Float and QString: Strange problem


  • Qt Champions 2017

    double coordinate;
    
    int degrees = int(coordinate);
    coordinate = (coordinate - degrees) * 60;
    int minutes = int(coordinate);
    int seconds = (coordinate - minutes) * 60;
    

    PS.
    Usually the seconds are taken as a floating point, not as int as here.



  • @kshegunov I use grade (int) and minutes (double) only.



  • @aha_1980 Thanks for your reply. If I add 0.5 + 0.499999999999 a failure also occurs as in this case with 0.999999999999999- My guess is that the start value is 60.99999999999999999999999999999 but the Qdebug shows it as 61.

    The problem is that 60 grade 60 minutes as a 61 ° replacement is unacceptable.


  • Qt Champions 2018

    @Marce

    Please give us compileable code and test data.

    Otherwise it's just guessing from our side.

    Regards



  • @Marce said in Float and QString: Strange problem:

    have a function that returns two double values: Latitude and Longitude and I want to convert it to the format: degrees (integer) and minutes (double).

    What about using the class QGeoCoordinate? In particular the toString() method... From documentation:

    Returns this coordinate as a string in the specified format.



  • @Marce said in Float and QString: Strange problem:

    if (min_long >= 60)
    {
    min_long -= 60;
    grade_long++;
    }

    if (min_lat >= 60)
    {
    min_lat -= 60;
    grade_lat++;
    }

    Is it possible min_lat or min_long are longer than 120?
    Consider changing to while loops to be more robust:

    while (min_long >= 60)
         {
          min_long -= 60;
          grade_long++;
         }
    
    
    while (min_lat >= 60)
         {
          min_lat -= 60;
          grade_lat++;
         }
    


  • @fcarney Thanks for your reply. It's not possible. I have added it on purpose so that the problem is well observed.



  • @Pablo-J.-Rogina Thanks for your reply. My goal is to express the coordinates in degrees in the format:
    gg mm.mmmmmmm
    g: degrees
    m: minutes
    and with that library it is not possible.



  • Works for me:

    #include <QCoreApplication>
    #include <QDebug>
    
    void double_string(double Long, double Lat){
        int grade_lat;
        int grade_long;
        double min_lat;
        double min_long;
    
        grade_lat = abs (Lat);
        grade_long = abs(Long);
        min_lat = 60*(abs (Lat) - grade_lat);
        min_long = 60*(abs (Long) - grade_long);
    
        qDebug() << "min_long:" << min_long;
        qDebug() << "min_lat:" << min_lat;
    
        if (min_long >= 60)
             {
              min_long -= 60;
              grade_long++;
             }
    
        if (min_lat >= 60)
             {
              min_lat -= 60;
              grade_lat++;
             }
    
        qDebug() << "long calculated: "+QString::number(Long,'f',7)+" grade_long: "+QString::number(grade_long,'f',7)+" min_long: "+QString::number(min_long,'f',7);
    
        qDebug() << "lat calculated: "+QString::number(Lat,'f',7)+" grade_lat: "+QString::number(grade_lat,'f',7)+" min_lat: "+QString::number(min_lat,'f',7);
    }
    
    int main(int argc, char *argv[])
    {
        QCoreApplication a(argc, argv);
    
        double_string(-31.0f,-61.0f);
        double_string(31.0f,61.0f);
        double_string(-31.0,-61.0);
        double_string(31.0,61.0);
    
        return a.exec();
    }
    

    Result:

    min_long: 0
    min_lat: 0
    "long calculated: -31.0000000 grade_long: 31.0000000 min_long: 0.0000000"
    "lat calculated: -61.0000000 grade_lat: 61.0000000 min_lat: 0.0000000"
    min_long: 0
    min_lat: 0
    "long calculated: 31.0000000 grade_long: 31.0000000 min_long: 0.0000000"
    "lat calculated: 61.0000000 grade_lat: 61.0000000 min_lat: 0.0000000"
    min_long: 0
    min_lat: 0
    "long calculated: -31.0000000 grade_long: 31.0000000 min_long: 0.0000000"
    "lat calculated: -61.0000000 grade_lat: 61.0000000 min_lat: 0.0000000"
    min_long: 0
    min_lat: 0
    "long calculated: 31.0000000 grade_long: 31.0000000 min_long: 0.0000000"
    "lat calculated: 61.0000000 grade_lat: 61.0000000 min_lat: 0.0000000"
    

  • Qt Champions 2017

    @Marce said in Float and QString: Strange problem:

    Thanks for your reply. It's not possible. I have added it on purpose so that the problem is well observed.

    I don't understand the problem. You've just said that the domain is -60 to +60, yet you feed invalid input to the algorithm. There's no algorithm that's consistently going to produce valid data out of invalid input.



