Sorting using qSort(), - if QList contains POINTERS

Hi guys,


How to use qSort() if the QList contains pointers like this...

Code:

---

QList < double* > _doubleList;  
//  this is for simple easyness , you can give QList<employee*>
 
_doubleList .append( new( 2.22) );
_doubleList .append( new( 1.22) );
_doubleList .append( new( 4.22) );
_doubleList .append( new( 3.22) );
 
qSort( _doubleList );           // This code will not sort the data properly

---

while using the above "_doubleList" the data is not sorting .
But if am using the list like this ...

Code:

---

QList < double > _doubleList;  
//  this is for simple easyness , you can give QList<employee*>
 
_doubleList .append(  2.22 );
_doubleList .append(  1.22 );
_doubleList .append( 4.22 );
_doubleList .append( 3.22 );
 
qSort( _doubleList );           // This code WILL  SORT the data properly

---

please tell me how to sort a QList which contains pointers.
thanks in advance

You have to implement your own lessThan operator like shown here:

http://doc.trolltech.com/4.3/qtalgorithms.html#qSort-2

can anybody help me to sort the following code.
My intention is to sort the sudent object in assending order( based on the _rollNo ).

Code:

---

class Student
{
public:
	Student() 
		{ cout<<"\n" << __FUNCTION__; }
	Student(int roolNo ):_rolNo( roolNo ) 
		{ cout<<"\n" << __FUNCTION__; }
	~Student() 
		{ cout<<"\n" << __FUNCTION__; }
	 bool toAssending( Student & s1 , Student & s2 )
		{
			cout<<"\n" << __FUNCTION__;
			return s1.getRollNo() < s2.getRollNo();
		}
	int getRollNo()
		{
		return _rolNo;
		}
private :
	 int _rolNo;
 
};
 
int main()
{
 
QList<Student*> studentList ;
	studentList.append( new Student(2) );
	studentList.append( new Student(4) );	
	studentList.append( new Student(3) );
	studentList.append( new Student(1) );
	studentList.append( new Student(5) );
	QListIterator<Student*> it( studentList );
	Student* val = 0;
 
	cout<<"\n   ****** Before SORT ***** " ;
	while( it.hasNext() )
	{
		val = it.next();		
		cout<<"\n" << val->getRollNo();
 
	}
 
        qSort( studentList.begin() , studentList.end(), toAssending );   //  please correct  syntax if am wrong
 
	cout<<"\n   ****** After SORT ***** " ;
	QListIterator<Student*> it_1( studentList );
	Student* val_1 = 0;
	while( it_1.hasNext() )
	{
		val_1 = it_1.next();				
		cout<<"\n" << val_1->getRollNo();
	}
 
	qDeleteAll( studentList );
 
 
 
return 0;
}

---

thanks

Make toAssending() a non-member function.

Code:

---

class Student
{
    ...
};
 
bool toAssending( const Student & s1 , const Student & s2 )
{
	cout<<"\n" << __FUNCTION__;
	return s1.getRollNo() < s2.getRollNo();
}
 
int main()
{
	...
        qSort( studentList.begin() , studentList.end(), toAssending );
	...
}

---

jpn,

I have tried the same code its not working. i am getting compilation error.
here is my code

Code:

---

class Student
{
public:
	Student() 
		{ cout<<"\n" << __FUNCTION__; }
	Student(int roolNo ):_rolNo( roolNo ) 
		{ cout<<"\n" << __FUNCTION__; }
	~Student() 
		{ cout<<"\n" << __FUNCTION__; }
	/* 
	bool toAssending( Student & s1 , Student & s2 )
		{
			cout<<"\n" << __FUNCTION__;
			return s1.getRollNo() < s2.getRollNo();
		}*/
	int getRollNo() const
		{
		return _rolNo;
		}
private :
	 int _rolNo;
 
};
 
bool toAssending(  const Student & s1 ,  const Student & s2 )
{
    cout<<"\n" << __FUNCTION__;
    return s1.getRollNo() < s2.getRollNo();
}
 
int main()
{
 QList<Student*> studentList ;
	studentList.append( new Student(2) );
	studentList.append( new Student(4) );	
	studentList.append( new Student(3) );
	studentList.append( new Student(1) );
	studentList.append( new Student(5) );
	QListIterator<Student*> it( studentList );
	Student* val = 0;
	cout<<"\n   ****** Before SORT ***** " ;
	while( it.hasNext() )
	{
		val = it.next();
		//cout<<"\n" << val;
		cout<<"\n" << val->getRollNo();
 
	}
 
qSort( studentList.begin() , studentList.end(), toAssending );  //here am using the global function. instead of class -member function.
 
	cout<<"\n   ****** After SORT ***** " ;
	QListIterator<Student*> it_1( studentList );
	Student* val_1 = 0;
	while( it_1.hasNext() )
	{
		val_1 = it_1.next();		
		cout<<"\n" << val_1;
		cout<<"\n" << val_1->getRollNo();
	}
 
	qDeleteAll( studentList );
 
return 0;
}

---

here is the compliation error.
please help me to resolve it.

