emptying a priority_queue efficiently

[J.M.]

I would like to use a data structure (e.g. from the STL) that always allows
me to retrieve the largest element. (I want to push in elements, and remove
the largest, push in further elements, etc.) It seems a priority_queue from
the STL would work fine. However at some point, I am finished (even though
the queue is not empty) and want to throw away the rest of the elements. It
seems, I have to do this using:

while (!Q.empty()) Q.pop();

Other classes offer a resize() or a clear() and my guess that this is much
more efficient than using a while-loop. However, these functions are not
available for a priority_queue. Any ideas? Thanks in advance for any help.

Jan

[Alf P. Steinbach]

* J.M.:

I would like to use a data structure (e.g. from the STL) that always allows
me to retrieve the largest element. (I want to push in elements, and remove
the largest, push in further elements, etc.) It seems a priority_queue from
the STL would work fine. However at some point, I am finished (even though
the queue is not empty) and want to throw away the rest of the elements. It
seems, I have to do this using:

while (!Q.empty()) Q.pop();

Other classes offer a resize() or a clear() and my guess that this is much
more efficient than using a while-loop. However, these functions are not
available for a priority_queue. Any ideas? Thanks in advance for any help.

template< typename T >
void makeEmpty( std::priority_q ueue<T>& q )
{
std::priority_q ueue<Tempty;
std::swap( q, empty );
}

Of course, you need to measure measure measure.

--
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[=?ISO-8859-1?Q?Ney_Andr=E9_de_Mello_Zunino?=]

J.M. escreveu:

I would like to use a data structure (e.g. from the STL) that always allows
me to retrieve the largest element. (I want to push in elements, and remove
the largest, push in further elements, etc.) It seems a priority_queue from
the STL would work fine. However at some point, I am finished (even though
the queue is not empty) and want to throw away the rest of the elements. It
seems, I have to do this using:

while (!Q.empty()) Q.pop();

Other classes offer a resize() or a clear() and my guess that this is much
more efficient than using a while-loop. However, these functions are not
available for a priority_queue. Any ideas? Thanks in advance for any help.

I might be missing something, but since you always wish to obtain the
largest element, I guess a set and an operator< for your element type
would give you what you need. You might use rbegin() or begin() to get
to the last element, depending on whether your operator< sorts elements
in ascending or descending order, respectivelly.

Regards,

--
Ney André de Mello Zunino
http://blog.zunino.eti.br/

[Marcus Kwok]

J.M. <jm************ ***@gmx.dewrote :

I would like to use a data structure (e.g. from the STL) that always allows
me to retrieve the largest element. (I want to push in elements, and remove
the largest, push in further elements, etc.) It seems a priority_queue from
the STL would work fine. However at some point, I am finished (even though
the queue is not empty) and want to throw away the rest of the elements. It
seems, I have to do this using:

while (!Q.empty()) Q.pop();

Other classes offer a resize() or a clear() and my guess that this is much
more efficient than using a while-loop. However, these functions are not
available for a priority_queue. Any ideas? Thanks in advance for any help.

Alf P. Steinbach gave you a good answer. Another idea is to use a
different container and call std::make_heap( ) on it, but consistently
maintaining the heap property may be less efficient than just using the
priority queue.

--
Marcus Kwok
Replace 'invalid' with 'net' to reply

[Marcus Kwok]

Marcus Kwok <ri******@gehen nom.invalidwrot e:

J.M. <jm************ ***@gmx.dewrote :

>I would like to use a data structure (e.g. from the STL) that always allows
me to retrieve the largest element. (I want to push in elements, and remove
the largest, push in further elements, etc.) It seems a priority_queue from
the STL would work fine. However at some point, I am finished (even though
the queue is not empty) and want to throw away the rest of the elements. It
seems, I have to do this using:

while (!Q.empty()) Q.pop();

Other classes offer a resize() or a clear() and my guess that this is much
more efficient than using a while-loop. However, these functions are not
available for a priority_queue. Any ideas? Thanks in advance for any help.

Alf P. Steinbach gave you a good answer. Another idea is to use a
different container and call std::make_heap( ) on it, but consistently
maintaining the heap property may be less efficient than just using the
priority queue.

After more investigation, <algorithmals o offers std::push_heap( ) and
std::pop_heap() in addition to std::make_heap( ), so maintaining the heap
property may not be as big of a hit as I thought. Of course, you will
need to measure for yourself if the performance is acceptable.

--
Marcus Kwok
Replace 'invalid' with 'net' to reply

[Mark P]

J.M. wrote:

I would like to use a data structure (e.g. from the STL) that always allows
me to retrieve the largest element. (I want to push in elements, and remove
the largest, push in further elements, etc.) It seems a priority_queue from
the STL would work fine. However at some point, I am finished (even though
the queue is not empty) and want to throw away the rest of the elements. It
seems, I have to do this using:

while (!Q.empty()) Q.pop();

Other classes offer a resize() or a clear() and my guess that this is much
more efficient than using a while-loop. However, these functions are not
available for a priority_queue. Any ideas? Thanks in advance for any help.