  • Tanks for your reply. My problem is that a function is returning (supposedly 61 degrees). I need to convert it to the format gg mm.mmmmmmm, that is, 61 ° 00.00000000min, but the attached code shows 60 ° 60.00000000min. Although, 60 ° + 60 minutes = 61 ° is poorly expressed in the QDebug. If I initialize the variable with 61.0 ° the result is correct so I assume that the variable is stored in 60.9999999999999999999999999.



  • I can reproduce:

        double_string(30.9999999999999999,60.9999999999999999);
        double_string(30.99999999999999,60.99999999999999);
        double_string(30.999999999999,60.999999999999);
    
    "long calculated: 31.0000000 grade_long: 31.0000000 min_long: 0.0000000"
    "lat calculated: 61.0000000 grade_lat: 61.0000000 min_lat: 0.0000000"
    "long calculated: 31.0000000 grade_long: 30.0000000 min_long: 60.0000000"
    "lat calculated: 61.0000000 grade_lat: 60.0000000 min_lat: 60.0000000"
    "long calculated: 31.0000000 grade_long: 30.0000000 min_long: 60.0000000"
    "lat calculated: 61.0000000 grade_lat: 60.0000000 min_lat: 60.0000000"
    

    You will need to round the number to a lesser precision than the displayed number of 7.

        Long = double(int(Long*1000000)/1000000.0);
        Lat = double(int(Lat*1000000)/1000000.0);
    

    Result:

    "lat calculated: 61.0000000 grade_lat: 61.0000000 min_lat: 0.0000000"
    "long calculated: 30.9999990 grade_long: 30.0000000 min_long: 59.9999400"
    "lat calculated: 60.9999990 grade_lat: 60.0000000 min_lat: 59.9999400"
    "long calculated: 30.9999990 grade_long: 30.0000000 min_long: 59.9999400"
    "lat calculated: 60.9999990 grade_lat: 60.0000000 min_lat: 59.9999400"
    

  • Qt Champions 2017

    @Marce said in Float and QString: Strange problem:

    I need to convert it to the format gg mm.mmmmmmm

    Which I gave you the solution for. Truncate once to get the degrees and then subtract the whole part, multiply by 60 to get the minutes and that's it.

    If I initialize the variable with 61.0 ° the result is correct so I assume that the variable is stored in 60.9999999999999999999999999

    You should realize first that flotaing point numbers are approximations. No real number can be represented exactly, and to add insult to injury not every rational number can be represented exactly. Take the simplest - 1/3, how do you suppose is this represented. Firstly, even in decimal it's 0.33(3), secondly the floating point numbers in computers are powers of 2. There are no common divisors of 2 and 3, so you can get an approximation of 0.33 only, much less something that's infinitely long sequence of 3s. Exactly representable numbers are 1/2 (0.5), 1/4 (0.25), 1/8 (0.125), so any number that can be stored is a sum of such things, nothing more, nothing less.

    @fcarney said in Float and QString: Strange problem:

    You will need to round the number to a lesser precision than the displayed number of 7.

    Long = double(int(Long*1000000)/1000000.0);
    Lat = double(int(Lat*1000000)/1000000.0);
    

    Before doing a multiplication, truncation and division, which is terribly inefficient, have you considered making a simple addition?

    Long += 0.0000005;
    

    Display up to 6 digits and you get your rounding correctly.


  • Moderators

    Theres also qRound to add my 2 cents :)



  • @kshegunov said in Float and QString: Strange problem:

    Long += 0.0000005;

    Doesn't that just move the problem though? If it was close to the rounding, adding to the value could put it right at the rounding location. Somehow the value needs to be rounded/truncated to less precision than the display precision or the problem will persist.



  • @fcarney

    If it was close to the rounding, adding to the value could put it right at the rounding location

    I don't get what you mean by that. So far as I am aware, like @kshegunov I believe that will always mean that truncating the result after the addition at 6 decimal places will give the correctly-rounded result.



  • @JonB said in Float and QString: Strange problem:

    I don't get what you mean by that.

    Here is a value in which the value output from the function will produce grade 60 min 60:

    60.99999999999999
    

    Yes, adding to that value will produce 61 instead. However if I have this value:

    60.99999949999999
    
    adding 0.0000005 will result in 60.99999999999999
    

    The problem is just moved to a different location.



  • @fcarney
    Somewhere we're talking at cross-purposes.

