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How to search for a specific character in a QString

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  • tomyT Offline
    tomyT Offline
    tomy
    wrote on last edited by
    #11

    There will not be any dot in other numbers, say, 12.21! Because we have used '0' as the precision!

    matthew.kuiashM 1 Reply Last reply
    0
    • tomyT tomy

      There will not be any dot in other numbers, say, 12.21! Because we have used '0' as the precision!

      matthew.kuiashM Offline
      matthew.kuiashM Offline
      matthew.kuiash
      wrote on last edited by matthew.kuiash
      #12

      @tomy Precisely. Your example was of an integer number, albeit packed into a double (there is adequate precision for your example)

      If you ask for extra precision you will get extra '0's after the decimal point. I don't know if this is what you want.

      What is it you are trying to do?! If you are justing searching for characters use indexOf. Is this is something to do with figuring out how many digits are required to display a number? In which case use math functions such as floor, ceiling and log10. If you want to know about the fractional parts use X-floor(X).

      The example you gave does work. Just not the way you want it too and that was not what you originally asked for.

      The legendary cellist Pablo Casals was asked why he continued to practice at age 90. "Because I think I'm making progress," he replied.

      tomyT 1 Reply Last reply
      1
      • SGaistS Offline
        SGaistS Offline
        SGaist
        Lifetime Qt Champion
        wrote on last edited by
        #13

        Also, why not use a QDoubleSpinBox to show your numbers ?

        Interested in AI ? www.idiap.ch
        Please read the Qt Code of Conduct - https://forum.qt.io/topic/113070/qt-code-of-conduct

        1 Reply Last reply
        1
        • tomyT tomy

          @SGaist
          Good solution, but what if we want to know "the index of that QChar" if exists?

          @jsulm
          Good thanks. How should I look for a specific method while apparently there are thousands methods on Qt documentations? For example I want the index of a specific char in a string (if that exists), what is an optimized way to look for it?

          PS: Of course I can use a simple for loop but when there is method already, why increase lines of code by loops!

          jsulmJ Offline
          jsulmJ Offline
          jsulm
          Lifetime Qt Champion
          wrote on last edited by
          #14

          @tomy How? Well, you know it is QString so take a look at its documentation. QString doesn't have "thousands" of methods.

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

          1 Reply Last reply
          0
          • matthew.kuiashM matthew.kuiash

            @tomy Precisely. Your example was of an integer number, albeit packed into a double (there is adequate precision for your example)

            If you ask for extra precision you will get extra '0's after the decimal point. I don't know if this is what you want.

            What is it you are trying to do?! If you are justing searching for characters use indexOf. Is this is something to do with figuring out how many digits are required to display a number? In which case use math functions such as floor, ceiling and log10. If you want to know about the fractional parts use X-floor(X).

            The example you gave does work. Just not the way you want it too and that was not what you originally asked for.

            tomyT Offline
            tomyT Offline
            tomy
            wrote on last edited by
            #15

            @matthew.kuiash

            What I need is showing a double result. It may be a very small negative/positive number (say, -0.000000001234) or a very big positive/negative number (say, +2500000000036540). It's range I should cover by code.

            I only need the 'number of' digits after the decimal point. Then I will use another method to use 'that number' precision for the output.

            @SGaist:
            Very good info, thanks.
            But the lineEdit there, is used to "show" the result of a calculation. (My calculator works fine but I just want to make it more precise).
            QDoubleSpinBox has two buttons and works in essence like an editable cadre, while I just need "to show" the result (and also save the current result for the next expressions).

            matthew.kuiashM Z 2 Replies Last reply
            0
            • VRoninV Offline
              VRoninV Offline
              VRonin
              wrote on last edited by VRonin
              #16
              double d = 12000000.0;
              QString s = QString::number(d,'f',std::numeric_limits<decltype(d)>::max_digits10);
              if(s.contains('.')){
              int i=s.size()-1;
              for(;s.at(i)=='0';--i){}
              s=s.left(i+ (s.at(i)=='.' ? 0:1));
              }
              

              Edit:

              alternative:

              // needs #include <cmath>
              double d = 12000000.0;
              double junk;
              int precision=0;
              for(double tester=d;!qFuzzyIsNull(std::modf(tester,&junk));++precision,tester*=10.0){}
              QString s = QString::number(d,'f',precision);
              

              "La mort n'est rien, mais vivre vaincu et sans gloire, c'est mourir tous les jours"
              ~Napoleon Bonaparte

              On a crusade to banish setIndexWidget() from the holy land of Qt

              tomyT 1 Reply Last reply
              3
              • VRoninV VRonin
                double d = 12000000.0;
                QString s = QString::number(d,'f',std::numeric_limits<decltype(d)>::max_digits10);
                if(s.contains('.')){
                int i=s.size()-1;
                for(;s.at(i)=='0';--i){}
                s=s.left(i+ (s.at(i)=='.' ? 0:1));
                }
                

                Edit:

                alternative:

                // needs #include <cmath>
                double d = 12000000.0;
                double junk;
                int precision=0;
                for(double tester=d;!qFuzzyIsNull(std::modf(tester,&junk));++precision,tester*=10.0){}
                QString s = QString::number(d,'f',precision);
                
                tomyT Offline
                tomyT Offline
                tomy
                wrote on last edited by tomy
                #17

                @VRonin
                Thank you.

