Why does one conversion fail but not the other?

[MScottM]

Here is a sample of my logfile (with header):

Date,Time(UTC),Bus Name,PGN#,PGN Priority,PGN Src. Addr.,PGN Dest. Addr.,Data Length,DB0,DB1,DB2,DB3,DB4,DB5,DB6,DB7
20151029,140003,NMEA2000,127250,2,01,ff,8,ff,f1,bc,ff,7f,ff,7f,fd
20151029,140040,NMEA2000,129025,2,03,ff,8,ab,23,8e,13,0e,8f,6b,b9

I've built QStringLists with the data that I need (DB0, DB1...)

And I'm trying to make it human readable. Here is my code to get at the GPS position in the log data:

void MainWindow::on_btnPosition_clicked()
{

    //BUILD LATITUDE HEX NUMBER FROM LOG DATA
    PGNListCount = PGNList.count();
    for (int i=0;i<PGNListCount;++i) {
        if (PGNList.at(i).contains("129025")) {
            DB30Data << DB3.at(i) + DB2.at(i) + DB1.at(i) + DB0.at(i);
            }
     }    
    qDebug() << "DB30data" << DB30Data.at(10);

    //BUILD LONGITUDE HEX NUMBER FROM LOG DATA
    PGNListCount = PGNList.count();
    qDebug() << "PGNListCount" << PGNListCount;
    for (int i=0;i<PGNListCount;++i) {
        if (PGNList.at(i).contains("129025")) {
            DB74Data << DB7.at(i) + DB6.at(i) + DB5.at(i) + DB4.at(i);

            }
     }
    qDebug() << "DB74Data" << DB74Data.at(10);



    //CONVERT LATITUDE HEX TO FLOAT
    int DB30DataCount = DB30Data.count();
    for (int i=0; i<DB30DataCount;++i) {
            latitudeFromHex = ((DB30Data.at(i).toInt(&ok, 16))*0.0000001);
            if (!ok){
                qDebug() << "latitude conversion failed";
                break;
            }
            Latitude.append(latitudeFromHex);
    }
    qDebug() << "Latitude" << QString::number(Latitude.at(10),'f',7);


    //CONVERT LONGITUDE HEX TO FLOAT
    int DB74DataCount = DB74Data.count();
    qDebug() << "DB74DataCount" << DB74DataCount;
    for (int i=0; i<DB74DataCount; ++i) {
            longitudeFromHex = ((DB74Data.at(i).toInt(&ok, 16))*0.0000001);
            if (!ok){
                qDebug() << "longitude conversion failed";
                break;
            }
            Longitude.append(longitudeFromHex);
    }
    qDebug() << "Longitude" << QString::number(Longitude.at(10),'f',10);

}

And here are the resulting debug statements:

Debugging starts
DB30data "138e23ab"
PGNListCount 1000
DB74Data "b96b8f1d"
Latitude "32.8082347"
DB74DataCount 200
longitude conversion failed
ASSERT failure in QVector<T>::at: "index out of range", file C:\Qt\5.5\mingw492_32\include/QtCore/qvector.h, line 393
Debugging has finished

Latitude and Longitude are declared as QVector<long double>
latitudeFromHex and longitudeFromHex are declared as long double

Why is the Latitude conversion working, but the Longitude conversion failing? This is a case where I need a nudge in the right direction - I haven't turned up anything in my own research.

Thanks for any help!

[kshegunov]

Have a look at the stack trace, it will point you to the exact line in your code where you overrun the vector.

[MScottM]

I tried setting a breakpoint and stepping through while watching the stack, but it doesn't crash anywhere - it just drops to the boolean check and breaks out. If I take the check out, it just loads the array with zeros.

[Eeli K]

@MScottM You have

    if (!ok){
                qDebug() << "longitude conversion failed";
                break;
            }
            Longitude.append(longitudeFromHex);
    }
    qDebug() << "Longitude" << QString::number(Longitude.at(10),'f',10);

What does it show if you change to

qDebug() << "longitude conversion failed at " << i;

and

qDebug() << "Longitude count " << Longitude.count();
qDebug() << "Longitude" << QString::number(Longitude.at(10),'f',10);

[MScottM]

@Eeli-K Thanks for the idea!

Debug statements:

DB30data "138e23ab"
PGNListCount 1000
DB74Data "b96b8f0e"
Latitude "32.8082347"
DB74DataCount 200
longitude conversion failed at 0
longitude count 0
ASSERT failure in QVector<T>::at: "index out of range", file C:\Qt\5.5\mingw492_32\include/QtCore/qvector.h, line 393
Debugging has finished

I put all three debug statements in the 'if' check like this:

if (!ok){
                qDebug() << "longitude conversion failed at " << i;
                qDebug() << "longitude count" << Longitude.count();
                qDebug() << "Longitude" << QString::number(Longitude.at(i),'f',10);
                break;
            }

[Eeli K]

@MScottM It's obvious that

longitudeFromHex = ((DB74Data.at(i).toInt(&ok, 16))*0.0000001);

doesn't work as you expect. Break it down to pieces, one new function call/assignment per line, and follow it with debugger or add a qDebug line after each line.

[MScottM]

Got it! I had to convert to LongLong:

longitudeFromHex = ((DB74Data.at(i).toLongLong(&ok, 16))*0.0000001);

did the trick.

Thanks for taking the time to post!