Skip to content
  • Categories
  • Recent
  • Tags
  • Popular
  • Users
  • Groups
  • Search
  • Get Qt Extensions
  • Unsolved
Collapse
Brand Logo
  1. Home
  2. Special Interest Groups
  3. C++ Gurus
  4. How to use Qt documentation?
Forum Updated to NodeBB v4.3 + New Features

How to use Qt documentation?

Scheduled Pinned Locked Moved Solved C++ Gurus
documentationqtendian
16 Posts 6 Posters 2.4k Views 1 Watching
  • Oldest to Newest
  • Newest to Oldest
  • Most Votes
Reply
  • Reply as topic
Log in to reply
This topic has been deleted. Only users with topic management privileges can see it.
  • P Please_Help_me_D
    22 Nov 2019, 11:25

    @JonB I don't have a gun :D
    but as far as I know (and according to my experience in Matlab) files can be stored in any type (int, float, double etc) and in either big- or little-endian
    Also my data maybe stored in IEEE format but there is also IBM format (it is old format but still actively used while storing SEGY files)

    J Online
    J Online
    JonB
    wrote on 22 Nov 2019, 11:27 last edited by JonB
    #6

    @Please_Help_me_D
    Apologies.
    See https://stackoverflow.com/questions/2945174/floating-point-endianness & https://stackoverflow.com/questions/2782725/converting-float-values-from-big-endian-to-little-endian. I think the latter shows you code to convert. Also https://stackoverflow.com/questions/1786137/c-serialization-of-the-floating-point-numbers-floats-doubles.

    P 1 Reply Last reply 22 Nov 2019, 11:37
    3
    • J JonB
      22 Nov 2019, 11:27

      @Please_Help_me_D
      Apologies.
      See https://stackoverflow.com/questions/2945174/floating-point-endianness & https://stackoverflow.com/questions/2782725/converting-float-values-from-big-endian-to-little-endian. I think the latter shows you code to convert. Also https://stackoverflow.com/questions/1786137/c-serialization-of-the-floating-point-numbers-floats-doubles.

      P Offline
      P Offline
      Please_Help_me_D
      wrote on 22 Nov 2019, 11:37 last edited by
      #7

      @JonB Thank you!
      First of all I'm trying to use built-in function. And I liked Qt built in function because it can deal with array, that is what I like :)
      What <quint32> mean in this example?

      quint32 a[] = {1, 2, 3, 4, 5};
      quint32 x[5];
      qToBigEndian<quint32>(a, 5,x);
      
      J 1 Reply Last reply 22 Nov 2019, 11:44
      0
      • P Please_Help_me_D
        22 Nov 2019, 11:07

        @mrjj thank you!
        In reality I also need to use Little/Big conversation with float data-type. Do you know how to solve that?

        K Offline
        K Offline
        KroMignon
        wrote on 22 Nov 2019, 11:42 last edited by
        #8

        @Please_Help_me_D said in How to use Qt documentation?:

        In reality I also need to use Little/Big conversation with float data-type. Do you know how to solve that?

        As far as I know, a float is stored as a 32 bits word and a double as 64 bits word.
        So my suggestion would be to cast your float value to quint32 and your double to quint64

        float floatBE = static_cast<float>(qToBigEndian(static_cast<quint32>(val));
        double doubleBE = static_cast<double>(qToBigEndian(static_cast<quint64>(valDouble));
        

        It is an old maxim of mine that when you have excluded the impossible, whatever remains, however improbable, must be the truth. (Sherlock Holmes)

        P A 2 Replies Last reply 22 Nov 2019, 11:53
        0
        • P Please_Help_me_D
          22 Nov 2019, 11:37

          @JonB Thank you!
          First of all I'm trying to use built-in function. And I liked Qt built in function because it can deal with array, that is what I like :)
          What <quint32> mean in this example?

          quint32 a[] = {1, 2, 3, 4, 5};
          quint32 x[5];
          qToBigEndian<quint32>(a, 5,x);
          
          J Online
          J Online
          JonB
          wrote on 22 Nov 2019, 11:44 last edited by
          #9

          @Please_Help_me_D
          quint32 is (presumably) Qt's type for an unsigned 32-bit integer.
          qToBigEndian<quint32> is written like that presumably because it's a template class/function.

          1 Reply Last reply
          0
          • K KroMignon
            22 Nov 2019, 11:42

            @Please_Help_me_D said in How to use Qt documentation?:

            In reality I also need to use Little/Big conversation with float data-type. Do you know how to solve that?

