[SOLVED] Sorting possible on a QVector that has a struct as elements

astodolski

I have a QVector<marker>MarkerMap();

marker is made up of:

typedef struct
{
    int x;
    int y;
    quint16 intensity;
}marker;

I would like to know if it's possible to sort the vector on the intensity member

Thanks

Chris Kawa

Yes, either implement operator< for it or pass a lambda to qSort:

typedef struct
{
    int x;
    int y;
    quint16 intensity;
} marker;

//Method 1:
bool operator<(const marker& a, const marker& b) { return a.intensity < b.intensity; }
//and then:
QVector<marker> foo;
foo << marker{1,1,10} << marker{1,1,5} << marker{1,1,23};
qSort(foo);

//Method 2:
QVector<marker> foo;
foo << marker{1,1,10} << marker{1,1,5} << marker{1,1,23};
qSort(foo.begin(), foo.end(), [](const marker& a, const marker& b) { return a.intensity < b.intensity; });

astodolski

Yes, either implement operator< for it or pass a lambda to qSort:

Thanks Chris for responding.

I'm sorry I should have mentioned that qSort is now deprecated as of Qt 5.3 which is what I am using. Can I use the above with std:sort instead of qSort.

Chris Kawa

Sure, it works the same.

astodolski

Sure, it works the same.

Hmm. OK because both "methods" did not compile.

Method 2 yields:

error: C3867: 'QVector<marker>::begin': function call missing argument list; use '&QVector<marker>::begin' to create a pointer to member

My call:

qSort(MarkerMap.begin, MarkerMap.end(), [](const marker& a, const marker& b) {return a.intensity < b.intensity;});

Chris Kawa

Are you sure you didn't write begin instead of begin() ?

Anyway I correct myself: sure, it works (almost) the same.
std::sort always takes iterators not the whole container so in first method pass begin() and end() to it instead of just the container.

astodolski

I left the parends out of the begin call. I'm trying it with qSort. Even though deprecated qSort still works (if used right that is).

Method 1 for some reason still wouldn't compile

Chris Kawa

#include <algorithm>
#include <QVector>

typedef struct
{
    int x;
    int y;
    quint16 intensity;
} marker;

bool operator<(const marker& a, const marker& b) { return a.intensity < b.intensity; }

int main()
{
    QVector<marker> foo;
    foo << marker{1,1,10} << marker{1,1,5} << marker{1,1,23};
    std::sort(foo.begin(), foo.end());
}

Works as expected.

astodolski

Thanks, I need to see why this will not compile:

bool operator<(const marker& a, const marker& b) { return a.intensity < b.intensity; }

yields:

C:\Users\Default.WIN-SHC18RVLNPH\Documents\Qt Projects\G14xx Dead Pixel Test\worker.cpp:31: error: C2601: 'operator <' : local function definitions are illegal
worker.cpp(27): this line contains a '{' which has not yet been matched

Update:

I moved the operator to the top of the implementation file. The error stating the def was a local def tipped me off.

Thanks

Chris Kawa

You need to start reading the error messages ;)
It says that you can't declare operator< function inside another function. See my example. That operator needs to be at global scope.