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Advice about the application I will be making? (I'm a beginner)

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  • mrjjM Offline
    mrjjM Offline
    mrjj
    Lifetime Qt Champion
    wrote on last edited by mrjj
    #41

    well
    just use the variable directly. no need to use function.
    im not sure what get_ch_tipic is as you dont have () so its not function call.

    However, if you do

    QString MyX; ( in .h as you shown)

    then anywhere you can just use it like

    MyX = "";
    or read it.

    alt text
    is just bad syntax.
    as you say get_ch_Topic is a object with the function getTopic and its not true.

    if you gave MainWindow a function called getTopic
    then just call it
    qDebug() << getTopic();

    Its important to understand that all you add to MainWindow class ( in .h)
    can be access from any functio that also a memebr .
    that is
    MainWindow::SomeFunction() {
    any variable from .h you can use here. no special syntax. just use it.
    }

    1 Reply Last reply
    3
    • D Offline
      D Offline
      dvlpr.bernard
      wrote on last edited by
      #42

      Thank you sir. I just don't know but still it doesn't work it just return "" not "WHILE". :(

      mrjjM 1 Reply Last reply
      0
      • D dvlpr.bernard

        Thank you sir. I just don't know but still it doesn't work it just return "" not "WHILE". :(

        mrjjM Offline
        mrjjM Offline
        mrjj
        Lifetime Qt Champion
        wrote on last edited by
        #43

        @dvlpr.bernard
        hi
        check code where you set it.
        Maybe you create other instance you set it on ?

        D 1 Reply Last reply
        0
        • mrjjM mrjj

          @dvlpr.bernard
          hi
          check code where you set it.
          Maybe you create other instance you set it on ?

          D Offline
          D Offline
          dvlpr.bernard
          wrote on last edited by
          #44

          @mrjj
          Hello sir,
          Where should I declare a global instance / object in my 3 files.
          mainwindow.h, main.cpp, or mainwindow.cpp?
          Ang how whenever I declare one because whenever I declare one e.g. (MainWindow get_ch_Topic;) it display an error "no previous extern declaration for non-static variable"

          jsulmJ 1 Reply Last reply
          0
          • D dvlpr.bernard

            @mrjj
            Hello sir,
            Where should I declare a global instance / object in my 3 files.
            mainwindow.h, main.cpp, or mainwindow.cpp?
            Ang how whenever I declare one because whenever I declare one e.g. (MainWindow get_ch_Topic;) it display an error "no previous extern declaration for non-static variable"

            jsulmJ Offline
            jsulmJ Offline
            jsulm
            Lifetime Qt Champion
            wrote on last edited by
            #45

            @dvlpr.bernard said in Advice about the application I will be making? (I'm a beginner):

            Where should I declare a global instance / object in my 3 files.

            Nowhere! Avoid global variables especially if those are derived from QObject!
            Why do you need global variables at all?

            https://forum.qt.io/topic/113070/qt-code-of-conduct

            D 1 Reply Last reply
            2
            • jsulmJ jsulm

              @dvlpr.bernard said in Advice about the application I will be making? (I'm a beginner):

              Where should I declare a global instance / object in my 3 files.

              Nowhere! Avoid global variables especially if those are derived from QObject!
              Why do you need global variables at all?

              D Offline
              D Offline
              dvlpr.bernard
              wrote on last edited by
              #46
              This post is deleted!
              D 1 Reply Last reply
              0
              • D dvlpr.bernard

                This post is deleted!

                D Offline
                D Offline
                dvlpr.bernard
                wrote on last edited by
                #47

                @jsulm
                Hello sir. So that I can access the string that I declared in other functions.
                How could I do it the right way?

                0_1556355716202_9f195507-308b-4328-a197-c9245cbddf46-image.png

                0_1556355767974_19f63b1f-35ae-4a4f-ab7a-ec88acd016fb-image.png 0_1556355591803_f0274585-e5f9-4e11-8428-9341f8853a75-image.png

                1 Reply Last reply
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                • mrjjM Offline
                  mrjjM Offline
                  mrjj
                  Lifetime Qt Champion
                  wrote on last edited by
                  #48

                  Hi
                  Put all the variables in the class!

