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Bug in conversion from double to integer (only some value)

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  • Christian EhrlicherC Offline
    Christian EhrlicherC Offline
    Christian Ehrlicher
    Lifetime Qt Champion
    wrote on last edited by
    #5

    I dobut there is a bug in such a simple thing.
    Please provide the output of the three toDouble() without multiplication.

    Qt Online Installer direct download: https://download.qt.io/official_releases/online_installers/
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    1 Reply Last reply
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    • J Offline
      J Offline
      JAHT
      wrote on last edited by
      #6

      The double value are right.

      QString s16 = "1.6";
      QString s17 = "1.7";
      QString s18 = "1.8";
      auto i16 = uint8_t(s16.toDouble() * 10);
      auto i17 = uint8_t(s17.toDouble() * 10);
      auto i18 = uint8_t(s18.toDouble() * 10);
      qDebug() << s16.toDouble() << s17.toDouble() << s18.toDouble();
      qDebug() << int(i16) << int(i17) << int(i18);
      

      10:27:17: Starting D:\TrabajoJose\DoubleConversion\build-DoubleConversion-Desktop_Qt_5_15_2_MinGW_32_bit-Debug\debug\DoubleConversion.exe ...
      1.6 1.7 1.8
      16 16 18

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      • Christian EhrlicherC Offline
        Christian EhrlicherC Offline
        Christian Ehrlicher
        Lifetime Qt Champion
        wrote on last edited by
        #7

        So no Qt problem here.

        Qt Online Installer direct download: https://download.qt.io/official_releases/online_installers/
        Visit the Qt Academy at https://academy.qt.io/catalog

        JonBJ 1 Reply Last reply
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        • Christian EhrlicherC Christian Ehrlicher

          So no Qt problem here.

          JonBJ Offline
          JonBJ Offline
          JonB
          wrote on last edited by JonB
          #8

          @Christian-Ehrlicher , @JAHT
          uin8_t(double) does floor not any kind of round, right? So a tiny difference where what shows as 1.7 is actually 1.6999999999999 and * 10 gives 16.999999999 would be enough for uint_t() to produce 16 rather than 17? Still not sure why 32-bit worse than 64-bit, are doubles not 64-bit in both (try printing sizeof(double) in each)? I think this code should not directly use uint_t(double), some sort of math round() first? Or add 0.5 like uint8_t(s17.toDouble() * 10 + 0.5);? I always add 0.5 (or 0.4999 depending on what you want to happen to x.5) before inting a double.

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          • J Offline
            J Offline
            JAHT
            wrote on last edited by
            #9

            Yes, it seems that that is the issue, I was wrong when saying the version used by my colleague that it worked, it is Qt 6.6.0 MinGW_64_bit

            JonBJ 1 Reply Last reply
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            • Christian EhrlicherC Offline
              Christian EhrlicherC Offline
              Christian Ehrlicher
              Lifetime Qt Champion
              wrote on last edited by Christian Ehrlicher
              #10

              Again: this has nothing to do with Qt but maybe with the compiler you're using (mingw version whatever). And @JonB is correct - you should round your number before otherwise it's truncated.

              Qt Online Installer direct download: https://download.qt.io/official_releases/online_installers/
              Visit the Qt Academy at https://academy.qt.io/catalog

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              • J JAHT

                Yes, it seems that that is the issue, I was wrong when saying the version used by my colleague that it worked, it is Qt 6.6.0 MinGW_64_bit

                JonBJ Offline
                JonBJ Offline
                JonB
                wrote on last edited by JonB
                #11

                @JAHT
                I am a bit lost as to whether you now say it does or does not work correctly in 64-bit MinGW. But in any case if you think about it I suggest you always add 0.5/0.4999 to a double before you int it if you want it rounded not floored.

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                • J Offline
                  J Offline
                  JAHT
                  wrote on last edited by
                  #12

                  @Christian-Ehrlicher @JonB

                  Thank you for your suggestion.

                  With 32-bit MinGW it doesn´t work and with 64-bit MinGW it works

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                  • JonBJ Offline
                    JonBJ Offline
                    JonB
                    wrote on last edited by JonB
                    #13

                    If 32-bit MinGW says unit_t(1.7 * 10 + 0.5) == 16 I would be surprised/shocked, and worry about anything (at least with floating point) compiled with it, but I cannot test....