Code:

---

main.cpp(311) : warning C4101: 'assending' : unreferenced local variable
c:\Qt\4.2.2\include\QtCore\../../src\corelib\tools\qalgorithms.h(355) : error C2664: 'bool (const Student &,const Student &)' : cannot convert parameter 2 from 'Student *' to 'const Student &'
        Reason: cannot convert from 'Student *' to 'const Student'
        No constructor could take the source type, or constructor overload resolution was ambiguous
        c:\Qt\4.2.2\include\QtCore\../../src\corelib\tools\qalgorithms.h(180) : see reference to function template instantiation 'void QAlgorithmsPrivate::qSortHelper<RandomAccessIterator,T,bool(__cdecl *)(const Student &,const Student &)>(RandomAccessIterator,RandomAccessIterator,const T & ,LessThan)' being compiled
        with
        [
            RandomAccessIterator=QList<Student *>::iterator,
            T=Student *,
            LessThan=bool (__cdecl *)(const Student &,const Student &)
        ]
        main.cpp(312) : see reference to function template instantiation 'void qSort<QList<T>::iterator,bool(__cdecl *)(const Student &,const Student &)>(RandomAccessIterator,RandomAccessIterator,LessThan)' being compiled
        with
        [
            T=Student *,
            RandomAccessIterator=QList<Student *>::iterator,
            LessThan=bool (__cdecl *)(const Student &,const Student &)
        ]
c:\Qt\4.2.2\include\QtCore\../../src\corelib\tools\qalgorithms.h(355) : error C2664: 'bool (const Student &,const Student &)' : cannot convert parameter 1 from 'Student *' to 'const Student &'
        Reason: cannot convert from 'Student *' to 'const Student'
        No constructor could take the source type, or constructor overload resolution was ambiguous
c:\Qt\4.2.2\include\QtCore\../../src\corelib\tools\qalgorithms.h(360) : error C2664: 'bool (const Student &,const Student &)' : cannot convert parameter 2 from 'Student *' to 'const Student &'
        Reason: cannot convert from 'Student *' to 'const Student'
        No constructor could take the source type, or constructor overload resolution was ambiguous
c:\Qt\4.2.2\include\QtCore\../../src\corelib\tools\qalgorithms.h(360) : error C2664: 'bool (const Student &,const Student &)' : cannot convert parameter 1 from 'Student *' to 'const Student &'
        Reason: cannot convert from 'Student *' to 'const Student'
        No constructor could take the source type, or constructor overload resolution was ambiguous
c:\Qt\4.2.2\include\QtCore\../../src\corelib\tools\qalgorithms.h(362) : error C2664: 'bool (const Student &,const Student &)' : cannot convert parameter 2 from 'Student *' to 'const Student &'
        Reason: cannot convert from 'Student *' to 'const Student'
        No constructor could take the source type, or constructor overload resolution was ambiguous
c:\Qt\4.2.2\include\QtCore\../../src\corelib\tools\qalgorithms.h(362) : error C2664: 'bool (const Student &,const Student &)' : cannot convert parameter 1 from 'Student *' to 'const Student &'
        Reason: cannot convert from 'Student *' to 'const Student'
        No constructor could take the source type, or constructor overload resolution was ambiguous
c:\Qt\4.2.2\include\QtCore\../../src\corelib\tools\qalgorithms.h(370) : error C2664: 'bool (const Student &,const Student &)' : cannot convert parameter 2 from 'Student *' to 'const Student &'
        Reason: cannot convert from 'Student *' to 'const Student'
        No constructor could take the source type, or constructor overload resolution was ambiguous
c:\Qt\4.2.2\include\QtCore\../../src\corelib\tools\qalgorithms.h(370) : error C2664: 'bool (const Student &,const Student &)' : cannot convert parameter 1 from 'Student *' to 'const Student &'
        Reason: cannot convert from 'Student *' to 'const Student'
        No constructor could take the source type, or constructor overload resolution was ambiguous
c:\Qt\4.2.2\include\QtCore\../../src\corelib\tools\qalgorithms.h(373) : error C2664: 'bool (const Student &,const Student &)' : cannot convert parameter 2 from 'Student *' to 'const Student &'
        Reason: cannot convert from 'Student *' to 'const Student'
        No constructor could take the source type, or constructor overload resolution was ambiguous
c:\Qt\4.2.2\include\QtCore\../../src\corelib\tools\qalgorithms.h(373) : error C2664: 'bool (const Student &,const Student &)' : cannot convert parameter 1 from 'Student *' to 'const Student &'
        Reason: cannot convert from 'Student *' to 'const Student'
        No constructor could take the source type, or constructor overload resolution was ambiguous
c:\Qt\4.2.2\include\QtCore\../../src\corelib\tools\qalgorithms.h(385) : error C2664: 'bool (const Student &,const Student &)' : cannot convert parameter 2 from 'Student *' to 'const Student &'
        Reason: cannot convert from 'Student *' to 'const Student'
        No constructor could take the source type, or constructor overload resolution was ambiguous
c:\Qt\4.2.2\include\QtCore\../../src\corelib\tools\qalgorithms.h(385) : error C2664: 'bool (const Student &,const Student &)' : cannot convert parameter 1 from 'Student *' to 'const Student &'
        Reason: cannot convert from 'Student *' to 'const Student'
        No constructor could take the source type, or constructor overload resolution was ambiguous
 
 
logFile - 12 error(s), 10 warning(s)