                Both have flaws. But also both have high level of precision.
                I think the flaws belong to the way numbers are stored in registers. And I think there is no completely (100%) accurate calculator in the world because of that.

                I chose the latter. Thanks for your help. I'm appreciative.

                1 Reply Last reply
                0
                • tomyT tomy

                  @matthew.kuiash

                  What I need is showing a double result. It may be a very small negative/positive number (say, -0.000000001234) or a very big positive/negative number (say, +2500000000036540). It's range I should cover by code.

                  I only need the 'number of' digits after the decimal point. Then I will use another method to use 'that number' precision for the output.

                  @SGaist:
                  Very good info, thanks.
                  But the lineEdit there, is used to "show" the result of a calculation. (My calculator works fine but I just want to make it more precise).
                  QDoubleSpinBox has two buttons and works in essence like an editable cadre, while I just need "to show" the result (and also save the current result for the next expressions).

                  matthew.kuiashM Offline
                  matthew.kuiashM Offline
                  matthew.kuiash
                  wrote on last edited by
                  #18

                  @tomy OK. You have plenty to go on here but I'll just caution you that the number of digits after the decimal point can be extremely high for doubles.

                  The legendary cellist Pablo Casals was asked why he continued to practice at age 90. "Because I think I'm making progress," he replied.

                  tomyT 1 Reply Last reply
                  0
                  • matthew.kuiashM matthew.kuiash

                    @tomy OK. You have plenty to go on here but I'll just caution you that the number of digits after the decimal point can be extremely high for doubles.

                    tomyT Offline
                    tomyT Offline
                    tomy
                    wrote on last edited by tomy
                    #19

                    @matthew.kuiash
                    I have used as high precision as possible for doubles using VRonin's code.
                    I think I'm at the end. The differences between what we expect and what is shown using the code may happen for low of high precisions because of the difference of the way we and the machine calculate expressions.

                    1 Reply Last reply
                    0
                    • tomyT tomy

                      @matthew.kuiash

                      What I need is showing a double result. It may be a very small negative/positive number (say, -0.000000001234) or a very big positive/negative number (say, +2500000000036540). It's range I should cover by code.

                      I only need the 'number of' digits after the decimal point. Then I will use another method to use 'that number' precision for the output.

                      @SGaist:
                      Very good info, thanks.
                      But the lineEdit there, is used to "show" the result of a calculation. (My calculator works fine but I just want to make it more precise).
                      QDoubleSpinBox has two buttons and works in essence like an editable cadre, while I just need "to show" the result (and also save the current result for the next expressions).

                      Z Offline
                      Z Offline
                      zapprox
                      wrote on last edited by
                      #20

                      Hi @tomy. QDoubleSpinBox can be used as you mentioned . First, to hide buttons call QAbstractSpinBox::setButtonSymbols(ButtonSymbols bs) and second call QAbstractSpinBox::setReadOnly(bool r).
                      Have a nice day.

                      VRoninV tomyT 2 Replies Last reply
                      0
                      • Z zapprox

                        Hi @tomy. QDoubleSpinBox can be used as you mentioned . First, to hide buttons call QAbstractSpinBox::setButtonSymbols(ButtonSymbols bs) and second call QAbstractSpinBox::setReadOnly(bool r).
                        Have a nice day.

                        VRoninV Offline
                        VRoninV Offline
                        VRonin
                        wrote on last edited by
                        #21

                        QDoubleSpinBox can be used as you mentioned

                        You still have to hard code the precision (number of decimals) shown so it does not solve the problem

                        The differences between what we expect and what is shown using the code may happen for low of high precisions

                        The maximum number of digits that can be represented in text is is std::numeric_limits<double>::digits10, you can't get meaningful results if you go beyond this limit

                        "La mort n'est rien, mais vivre vaincu et sans gloire, c'est mourir tous les jours"
                        ~Napoleon Bonaparte

                        On a crusade to banish setIndexWidget() from the holy land of Qt

                        1 Reply Last reply
                        1
                        • Z zapprox

                          Hi @tomy. QDoubleSpinBox can be used as you mentioned . First, to hide buttons call QAbstractSpinBox::setButtonSymbols(ButtonSymbols bs) and second call QAbstractSpinBox::setReadOnly(bool r).
                          Have a nice day.