            As far as I know, a float is stored as a 32 bits word and a double as 64 bits word.
            So my suggestion would be to cast your float value to quint32 and your double to quint64

            float floatBE = static_cast<float>(qToBigEndian(static_cast<quint32>(val));
            double doubleBE = static_cast<double>(qToBigEndian(static_cast<quint64>(valDouble));
            
            P Offline
            P Offline
            Please_Help_me_D
            wrote on 22 Nov 2019, 11:53 last edited by
            #10

            @KroMignon thank you! that should work, few minuts I'm going to try it
            @JonB please correct me if I misunderstood you, we need to write datatype in brackets <> to point that incoming (or outcoming??) parameter is in quint32 because qToBigEndian is a template function?
            I just did:

                double y[] = {1.442, 2.34, 1.56, 2.66, 68.88};
                double yy[5];
                qToLittleEndian<double>(y, 5, yy);
            

            and it doesn't output the error but I need to check the correctness of this

            J 1 Reply Last reply 22 Nov 2019, 11:57
            0
            • P Please_Help_me_D
              22 Nov 2019, 11:53

              @KroMignon thank you! that should work, few minuts I'm going to try it
              @JonB please correct me if I misunderstood you, we need to write datatype in brackets <> to point that incoming (or outcoming??) parameter is in quint32 because qToBigEndian is a template function?
              I just did:

                  double y[] = {1.442, 2.34, 1.56, 2.66, 68.88};
                  double yy[5];
                  qToLittleEndian<double>(y, 5, yy);
              

              and it doesn't output the error but I need to check the correctness of this

              J Online
              J Online
              JonB
              wrote on 22 Nov 2019, 11:57 last edited by
              #11

              @Please_Help_me_D
              It's not a "parameter" in the sense of passing to a function. You really need to go Google for C++ template if you wish to understand C++ templates.

              P 1 Reply Last reply 22 Nov 2019, 12:14
              2
              • J JonB
                22 Nov 2019, 11:57

                @Please_Help_me_D
                It's not a "parameter" in the sense of passing to a function. You really need to go Google for C++ template if you wish to understand C++ templates.

                P Offline
                P Offline
                Please_Help_me_D
                wrote on 22 Nov 2019, 12:14 last edited by
                #12

                @JonB ok I look for that
                I tryed to convert double precision point in Qt and in Matlab to check the correctness of little/big endian conversation. So in Qt:

                    double y[] = {1.442, 2.34, 1.56, 2.66, 68.88};
                    double yy[5];
                    qToLittleEndian<double>(y, 5, yy);
                

                yy = {-1.291069932956366e+151, -2.242484837845019e-38, -1.498274909768364e+261, 1.217960639251452e+43, -2.242484835660769e-38};

                In Matlab:

                // Matlab code
                y = [1.442, 2.34, 1.56, 2.66, 68.88];
                yy = swapbytes(y); // should rearrange the byte order [swapbyte_documentation](https://www.mathworks.com/help/matlab/ref/swapbytes.html)
                

                yy = [-1.29106993295637e+151, -2.24248483784502e-38, -1.49827490976836e+261, 1.21796063925145e+43, -2.24248483566077e-38];

                The Matlab result is very close to Qt result. So I think that Qt function qToLittleEndian works not only with qType but with other standart types as well

                1 Reply Last reply
                0
                • fcarneyF Offline
                  fcarneyF Offline
                  fcarney
                  wrote on 22 Nov 2019, 16:44 last edited by
                  #13

                  @Please_Help_me_D said in How to use Qt documentation?:

                  I need to check the correctness of this

                  Look up how a given value is stored in big endian and little endian. Then print out the bytes of your variable as hex. Do this before and after conversion. You should be able to see that it rearranges the bytes in an expected way. It might be worthwhile to create your own function to do this as well. So you understand more what is going on. It might also be worthwhile to templatize your function so you can see how templates work too.

                  C++ is a perfectly valid school of magic.

                  P 1 Reply Last reply 22 Nov 2019, 21:07
                  1
                  • fcarneyF fcarney
                    22 Nov 2019, 16:44

                    @Please_Help_me_D said in How to use Qt documentation?:

                    I need to check the correctness of this

                    Look up how a given value is stored in big endian and little endian. Then print out the bytes of your variable as hex. Do this before and after conversion. You should be able to see that it rearranges the bytes in an expected way. It might be worthwhile to create your own function to do this as well. So you understand more what is going on. It might also be worthwhile to templatize your function so you can see how templates work too.

                    P Offline
                    P Offline
                    Please_Help_me_D
                    wrote on 22 Nov 2019, 21:07 last edited by
                    #14

                    @fcarney thank you. I already did it with the help of Matlab above. I just converted the same numbers of double precision from Big to Little and the result was the same.
                    I understand how it works, it is clear from the definition of Little/Big endian and definition of byte

                    1 Reply Last reply
                    1
                    • K KroMignon
                      22 Nov 2019, 11:42

                      @Please_Help_me_D said in How to use Qt documentation?:

                      In reality I also need to use Little/Big conversation with float data-type. Do you know how to solve that?