                  class MainWindow : public QMainWindow
                  {
                  Q_OBJECT

                  public:
                  explicit MainWindow(QWidget *parent = nullptr);
                  ~MainWindow();
                  ...
                  private:
                  Ui::MainWindow *ui;
                  QString ch_Dif_Lvl; ///// HERE !
                  QString fc_Topic;
                  };

                  then you can just use them

                  void MainWindow::on_pushButton_clicked()
                  {
                      ch_Dif_Lvl = "BEGINNER";
                     fc_Topic = "Topic";
                  }
                  
                  
                  D 1 Reply Last reply
                  3
                  • mrjjM mrjj

                    Hi
                    Put all the variables in the class!

                    class MainWindow : public QMainWindow
                    {
                    Q_OBJECT

                    public:
                    explicit MainWindow(QWidget *parent = nullptr);
                    ~MainWindow();
                    ...
                    private:
                    Ui::MainWindow *ui;
                    QString ch_Dif_Lvl; ///// HERE !
                    QString fc_Topic;
                    };

                    then you can just use them

                    void MainWindow::on_pushButton_clicked()
                    {
                        ch_Dif_Lvl = "BEGINNER";
                       fc_Topic = "Topic";
                    }
                    
                    
                    D Offline
                    D Offline
                    dvlpr.bernard
                    wrote on last edited by
                    #49

                    @mrjj
                    Thank you. God bless your kindness to help other people.

                    mrjjM 1 Reply Last reply
                    3
                    • D dvlpr.bernard

                      @mrjj
                      Thank you. God bless your kindness to help other people.

                      mrjjM Offline
                      mrjjM Offline
                      mrjj
                      Lifetime Qt Champion
                      wrote on last edited by
                      #50

                      @dvlpr.bernard
                      Thank you :)
                      I hope the project is progressing fine.

                      D 1 Reply Last reply
                      2
                      • mrjjM mrjj

                        @dvlpr.bernard
                        Thank you :)
                        I hope the project is progressing fine.

                        D Offline
                        D Offline
                        dvlpr.bernard
                        wrote on last edited by dvlpr.bernard
                        #51

                        @mrjj
                        Good day sir.

                        I have been trying to get the data from my database it is already connected but whenever the query executes qDebug() returns "QSqlQuery::exec: database not open".

                        Also I tried to replace this query
                        qry.exec("select Question from mydatabase where QQuantity = 4 and Topic = 'FOR' and Difficulty_Level = 'ADVANCED'")
                        with variable and string name
                        qry.exec("select Question from mydatabase where QQuantity = '"+quantity+"'and Topic = '"ch_topic"' and Difficulty_Level = '"ch_dif_lvl"'")
                        but it didn't work also.

                        0_1556616460603_2081b023-6b26-4c79-8e38-65ee64c55338-image.png

                        0_1556617041288_2d77d958-9ce6-46e9-add1-b2f1cdb76d81-image.png

                        1 Reply Last reply
                        0
                        • mrjjM Offline
                          mrjjM Offline
                          mrjj
                          Lifetime Qt Champion
                          wrote on last edited by
                          #52

                          Hi
                          I cant see anything wrong.
                          Show how code is before the if ( mydb.open )

                          D 1 Reply Last reply
                          1
                          • mrjjM mrjj

                            Hi
                            I cant see anything wrong.
                            Show how code is before the if ( mydb.open )

                            D Offline
                            D Offline
                            dvlpr.bernard
                            wrote on last edited by
                            #53

                            The code above is inside of this button function
                            0_1556624873643_3c1572d4-d1c4-4bcb-be5b-7c7a6701dfb4-image.png

                            Here's the declaration of the database
                            0_1556624942089_c2a56415-3c65-44f0-bc2b-e108db11ebed-image.png
                            0_1556624974330_fa46ea4d-303f-475f-b64e-b252d5391cfb-image.png

                            JonBJ 1 Reply Last reply
                            0
                            • D dvlpr.bernard

                              The code above is inside of this button function
                              0_1556624873643_3c1572d4-d1c4-4bcb-be5b-7c7a6701dfb4-image.png

                              Here's the declaration of the database
                              0_1556624942089_c2a56415-3c65-44f0-bc2b-e108db11ebed-image.png
                              0_1556624974330_fa46ea4d-303f-475f-b64e-b252d5391cfb-image.png

                              JonBJ Online
                              JonBJ Online
                              JonB
                              wrote on last edited by
                              #54

                              @dvlpr.bernard

                              • I notice that your Output window shows two occurrences of your message Connected.... Why is that? Are you connecting more than once?? [In a separate issue, in the long run you absolutely will not want to open() and then close() the database for each query. The whole point is to connect once, keep the connection open while you execute all your queries, and close when you are completely done with the database.]