---

Right, the comparator function should naturally take pointers since that's what the container contains:

Code:

---

bool toAssending( const Student * s1 , const Student * s2 )
{
    cout<<"\n" << __FUNCTION__;
    return s1->getRollNo() < s2->getRollNo();
}

---

could you please me , how this qSort() is working .
Are we passing function pointer to qSort() ...??



thanks

Quote:

---

Originally Posted by joseph

could you please me , how this qSort() is working.

---

You can always check Qt sources for implementation details. ;) You can find qSort() implementation in src/corelib/tools/qalgorithms.h.

Quote:

---

Are we passing function pointer to qSort() ...??

---

Yes, qSort() calls the passed function to compare items.

First of all thanks jpn,
But please see the below code for compare <double*>

Code:

---

bool putDoubleListInAscendingOrder( const double * d1 , const double * d2 )
{
	cout<<"\n" << __FUNCTION__;
	return *d1 < *d2;
}
 
int main()
{
 
QList<double*> _doubleList ;
	_doubleList.append( new double(5.12) );
	_doubleList.append( new double(2.22) );
	_doubleList.append( new double(3.22) );
	_doubleList.append( new double(1.22) );
	_doubleList.append( new double(4.22) );
 
 
QListIterator<double*> it( _doubleList );
	double* d = 0;
	cout<<"\n   ****** Before SORT ***** " ;
	while( it.hasNext() )
	{
		d = it.next();		
		cout<<"\n" << *d;		
	}
        cout<<"\n   ****** After SORT ***** " ;
	qSort( _doubleList.begin(), _doubleList.end() , putDoubleListInAscendingOrder );
        it. toFront();  //  Note tat i am using the same iterator
                            //  According to assistant , iterator will move before 1-st item
                            // if i am calling  it.next(), then it is sapposed to return the first item in
                            // _doubleList .
 
        d= it.next();  
        cout<<"\n d :"  << *d ;   //  Here the expected result is "1.22"
 
// N:B => But am getting the reuslt of cout is =  "5.12"  ...why..????
 
 
return 0;
}

---

but if am changing the code as below. ( with different iterator or _doubleList.first() will return the expected result ie; "1.22" )

Code:

---

int main()
{
 
QList<double*> _doubleList ;
	_doubleList.append( new double(5.12) );
	_doubleList.append( new double(2.22) );
	_doubleList.append( new double(3.22) );
	_doubleList.append( new double(1.22) );
	_doubleList.append( new double(4.22) );
 
 
QListIterator<double*> it( _doubleList );
	double* d = 0;
	cout<<"\n   ****** Before SORT ***** " ;
	while( it.hasNext() )
	{
		d = it.next();		
		cout<<"\n" << *d;		
	}
        cout<<"\n   ****** After SORT ***** " ;
	qSort( _doubleList.begin(), _doubleList.end() , putDoubleListInAscendingOrder );
 
        QListIterator<double*> it_2 (  _doubleList )
        it_2. next()     ; 
        d= it_2. next();
        cout<<"\n d :"  << *d ;   //  Here the result is "1.22"
 
// could you please explain why ...?? or If my understanding is wrong please correct me..
 
return 0;
}

---

'it' is invalid after qSort()

Indeed, as QListIterator docs state:

Quote:

---

QListIterator<T> allows you to iterate over a QList<T> (or a QQueue<T>). If you want to modify the list as you iterate over it, use QMutableListIterator<T> instead.

---

Also, may I ask what's the point of allocating doubles on the heap? Notice that QList is an implicitly shared class.

Quote:

---

Originally Posted by jpn

Indeed, as QListIterator docs state:

Also, may I ask what's the point of allocating doubles on the heap? Notice that QList is an implicitly shared class.

---

Thanks for you information.
yes you are right. There is no need for <double*>.
But it was just an example.:)


I have tried with QMutableListIterator . its working perfectly
thanks

Check this & use functors!

http://stackoverflow.com/questions/1...01277#18301277

I am sure that 6 years ago, your advice would have been useful. Meanwhile, the OP's original project was canceled, the company went under, and he's had 4 different jobs since then.

Functors are nice, but when all you need is a simple static function that takes two pointers to double as arguments and returns a bool, adding all that overhead is overkill. And will the person who inherits this code take a look and ask, "Why the heck did they use functors when a static function would have worked just as well? Must be reading too many advanced C++ books".

Been there, done that. Functors have their uses and many C++ generic programming template constructs could not be implemented without them. Sorting a list of pointers by the value of the pointed-to instance isn't one of them.