Best way to sort one user-defined struct

[stella1016]

I have a user-defined data type, say "people". It has parameter: id (unique), number_of_books, age, weight. The data set is built.
Now I want to have the ordered id with following criterias:
1. number_of_books
2. age
3. weight

which means, the order mainly depends on number_of_books, then age, then weight.

How could I implement this by using QT?
I am thinking about qSort(), but it has only one key to sort, right?
Thanks a lot~

[Le_B]

qSort use the less than operator so you must implement one for your struct and the use qSort

[stella1016]

Thanks. I tried, but there is error. Do you know the problem? Thanks again.

qSort(before_sort.begin(), before_sort.end(), qGreater<people>);
 
bool MyClass::qGreater(people p1, people p2)
{
	return true;
}

qSort(before_sort.begin(), before_sort.end(), qGreater<people>);

bool MyClass::qGreater(people p1, people p2)
{
	return true;
}

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qSort use the less than operator so you must implement one for your struct and the use qSort

[yeye_olive]

I could not find a clear confirmation of this in the docs, but it seems to me that qGreater<T>() just calls operator>() for operands of type T. If you want to use qGreater() you have to implement the > operator for your class.

Alternatively you can:
- define the operator < for your class and call qSort(begin, end);
- pick any name for your comparison function, like bool myComparisonFunction(const T &lhs, const T &rhs) and call qSort(begin, end, myComparisonFunction).

In any case all you have to do is implement in your comparison function the lexicographical comparison you described.

[Le_B]

exactly in your people struct just put:

bool operator <(const T& b) const
{
// put you code here
}

bool operator <(const T& b) const
{
// put you code here
}

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and then use :

QList<people> lst_people;
qSort(lst_people);

QList<people> lst_people;
qSort(lst_people);

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[Lykurg]

or without modifying the struct use

qSort(before_sort.begin(), before_sort.end(), MyClass::qGreater);
bool MyClass::qGreater(const people& p1, const people& p2)
{
	return true;
}

qSort(before_sort.begin(), before_sort.end(), MyClass::qGreater);
bool MyClass::qGreater(const people& p1, const people& p2)
{
	return true;
}

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[amleto]

are you sure that will work with member function as comparer?


e:
just tested, and it wont work.

needs to be a free function. or maybe a static.

[yeye_olive]

Right, it should be a standalone function or a static method.

[stella1016]

Thanks a lot for all the replies!
The following code works. That's right, the compare method has to be static, should be not as a member function of this class.

bool qGreater(people p1, people p2)
{
	return true;
}
 
QList<people> MyClass::sorting_people()
{	
        ...
	qSort(before_sort.begin(), before_sort.end(), qGreater);
	...
}

bool qGreater(people p1, people p2)
{
	return true;
}

QList<people> MyClass::sorting_people()
{	
        ...
	qSort(before_sort.begin(), before_sort.end(), qGreater);
	...
}

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[Lykurg]

Also member functions can be static! So instead polluting the global namespace I would at least create a namespace for comparing function. And as for your case if you have a function MyClass::sorting_people, I would probably go for a static member function called MyClass::sorting_people_helper or something like that.

[Pandimensional]

Hi

I cant quite understand, I have hte following:

QList<QWidget *> tempList = tempLegend->findChildren<QWidget *>();
qSort(tempList.begin(), tempList.end(), qGreater);

but this is in a PlotWindow::PlotWindow constructor. The following error is generated:

missing template arguements before ) token.