                          tomyT Offline
                          tomyT Offline
                          tomy
                          wrote on last edited by
                          #22

                          @zapprox
                          You pointed to a good matter —hiding the buttons of that widget. As SGaist had suggested it firstly, I would like to use it because I thought it would solve the problems of those double numbers we were involving since above posts. But when VRonin subsequently said, it seems that we don't get more benefits of it compared to the chosen code.

                          Thank you all.

                          1 Reply Last reply
                          0
                          • kshegunovK Offline
                            kshegunovK Offline
                            kshegunov
                            Moderators
                            wrote on last edited by
                            #23

                            Didn't we discuss these precision things recently, I have a distinct impression we are running a loop ...
                            This thread comes to mind.

                            Read and abide by the Qt Code of Conduct

                            tomyT 1 Reply Last reply
                            0
                            • kshegunovK kshegunov

                              Didn't we discuss these precision things recently, I have a distinct impression we are running a loop ...
                              This thread comes to mind.

                              tomyT Offline
                              tomyT Offline
                              tomy
                              wrote on last edited by
                              #24

                              @kshegunov
                              No.
                              The earlier issue was about the way that makes us able to show our result (say, 1000000) in real number mode not scientific.
                              But after that, (this thread), we should be able to show the number of the precision suitable for each number as a result.
                              We don't want to e.g., show 12.25 as 12.25000000 (just because we are able to show number in real mode). For the matter of accuracy, the user of the app expects to see the result in a professional way.
                              Hope this helps.

                              PS: I wish we wouldn't have anything named "storing numbers in binary mode" and "scientific notation". ;)

                              1 Reply Last reply
                              0
                              • VRoninV Offline
                                VRoninV Offline
                                VRonin
                                wrote on last edited by
                                #25

                                I did not read the qFuzzyCompare documentation. shame on me. the compare to 0 should be avoided, corrected the snippet above

                                "La mort n'est rien, mais vivre vaincu et sans gloire, c'est mourir tous les jours"
                                ~Napoleon Bonaparte

                                On a crusade to banish setIndexWidget() from the holy land of Qt

                                tomyT 1 Reply Last reply
                                0
                                • tomyT Offline
                                  tomyT Offline
                                  tomy
                                  wrote on last edited by tomy
                                  #26

                                  I use this:

                                     double tester = d;
                                      double junk;
                                      int precision = 0;
                                  
                                      for(;!qFuzzyCompare(0.0,std::modf(tester,&junk));++precision,tester*=10.0){}
                                      QString s = QString::number(d,'f',precision);
                                      return s;
                                  

                                  How might this face a problem please?
                                  I want to test it on the app.

                                  1 Reply Last reply
                                  0
                                  • VRoninV VRonin

                                    I did not read the qFuzzyCompare documentation. shame on me. the compare to 0 should be avoided, corrected the snippet above

                                    tomyT Offline
                                    tomyT Offline
                                    tomy
                                    wrote on last edited by
                                    #27

                                    @VRonin
                                    Hi VRonin,

                                    If you see any problem with the code written above in my prior post compared to your edited one, please point to it. If there is a flaw, I will use the edited one in the app.

                                    1 Reply Last reply
                                    0
                                    • VRoninV Offline
                                      VRoninV Offline
                                      VRonin
                                      wrote on last edited by VRonin
                                      #28

                                      from http://doc.qt.io/qt-5/qtglobal.html#qFuzzyCompare

                                      Note that comparing values where either p1 or p2 is 0.0 will not work

                                      so instead of qFuzzyCompare(0.0,std::modf(tester,&junk)) use qFuzzyIsNull(std::modf(tester,&junk))

                                      "La mort n'est rien, mais vivre vaincu et sans gloire, c'est mourir tous les jours"
                                      ~Napoleon Bonaparte

                                      On a crusade to banish setIndexWidget() from the holy land of Qt

                                      1 Reply Last reply
                                      0
                                      • tomyT Offline
                                        tomyT Offline
                                        tomy
                                        wrote on last edited by tomy
                                        #29

                                        I don't know why you have thought that the decimal part will always be zero.

                                        I've used this:

                                        qFuzzyCompare(1.0,1.0+std::modf(tester,&junk))
                                        
                                        1 Reply Last reply
                                        0
                                        • VRoninV Offline
                                          VRoninV Offline
                                          VRonin
                                          wrote on last edited by
                                          #30

                                          that will work too but the loop must go on untill the decimal part is 0 so qFuzzyIsNull is appropriate.

                                          "La mort n'est rien, mais vivre vaincu et sans gloire, c'est mourir tous les jours"
                                          ~Napoleon Bonaparte

                                          On a crusade to banish setIndexWidget() from the holy land of Qt

                                          1 Reply Last reply
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