Jan

Besides previously given answers... you could design things so that the
priority_queue goes out of scope when you're done with it or, if that's
not possible, dynamically allocate it and attach it to a smart pointer
which you can manually delete as soon as convenient. (Implicit in this
is also the option to simply do nothing and wait for the queue to fall
out of scope, but I gather that this is not sufficient for you.)

[J.M.]

Mark P schrieb:

J.M. wrote:

>I would like to use a data structure (e.g. from the STL) that always
allows me to retrieve the largest element. (I want to push in elements,
and remove the largest, push in further elements, etc.) It seems a
priority_que ue from the STL would work fine. However at some point, I am
finished (even though the queue is not empty) and want to throw away the
rest of the elements. It seems, I have to do this using:

while (!Q.empty()) Q.pop();

Other classes offer a resize() or a clear() and my guess that this is
much more efficient than using a while-loop. However, these functions are
not available for a priority_queue. Any ideas? Thanks in advance for any
help.

Jan

Besides previously given answers... you could design things so that the
priority_queue goes out of scope when you're done with it or, if that's
not possible,

Not possible, it is part of another class, and I want to refill it starting
with an empty queue.,,

dynamically allocate it and attach it to a smart pointer
which you can manually delete as soon as convenient.

Ok, that would work of course, I was just hoping that there was something
more elegant and in "spirit" with the STL of just clearing a container. I
think Alf's solution is easiest and seems to reduce the time quite a bit.
(It only requires me changing one line of code ;-)))

Thanks,

Jan

(Implicit in this
is also the option to simply do nothing and wait for the queue to fall
out of scope, but I gather that this is not sufficient for you.)

[J.M.]

Marcus Kwok schrieb:

Marcus Kwok <ri******@gehen nom.invalidwrot e:

>J.M. <jm************ ***@gmx.dewrote :

>>I would like to use a data structure (e.g. from the STL) that always
allows me to retrieve the largest element. (I want to push in elements,
and remove the largest, push in further elements, etc.) It seems a
priority_queu e from the STL would work fine. However at some point, I am
finished (even though the queue is not empty) and want to throw away the
rest of the elements. It seems, I have to do this using:

while (!Q.empty()) Q.pop();

Other classes offer a resize() or a clear() and my guess that this is
much more efficient than using a while-loop. However, these functions
are not available for a priority_queue. Any ideas? Thanks in advance for
any help.

Alf P. Steinbach gave you a good answer. Another idea is to use a
different container and call std::make_heap( ) on it, but consistently
maintaining the heap property may be less efficient than just using the
priority queue.

After more investigation, <algorithmals o offers std::push_heap( ) and
std::pop_heap() in addition to std::make_heap( ), so maintaining the heap
property may not be as big of a hit as I thought. Of course, you will
need to measure for yourself if the performance is acceptable.

Well, I suppose all of that is feasible, but I was looking for a "quick fix"
and I think Alf's suggestion works best in that regard.

Thx

Jan

[J.M.]

Alf P. Steinbach schrieb:

* J.M.:

>I would like to use a data structure (e.g. from the STL) that always
allows me to retrieve the largest element. (I want to push in elements,
and remove the largest, push in further elements, etc.) It seems a
priority_que ue from the STL would work fine. However at some point, I am
finished (even though the queue is not empty) and want to throw away the
rest of the elements. It seems, I have to do this using:

while (!Q.empty()) Q.pop();

Other classes offer a resize() or a clear() and my guess that this is
much more efficient than using a while-loop. However, these functions are
not available for a priority_queue. Any ideas? Thanks in advance for any
help.

template< typename T >
void makeEmpty( std::priority_q ueue<T>& q )
{
std::priority_q ueue<Tempty;
std::swap( q, empty );
}

Seems to work quite well. Thx.

Jan

>
Of course, you need to measure measure measure.

[J.M.]

J.M. schrieb:

Alf P. Steinbach schrieb:

>* J.M.:

>>I would like to use a data structure (e.g. from the STL) that always
allows me to retrieve the largest element. (I want to push in elements,
and remove the largest, push in further elements, etc.) It seems a
priority_queu e from the STL would work fine. However at some point, I am
finished (even though the queue is not empty) and want to throw away the
rest of the elements. It seems, I have to do this using:

while (!Q.empty()) Q.pop();

Other classes offer a resize() or a clear() and my guess that this is
much more efficient than using a while-loop. However, these functions
are not available for a priority_queue. Any ideas? Thanks in advance for
any help.

template< typename T >
void makeEmpty( std::priority_q ueue<T>& q )
{
std::priority_q ueue<Tempty;
std::swap( q, empty );
}

Seems to work quite well. Thx.

Jan

Addendum:

It seems the following is even faster:

template< typename T >
void makeEmpty( std::priority_q ueue<T>& q )
{
std::priority_q ueue<Tempty;
q = empty;
}

>>
Of course, you need to measure measure measure.