priority_queue predicate

[thomas]

priority_queue usually uses the greater<int> predicate function.

But as you know, we don't always use priority_queue<int>. Actually we
may need the "priority_queue<pair<int,int>, vector<pair<int,int> >,
cmp> hp;" thing.

My question is how should I write the "cmp" function?

I tried this one:

bool cmp(pair<int,int> &x, pair<int,int> &y){
return x.second < y.second;
}

but it doesn't work while it usually makes sense for "sort" predicate.

Any comments? Thanks in advance.

 

[thomas]

However, it should probably take its arguments by const reference or by
value. But since you haven't posted real code, nor provided any details
about what "doesn't work" means, it's not possible to tell what, if
anything, is wrong.

--
  Pete
Roundhouse Consulting, Ltd. (www.versatilecoding.com) Author of "The
Standard C++ Library Extensions: a Tutorial and Reference
(www.petebecker.com/tr1book)- Hide quoted text -

- Show quoted text -

-------code-----
#include<iostream>
#include<vector>
#include<map>
#include<set>
#include<cstdlib>
#include<cmath>
#include<list>
#include<stack>
#include<queue>

using namespace std;

bool cmp(const pair<PII,int> &x, const pair<PII,int> &y){
return x.second < y.second;
}
priority_queue<pair<PII, int>, vector<pair<PII,int> >, cmp>
heap; //pair<pair<node I, node j>, weight>

int main(){

}
---------end---------

for simplicity, you can compile the code above.
I'm using vc8, and got the errors:
----------------------
------ Build started: Project: pku, Configuration: Debug Win32 ------
Compiling...
a.cpp
...\a.cpp(14) : error C2065: 'PII' : undeclared identifier
...\a.cpp(17) : error C3203: 'pair' : unspecialized class template
can't be used as a template argument for template parameter '_Ty',
expected a real type
...\a.cpp(17) : error C3203: 'pair' : unspecialized class template
can't be used as a template argument for template parameter '_Ty',
expected a real type
...\a.cpp(17) : error C2923: 'std:riority_queue' : 'cmp' is not a
valid template type argument for parameter '_Pr'
..\a.cpp(14) : see declaration of 'cmp'
...\a.cpp(17) : error C2133: 'heap' : unknown size
...\a.cpp(17) : error C2512: 'std:riority_queue' : no appropriate
default constructor available
------------------------

 

[thomas]

-------code-----
#include<iostream>
#include<vector>
#include<map>
#include<set>
#include<cstdlib>
#include<cmath>
#include<list>
#include<stack>
#include<queue>

using namespace std;

bool cmp(const pair<PII,int> &x, const pair<PII,int> &y){
        return x.second < y.second;}

priority_queue<pair<PII, int>, vector<pair<PII,int> >, cmp>
heap;   //pair<pair<node I, node j>, weight>

int main(){

}

---------end---------

for simplicity, you can compile the code above.
I'm using vc8, and got the errors:
----------------------
------ Build started: Project: pku, Configuration: Debug Win32 ------
Compiling...
a.cpp
..\a.cpp(14) : error C2065: 'PII' : undeclared identifier
..\a.cpp(17) : error C3203: 'pair' : unspecialized class template
can't be used as a template argument for template parameter '_Ty',
expected a real type
..\a.cpp(17) : error C3203: 'pair' : unspecialized class template
can't be used as a template argument for template parameter '_Ty',
expected a real type
..\a.cpp(17) : error C2923: 'std:riority_queue' : 'cmp' is not a
valid template type argument for parameter '_Pr'
        ..\a.cpp(14) : see declaration of 'cmp'
..\a.cpp(17) : error C2133: 'heap' : unknown size
..\a.cpp(17) : error C2512: 'std:riority_queue' : no appropriate
default constructor available
------------------------- Hide quoted text -

- Show quoted text -

-------------code-----------
#include<iostream>
#include<vector>
#include<map>
#include<set>
#include<cstdlib>
#include<cmath>
#include<list>
#include<stack>
#include<queue>

using namespace std;

#define PII pair<int,int>


bool cmp(const pair<PII,int> &x, const pair<PII,int> &y){
return x.second < y.second;
}
priority_queue<pair<PII, int>, vector<pair<PII,int> >, cmp>
heap; //pair<pair<node I, node j>, weight>

int main(){

}

 

[Fraser Ross]

The headers required are queue, utility and vector.
A typedef is preferable. Two of them would be useful.
typedef std:air<int,int> PII;
Comparisons can be done on second but first would be more akin to the
associative containers.
The third argument must be a type. It must be this: bool (*)(element
const &x, element const &y).
A constructor that sets the comparison function must be invoked i.e.
heap(cmp)

A functor is easier to use than a function when you know how to use
them. I would rewrite the code to use a functor and to sort on first
instead of second.

Fraser.

 

[Stefan Ram]

The headers required are queue, utility and vector.

In a class, I explained that using certain names and
operators requires certain include directives.

Then I was asked if it would be possible to always
copy /all/ standard include directives to the start
of a program. I answered that no one does this, but
that it would be possible.

Is there a reason not to do so (except for a possible
increase in compilation time)?

 

[Maxim Yegorushkin]

priority_queue usually uses the greater<int> predicate function.

But as you know, we don't always use priority_queue<int>. Actually we
may need the "priority_queue<pair<int,int>, vector<pair<int,int> >,
cmp> hp;" thing.

My question is how should I write the "cmp" function?

I tried this one:

bool cmp(pair<int,int> &x, pair<int,int> &y){
    return x.second < y.second;

}

but it doesn't work while it usually makes sense for "sort" predicate.

std::sort() accepts the predicate by value and deduces its type, so
both function pointers and function objects (with operator()) work.

std:riority_queue<>, on the other hand, does not deduce the template
types, so that you have to specify it explicitly. The type of your
predicate is a function pointer:

typedef priority_queue<
pair<int,int>

> Q;

Constructors of the queue use a default-initialised value for the
predicate, which in your case is a NULL function pointer. Obviously,
you need to pass a pointer to you predicate function explicitly when
constructing the queue:

Q q(cmp);

It may be a bit easier to make your predicate a class, so that you
don't have to pass it into constructors:

struct cmp2
{
bool operator()(pair<int,int> const&, pair<int,int> const&)
const;
};

typedef priority_queue<
pair<int,int>

> Q2;

Q2 q2; // <--- default-initialised cmp2 works

Please also note, that when you use function pointers the calls
through a function pointer can not be inlined. With function objects
they can.