                              • [...] but it didn't work also.

                              What does that mean? Error message? Behaviour? What?? Please help us to help you by supplying useful information.

                              D 1 Reply Last reply
                              1
                              • JonBJ JonB

                                @dvlpr.bernard

                                • I notice that your Output window shows two occurrences of your message Connected.... Why is that? Are you connecting more than once?? [In a separate issue, in the long run you absolutely will not want to open() and then close() the database for each query. The whole point is to connect once, keep the connection open while you execute all your queries, and close when you are completely done with the database.]

                                • [...] but it didn't work also.

                                What does that mean? Error message? Behaviour? What?? Please help us to help you by supplying useful information.

                                D Offline
                                D Offline
                                dvlpr.bernard
                                wrote on last edited by
                                #55

                                @JonB @mrjj
                                if (query.exec( ))
                                always returns a false value thats why the query never gets executed.
                                0_1556682147823_32f7bcf9-5b1b-405b-b851-94dd8fd9cf15-image.png
                                0_1556682115403_04ffb565-381f-4719-8260-9fac15c4cc3b-image.png

                                1 Reply Last reply
                                0
                                • D Offline
                                  D Offline
                                  dvlpr.bernard
                                  wrote on last edited by
                                  #56

                                  I played around with the code and this error return.

                                  0_1556684436755_30f80dab-2418-4b5f-8d3a-5ac9047df238-image.png

                                  1 Reply Last reply
                                  0
                                  • D Offline
                                    D Offline
                                    dvlpr.bernard
                                    wrote on last edited by
                                    #57

                                    0_1556685583290_a11aff6b-ff9a-4e92-828b-3de115afe4c9-image.png

                                    JonBJ KillerSmathK 2 Replies Last reply
                                    0
                                    • mrjjM Offline
                                      mrjjM Offline
                                      mrjj
                                      Lifetime Qt Champion
                                      wrote on last edited by mrjj
                                      #58

                                      hi
                                      are you sure the table name is mydatabase ?
                                      seems wrong. unless the table is called taht.
                                      But it seems you used the name of the database.

                                      and it even says unknown table.
                                      so use table name instead.

                                      1 Reply Last reply
                                      3
                                      • D dvlpr.bernard

                                        0_1556685583290_a11aff6b-ff9a-4e92-828b-3de115afe4c9-image.png

                                        JonBJ Online
                                        JonBJ Online
                                        JonB
                                        wrote on last edited by JonB
                                        #59

                                        @dvlpr.bernard

                                        no such table: mydatabase

                                        @mrjj wrote:

                                        are you sure the table name is mydatabase ?

                                        Are you getting your database vs your tables mixed up in your head? You work with one database at a time. In SQLite that corresponds to the file you pass to setDatabaseName(), and is what gets opened via mydb.open(). A database may contain many tables, each with their own name. It is the table name you mention in SQL select (and most other) statements. You previously created your table(s) via some kind of CREATE TABLE name ... statement, and that name is what you need in your query. It would be "unusual" for you to have named a table mydatabase.

                                        You might like to do yourself a favour, to help with all the SQLite queries you're trying to write. Download one of the (free) "workbenches" for SQLite. These are GUI applications which will let you see what is in your database; it will also let you construct and play with queries, till you are ready to copy them over to your Qt code. Google for sqlite workbench, there seem to be several, e.g. https://www.sqlite-workbench.com/, https://sqlitestudio.pl, https://alternativeto.net/software/sqlite-manager/. I suggest you take the time to pick one, in the long run I think the effort will be well worthwhile for you.

                                        1 Reply Last reply
                                        2
                                        • D Offline
                                          D Offline
                                          dvlpr.bernard
                                          wrote on last edited by
                                          #60

                                          Good day.
                                          So here is my database named codeanaldb with a table name "mydatabase". I didn't know why qt couldn't recognize it.

                                          0_1556721420905_3aa67a8d-a82e-4456-9548-1500489ec844-image.png

                                          And here is the same query that I copy paste to qt but suddenly didn't work.

                                          0_1556721664803_4baa7258-307d-4a59-9dbb-68db56cb80f8-image.png

                                          JonBJ 1 Reply Last reply
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