How to sort a 2D QVector<Qvector>double>> by the first column

Tomax63

My QVector<Qvector<double>> looks ike this:

{{100, 50}, {150, 40}, {50, 58}, {75, 60}}

I want it to look like

{{50, 58}, {75, 60}, {100, 50}, {150, 40}}

How can I sort it by the first column?

Thomas

VRonin

std::sort(
    vect.begin()
    ,vect.end()
    ,[](const QVector<double>& left,const QVector<double>& right)->bool{
        if(left.empty()) 
            return true; 
        if(right.empty())
            return true;
        return left.first()<right.first();
    }
);

Tomax63

Wow, thats fast! Thanks!

    sort(erhalten.begin(),erhalten.end() ,[](const QVector<double>& left,const QVector<double>& right)->bool{
            if(left.empty())
                return true;
            if(right.empty())
                return true;
            return left.first()<right.first();
        }
    );

Sadly i get 7 error messages like:

C:\Qt\Tools\mingw530_32\i686-w64-mingw32\include\c++\bits\stl_algo.h:1847: Fehler: passing 'const QVector<double>' as 'this' argument discards qualifiers [-fpermissive]
*__first = _GLIBCXX_MOVE(__val);
^
C:\Qt\Tools\mingw530_32\i686-w64-mingw32\include\c++\bits\stl_heap.h:218: Fehler: passing 'const QVector<double>' as 'this' argument discards qualifiers [-fpermissive]
(__first + __holeIndex) = _GLIBCXX_MOVE((__first + __secondChild));
^
C:\Qt\Tools\mingw530_32\i686-w64-mingw32\include\c++\bits\stl_heap.h:224: Fehler: passing 'const QVector<double>' as 'this' argument discards qualifiers [-fpermissive]
(__first + __holeIndex) = _GLIBCXX_MOVE((__first
^

JohanSolo

~~Remove the const then.~~

Tomax63

Nope, still more errors (10)

C:\Qt\Tools\mingw530_32\i686-w64-mingw32\include\c++\bits\stl_algo.h:1847: Fehler: passing 'const QVector<double>' as 'this' argument discards qualifiers [-fpermissive]
__first = _GLIBCXX_MOVE(__val);
^
C:\Qt\Tools\mingw530_32\i686-w64-mingw32\include\c++\bits\predefined_ops.h:123: Fehler: no match for call to '(MainWindow::get_real(QVector<QVector<double> >)::<lambda(QVector<double>&, QVector<double>&)>) (const QVector<double>&, const QVector<double>&)'
{ return bool(_M_comp(__it1, __it2)); }
^
C:\Qt\Tools\mingw530_32\i686-w64-mingw32\include\c++\bits\predefined_ops.h:123: Fehler: binding 'const QVector<double>' to reference of type 'QVector<double>&' discards qualifiers
C:\Qt\Tools\mingw530_32\i686-w64-mingw32\include\c++\bits\predefined_ops.h:123: Fehler: binding 'const QVector<double>' to reference of type 'QVector<double>&' discards qualifiers
{ return bool(_M_comp(__it1, *__it2)); }
^

and 6 more

JohanSolo

Ah, sorry, I feel you're calling the aforementioned code in a const method of MainWindow. Does this make sense?

VRonin

To sort you need to modify the array. is erhalten const?

Tomax63

I have no idea whether that makes sense. I just copied the code into my routine where i want to process the Qvector.

How else should I use the code?

VRonin

Correction:

std::sort(erhalten.begin(),erhalten.end() ,[](const QVector<double>& left,const QVector<double>& right)->bool{
            if(left.empty() && right.empty())
                return false;
            if(left.empty())
                return true;
            if(right.empty())
                return false;
            return left.first()<right.first();
        }
    );

Tomax63

Yes, that was the problem. my array was const. Sorry for that easy error.

Thanks a lot!