Qt-interest Archive, March 2008
Using the qsort less than function
Message 1 in thread
Using Qt 4.4 beta 1 on windows XP:
I would like to use qSort on a QList to sort according to some custom
requirements. The problem is, that the less than function needs to be able
to access some variables to determine the right sort order. My thought was
to implement the compare function as a member function in my class, or
something like that, but that results in a compiler error:
c:/Devel/Qt/4.4.0-beta1/include/QtCore/../../src/corelib/tools/qalgorithms.h:381:
error: must use .* or ->* to call pointer-to-member function in `lessThan
(...)'
c:/Devel/Qt/4.4.0-beta1/include/QtCore/../../src/corelib/tools/qalgorithms.h:206:
instantiated from `void qSort(RandomAccessIterator, RandomAccessIterator,
LessThan) [with RandomAccessIterator = QList<QDomElement>::iterator,
LessThan = bool (PatentDataExtractor::*)(const QDomElement&, const
QDomElement&)]'
src\patentdataextractor.cpp:57: instantiated from here
c:/Devel/Qt/4.4.0-beta1/include/QtCore/../../src/corelib/tools/qalgorithms.h:386:
error: must use .* or ->* to call pointer-to-member function in `lessThan
(...)'
(...)
The documentation on qSort is rather sparse, and shows little examples. Is
there another way to accomplish my special sort function? Could somebody
point me to some information on how to accomplish this, or perhaps give a
small code example on how to write the arguments to qSort if I want to use a
member function?
Thanks!
André
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Message 2 in thread
André Somers wrote:
> Using Qt 4.4 beta 1 on windows XP:
>
> I would like to use qSort on a QList to sort according to some custom
> requirements. The problem is, that the less than function needs to be
> able to access some variables to determine the right sort order. My
> thought was to implement the compare function as a member function in my
> class, or something like that, but that results in a compiler error:
Can you post the offending code ?
--
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Message 3 in thread
"Stephen Collyer" <scollyer@xxxxxxxxxxxxxxxx> wrote in message
news:47DAA19E.8050707@xxxxxxxxxxxxxxxxxxx
> André Somers wrote:
>> Using Qt 4.4 beta 1 on windows XP:
>>
>> I would like to use qSort on a QList to sort according to some custom
>> requirements. The problem is, that the less than function needs to be
>> able to access some variables to determine the right sort order. My
>> thought was to implement the compare function as a member function in my
>> class, or something like that, but that results in a compiler error:
>
> Can you post the offending code ?
The code goes (something) like this:
QList<QDomElement> PatentDataExtractor::getSortedBiblioNodes() {
//(...)
QList<QDomElement> bel;
//fill the list
qSort(bel.begin(),bel.end(), PatentDataExtractor::biblioNodeLessThan);
return bel;
}
bool PatentDataExtractor::biblioNodeLessThan(const QDomElement& e1, const
QDomElement& e2) {
//we always put the biblioentry of the document we were looking for on top
if (e1.elementsByTagName("B111EP").at(0).toElement().text() ==
m_magicSortKey)
return true;
if (e2.elementsByTagName("B111EP").at(0).toElement().text() ==
m_magicSortKey)
return false;
// .... and so on
}
I hope this helps? I'm gratefull for any suggestions.
André
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Message 4 in thread
André Somers wrote:
> The code goes (something) like this:
>
>
> QList<QDomElement> PatentDataExtractor::getSortedBiblioNodes() {
> //(...)
>
> QList<QDomElement> bel;
>
> //fill the list
> qSort(bel.begin(),bel.end(), PatentDataExtractor::biblioNodeLessThan);
> return bel;
> }
If biblioNodeLessThan() is a member function, I guess you will
have to use something like the Boost function library to produce
a function callback that is fully bound to the current object.
At the moment, you are passing what is essentially an offset
into an object; you need to give qSort a callback to a member
function associated with the current object. Take a look at
Boost function to see how to do that. You'll probably need to
use std::mem_ptr and std::bind1st, but I'd have to look up the
details to get the syntax right.
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Message 5 in thread
Perhaps there is a detail I am missing, but wouldn't qsort "just work" if
you implemented the operator< for your class?
Evan Teran
Stephen Collyer wrote:
> André Somers wrote:
>
>> The code goes (something) like this:
>>
>>
>> QList<QDomElement> PatentDataExtractor::getSortedBiblioNodes() {
>> //(...)
>>
>> QList<QDomElement> bel;
>>
>> //fill the list
>> qSort(bel.begin(),bel.end(), PatentDataExtractor::biblioNodeLessThan);
>> return bel;
>> }
>
> If biblioNodeLessThan() is a member function, I guess you will
> have to use something like the Boost function library to produce
> a function callback that is fully bound to the current object.
> At the moment, you are passing what is essentially an offset
> into an object; you need to give qSort a callback to a member
> function associated with the current object. Take a look at
> Boost function to see how to do that. You'll probably need to
> use std::mem_ptr and std::bind1st, but I'd have to look up the
> details to get the syntax right.
>
--
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Message 6 in thread
"André Somers" <andre@xxxxxxxxxxxxxxxx> wrote in message
news:fre958$j8k$1@xxxxxxxxxxxxxxxx
>>> I would like to use qSort on a QList to sort according to some custom
>>> requirements. The problem is, that the less than function needs to be
>>> able to access some variables to determine the right sort order. My
>>> thought was to implement the compare function as a member function in my
>>> class, or something like that, but that results in a compiler error:
For now, I resorted to replacing the qSort call with a small, naive sorting
routine of my own. This is not a big problem, since the list is not too long
and sorting is not a big part of the time the code takes:
int swapsMade = 0, cnt =0;
int iterations = 0;
cnt = bel.count();
if (bel.count() > 1) {
do {
swapsMade = 0;
for (int i=0; i < cnt - 1; i++) {
if (biblioNodeLessThan(bel.at(i+1), bel.at(i))) {
swapsMade ++;
bel.swap(i, i+1);
//qDebug() << "swap " << i << i+1;
}
}
iterations++;
} while (swapsMade != 0);
} qDebug()
<< "processing order (sorting took" << iterations << "iterations)" ;
It just keeps on looping through the list untill there are no more swaps of
neighbouring elements made.
I am still interested in how to get the qSort approach to work, but at least
I can continue for now.
Thanks,
André
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Message 7 in thread
André Somers wrote:
> [...]
> bool PatentDataExtractor::biblioNodeLessThan(const QDomElement& e1,
> const QDomElement& e2) {
> //we always put the biblioentry of the document we were looking
> for on top
> if (e1.elementsByTagName("B111EP").at(0).toElement().text() ==
> m_magicSortKey)
> return true;
> if (e2.elementsByTagName("B111EP").at(0).toElement().text() ==
> m_magicSortKey)
> return false;
>
> // .... and so on
> }
> [...]
How about making the method a non-member friend of the class? Have not
tried it, though.
Regards
Kai Koehne
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