    60.99999949999999 truncate at 6 dps => 60.999999 => correct
    60.99999959999999 truncate at 6 dps => 60.999999 => wrong (assuming it's supposed to round to nearest 6 dps)

    OTOH :
    60.99999949999999 + 0.0000005 = 60.99999999999999 truncate at 6 dps => 60.999999 => correct
    60.99999959999999 + 0.0000005 = 61.00000099999999 truncate at 6 dps => 61.000000 => correct too (assuming it's supposed to round to nearest 6 dps)



  • @JonB said in Float and QString: Strange problem:

    truncate at 6 dps

    That is the issue right there:

        double testvalue = 0.99999999999999;
        qInfo() << QString::number(testvalue,'f',14);
        qInfo() << QString::number(testvalue,'f',12);
        qInfo() << QString::number(testvalue,'f',7);
        qInfo() << QString::number(testvalue,'f',6);
    

    Result:

    "0.99999999999999"
    "1.000000000000"
    "1.0000000"
    "1.000000"
    

    The QString::number is not just truncating, but rounding as well.


  • Qt Champions 2017

    @fcarney said in Float and QString: Strange problem:

    The QString::number is not just truncating, but rounding as well.

    Right, meaning you don't need to do anything when using QString::number, right? :)

    Here is a value in which the value output from the function will produce grade 60 min 60:

    0_1558543658032_Screenshot_20190522_194630.png

    The value is exactly correct. What you mean, I presume, is that when you display it with QString it gets rounded. You could tryo to change the rounding mode and use asprintf instead. Something like this:

    const int rmode = fegetround( );
    fesetround(FE_TOWARDZERO);
    QString result = QString::asprintf("%.6f", &minutes);
    fesetround(rmode);
    


  • @kshegunov said in Float and QString: Strange problem:

    Right, meaning you don't need to do anything when using QString::number, right? :)

    No, because QString::number is producing:

    "long calculated: -61.0000000 grade_long: 60.0000000 min_long: 60.0000000"
    instead of:
    "long calculated: -61.0000000 grade_long: 60.0000000 min_long: 59.9999999"
    

  • Qt Champions 2017

    @fcarney said in Float and QString: Strange problem:

    No, because QString::number is producing

    Edited the post.



  • @kshegunov said in Float and QString: Strange problem:

    const int rmode = fegetround( );
    fesetround(FE_TOWARDZERO);
    QString result = QString::asprintf("%.6f", &minutes);
    fesetround(rmode);

    I was wondering if there was a way to do that. I noticed sprintf did the same thing as number did. So all I could think of was the math route or doing post string output:

        QString tstr = QString::number(testvalue,'f',16);
        QStringList tstrs = tstr.split('.');
        tstrs[1].resize(7);
        QString tstr2 = tstrs[0] + '.' + tstrs[1];
        qInfo() << tstr;
        qInfo() << tstr2;
    

    I think your way of changing the rounding behavior is better though.


  • Qt Champions 2017

    Yeah, but there are two problems with it: It can be finicky and secondly you can't load it directly into QString (turns out). So you need to do something like this:

    #include <fenv.h>
    #include <cmath>
    
    #include <QString>
    
    int main(int, char **)
    {
    
        double coordinate = 60.99999999999999;
    
        int degree = coordinate;
        coordinate = (coordinate - degree) * 60;
    
        char minuteString[100] = {0};
    
        const int rmode = fegetround();
        fesetround(FE_TOWARDZERO);
        sprintf(minuteString, "%.6f", coordinate);
        fesetround(rmode);
    
        QString minutes = QString::fromLatin1(minuteString);
    
        return 0;
    }
    

    Another option would be to do something like this:

    int degree = coordinate;
    coordinate = (coordinate - degree) * 60;
    
    if (int(coordinate) >= 60)
        coordinate -= 0.0000005; // Round down
    
    QString minute = QString::number(coordinate, 'f', 6);
    

    Or better yet:

    coordinate += 0.0000005 / 60; // Round the minutes before doing whatever it is.
    
    int degree = coordinate;
    coordinate = (coordinate - degree) * 60;
    
    QString minute = QString::number(coordinate, 'f', 6);
    


  • @kshegunov said in Float and QString: Strange problem:

    coordinate += 0.0000005 / 60;

    Isn't this just adding an offset though? Moving the problem somewhere else? If its not then please explain how it works.

    Edit:
    This solution makes a lot of sense though:

    if (int(coordinate) >= 60)
        coordinate -= 0.0000005; // Round down
    

  • Qt Champions 2017

    @fcarney said in Float and QString: Strange problem:

    Isn't this just adding an offset though? Moving the problem somewhere else? If its not then please explain how it works.

    No, it's not moving it somewhere else, but my example is incomplete. Sorry about that.

    coordinate += 0.0000005 / 60; // Round the minutes before doing whatever it is.
    
    int degree = coordinate;
    coordinate = (coordinate - degree) * 60 - 0.0000005; //< This last part was missed, sorry about that.
    
    QString minute = QString::number(coordinate, 'f', 6);
    

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