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                    • J Offline
                      J Offline
                      JAHT
                      wrote on last edited by JAHT
                      #14

                      Yes with:

                      QString s16 = "1.6";
                       QString s17 = "1.7";
                       QString s18 = "1.8";
                       auto i16 = uint8_t(s16.toDouble() * 10+0.5);
                       auto i17 = uint8_t(s17.toDouble() * 10+0.5);
                       auto i18 = uint8_t(s18.toDouble() * 10+0.5);
                       qDebug() << s16.toDouble() << s17.toDouble() << s18.toDouble();
                       qDebug() << int(i16) << int(i17) << int(i18);
                      

                      The result is as expected with MingW_32

                      11:22:25: Starting D:\TrabajoJose\DoubleConversion\build-DoubleConversion-Desktop_Qt_5_15_2_MinGW_32_bit-Debug\debug\DoubleConversion.exe ...
                      1.6 1.7 1.8
                      16 17 18

                      Thank you very much @Christian-Ehrlicher @JonB

                      Christian EhrlicherC 1 Reply Last reply
                      1
                      • J JAHT

                        Yes with:

                        QString s16 = "1.6";
                         QString s17 = "1.7";
                         QString s18 = "1.8";
                         auto i16 = uint8_t(s16.toDouble() * 10+0.5);
                         auto i17 = uint8_t(s17.toDouble() * 10+0.5);
                         auto i18 = uint8_t(s18.toDouble() * 10+0.5);
                         qDebug() << s16.toDouble() << s17.toDouble() << s18.toDouble();
                         qDebug() << int(i16) << int(i17) << int(i18);
                        

                        The result is as expected with MingW_32

                        11:22:25: Starting D:\TrabajoJose\DoubleConversion\build-DoubleConversion-Desktop_Qt_5_15_2_MinGW_32_bit-Debug\debug\DoubleConversion.exe ...
                        1.6 1.7 1.8
                        16 17 18

                        Thank you very much @Christian-Ehrlicher @JonB

                        Christian EhrlicherC Offline
                        Christian EhrlicherC Offline
                        Christian Ehrlicher
                        Lifetime Qt Champion
                        wrote on last edited by
                        #15

                        @JAHT said in Bug in conversion from double to integer (only some value):

                        +0.5

                        use a proper round function or you will get wrong results with negative numbers.

                        Qt Online Installer direct download: https://download.qt.io/official_releases/online_installers/
                        Visit the Qt Academy at https://academy.qt.io/catalog

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                        • JonBJ Offline
                          JonBJ Offline
                          JonB
                          wrote on last edited by
                          #16

                          Great that is working now. But can you please write s16.toDouble() * 10 + 0.5 rather than * 10+0.5 as that just encourages mis-reading :)

                          @Christian-Ehrlicher said in Bug in conversion from double to integer (only some value):

                          use a proper round function or you will get wrong results with negative numbers.

                          Never thought about that as I don't have negative numbers! What would you suggest? Because I am worried that round() from the math library delivers its result as a double not an int. Then if the conversion of that from double to int still has some tiny floating point precision issue, as your tests claim to show, I am wondering if that still might produce 16 where it ought be 17...?

                          S 1 Reply Last reply
                          0
                          • JonBJ JonB

                            Great that is working now. But can you please write s16.toDouble() * 10 + 0.5 rather than * 10+0.5 as that just encourages mis-reading :)

                            @Christian-Ehrlicher said in Bug in conversion from double to integer (only some value):

                            use a proper round function or you will get wrong results with negative numbers.

                            Never thought about that as I don't have negative numbers! What would you suggest? Because I am worried that round() from the math library delivers its result as a double not an int. Then if the conversion of that from double to int still has some tiny floating point precision issue, as your tests claim to show, I am wondering if that still might produce 16 where it ought be 17...?

                            S Offline
                            S Offline
                            SimonSchroeder
                            wrote on last edited by
                            #17

                            @JonB said in Bug in conversion from double to integer (only some value):

                            Because I am worried that round() from the math library delivers its result as a double not an int.

                            Well, there are also lround and llround (https://en.cppreference.com/w/cpp/numeric/math/round). Hopefully, these two functions don't provide any problems with corner cases.

                            JonBJ 1 Reply Last reply
                            1
                            • S SimonSchroeder

                              @JonB said in Bug in conversion from double to integer (only some value):

                              Because I am worried that round() from the math library delivers its result as a double not an int.

                              Well, there are also lround and llround (https://en.cppreference.com/w/cpp/numeric/math/round). Hopefully, these two functions don't provide any problems with corner cases.

                              JonBJ Offline
                              JonBJ Offline
                              JonB
                              wrote on last edited by
                              #18

                              @SimonSchroeder I didn't know about std::lround(), that should be acceptable if preferred to adding 0.5 and then taking int.

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