

I'm studying C# but I got to the topic of Delegates and Events, I already know what a delegate is and how it works:

The delegate is like a "wrapper" for functions, where we can reference a method via an instance of the delegate.

I searched on the MS page, in various tutorials and also in this question What are Delegates in C#?

I have an example for delegate use like this: (ignore the namespace name)

using System;



namespace AdicionEliminacionReferenciasDelegadas

{

delegate void prueba();//contrato, debe tener la misma firma que las funciones que va a recibir

class Program
{
    static void Main(string[] args)
    {




    prueba Test = new prueba(metods.metod1);
        Test += metods.metod2;//apilacion de referencias en la instancia Test, 

        Test();//llamado secuencial de la apilacion (ejecuta las funciones referencias dentro deTest)
        Console.WriteLine("--------------------------------------------------------------------\n Llamado entre clases: \n");

        ejemplo obj = new ejemplo();
        obj.addDelegateStack(metods.metod1);//se añade un metodo de otra clase(estaba probando entre clases)
        obj.addDelegateStack(metods.metod2);
        obj.ejecutarDeleg(); //ejecuta al atributo Test (miembro de clase) donde están contenidas las referencias, de los 2 metodos agregados arriba

        Console.WriteLine("\nEliminacion de metodo 1, el cual se busca y se elimina del ArrayList interno del contenedor delegate .... \n");
        obj.deleteDelegateStack(metods.metod1);

        obj.ejecutarDeleg();//vuelve a ejecutar ese miembro, para comprobar que se elimina una referencia especifica




        Console.ReadLine();
    }


}


class ejemplo
{
    public prueba Test;




    public void addDelegateStack(prueba newDeleg)
    {
        Test += newDeleg;
    }

    public void deleteDelegateStack(prueba newDEleg)
    {
        Test -= newDEleg;
    }

    public void ejecutarDeleg()
    {
        Test();
    }



}



static class metods
{
    public static void metod1()
    {
        Console.WriteLine("Metodo 1, Estás en clase metods");
    }

    public static void metod2()
    {
        Console.WriteLine("Metodo 2, sigues en clase metods");
    }
}



}

With this example I was able to understand how it works and apply it to use between classes

Now yes, my question is:

What is the event for ?

According to Microsoft and any human logic, an event is a message sent to notify that something has happened. But if I already have the delegate , with which I can "send" functions between objects, by means of an instance of the delegate, then anything could be notified to and from any class, right?

So how is the use of event ?

*By the way, I already know that there are a couple of questions on this topic, C#- Understanding events , eventhandlers and delegates , but I can't understand them, for that reason, I open another question...

Thank you!