                      As far as I know, a float is stored as a 32 bits word and a double as 64 bits word.
                      So my suggestion would be to cast your float value to quint32 and your double to quint64

                      float floatBE = static_cast<float>(qToBigEndian(static_cast<quint32>(val));
                      double doubleBE = static_cast<double>(qToBigEndian(static_cast<quint64>(valDouble));
                      
                      A Offline
                      A Offline
                      aha_1980
                      Lifetime Qt Champion
                      wrote on 22 Nov 2019, 21:18 last edited by
                      #15

                      @KroMignon said in How to use Qt documentation?:

                      @Please_Help_me_D said in How to use Qt documentation?:

                      In reality I also need to use Little/Big conversation with float data-type. Do you know how to solve that?

                      As far as I know, a float is stored as a 32 bits word and a double as 64 bits word.
                      So my suggestion would be to cast your float value to quint32 and your double to quint64

                      float floatBE = static_cast<float>(qToBigEndian(static_cast<quint32>(val));
                      double doubleBE = static_cast<double>(qToBigEndian(static_cast<quint64>(valDouble));
                      

                      nope, that does NOT work! Casting float to int truncates it. What you can do instead, is to reinterpret_cast the memory occupied by the float - or use a union. We currently have a similar topic in the forum on this.

                      Regards

                      Qt has to stay free or it will die.

                      1 Reply Last reply
                      3
                      • P Offline
                        P Offline
                        Please_Help_me_D
                        wrote on 20 Jan 2020, 16:11 last edited by Please_Help_me_D
                        #16

                        I have written the following code:

                            if (fileEndian == "Little"){
                                for(quint32 i = 0; i < nTrc; i++){
                                    FFID(i) = *util::bit_cast<qint32*>(qFromLittleEndian(qFile->map(3608+i*bytesPerTrc, 1)));
                                }
                            } else if (fileEndian == "Big"){
                                for(quint32 i = 0; i < nTrc; i++){
                                    FFID(i) = *util::bit_cast<qint32*>(qFromBigEndian(qFile->map(3608+i*bytesPerTrc, 1)));
                                }
                            }
                        

                        It gives me errors:

                        • readsegy.obj:-1: ошибка: LNK2019: unresolved external symbol "unsigned char * __cdecl qbswap<unsigned char *>(unsigned char *)" (??$qbswap@PEAE@@YAPEAEPEAE@Z) referenced in function "unsigned char * __cdecl qFromBigEndian<unsigned char *>(unsigned char *)" (??$qFromBigEndian@PEAE@@YAPEAEPEAE@Z)
                        • debug\ReadSegy.exe:-1: ошибка: LNK1120: 1 unresolved externals

                        The compilator output:
                        jom: C: \ Users \ tasik \ Documents \ Qt_Projects \ build-ReadSegy-Desktop_x86_windows_msvc2017_pe_64bit-Debug \ Makefile.Debug [debug \ ReadSegy.exe] Error 1120
                        jom: C: \ Users \ tasik \ Documents \ Qt_Projects \ build-ReadSegy-Desktop_x86_windows_msvc2017_pe_64bit-Debug \ Makefile [debug] Error 2
                        19:05:31: The process "C: \ Qt \ Tools \ QtCreator \ bin \ jom.exe" ended with code 2.
                        Error during assembly / deployment of ReadSegy project (bundle: Desktop (x86-windows-msvc2017-pe-64bit))
                        During the execution of the "Assembly"

                        Actually the problem was that I didn't know the line which throws these errors but by intuation I just commented the BigEndian part of the code and it works:

                            if (fileEndian == "Little"){
                                for(quint32 i = 0; i < nTrc; i++){
                                    FFID(i) = *util::bit_cast<qint32*>(qFromLittleEndian(qFile->map(3608+i*bytesPerTrc, 1)));
                                }
                            } else if (fileEndian == "Big"){/*
                                for(quint32 i = 0; i < nTrc; i++){
                                    FFID(i) = *util::bit_cast<qint32*>(qFromBigEndian(qFile->map(3608+i*bytesPerTrc, 1)));
                                }*/
                            }
                        

                        I use little endian Windows 10 x64, Qt 5.14.0, MSVC 2017 x64.
                        Why do I can use qFromLittleEndian but I can't qFromBigEndian??

                        By the way the endian of my file is LITTLE now

                        I think I just found a solution. If I change the order of performing bit_cast and qFromBigEndian it works:

                            if (fileEndian == "Little"){
                                for(quint32 i = 0; i < nTrc; i++){
                                    FFID(i) = qFromLittleEndian(*util::bit_cast<qint32*>(qFile->map(3608+i*bytesPerTrc, 1)));
                                }
                            } else if (fileEndian == "Big"){
                                for(quint32 i = 0; i < nTrc; i++){
                                    FFID(i) = qFromBigEndian(*util::bit_cast<qint32*>(qFile->map(3608+i*bytesPerTrc, 1)));
                                }
                            }
                        

                        I don't understand why but that works fine

                        1 Reply Last reply
                        0

                        • Login

                        • Login or register to search.
                        • First post
                          Last post
                        0
                        • Categories
                        • Recent
                        • Tags
                        • Popular
                        • Users
                        • Groups
                        • Search
                        • Get Qt Extensions
                        • Unsolved