dev-cpp-users

[Dev-C++] STL sort() problem

[Abhijit S. <mu...@gm...>]

Greets.

I'm using a vector of a structure pointer (I use dynamic allocation), like
so:

    vector<PlsLine *> lines;

and PlsLine is a structure:

typedef struct _PlsLine
{
    char    *fn;            // Full filename (c:\foo\bar.mp3)
    char    *filename;      // Filename part only (bar.mp3)
    int     size;           // Filesize
} PlsLine;

I need to sort this, and I call sort() like this:

    sort(lines.begin(), lines.end());

However, sort() uses the comparison operators (< and >), and I can't define
them for pointers... The compiler flags this:

    bool operator <(const PlsLine *&left, const PlsLine *&right)

as an error. It says it should be a class or enumerated type. I tried doing
this:

    bool operator <(const PlsLine &left, const PlsLine &right)

and the compile worked, but it doesn't seem to sort.
Any work-around?
___________________________________________________________

Abhijit Shylanath

E-mail:   mu...@gm... || ibr...@bi...
Web-site: http://mudeth.tripod.com/

Re: [Dev-C++] STL sort() problem

[Eddy L O J. <ed...@kl...>]

2002-08-18 10:46:45, "Abhijit Shylanath" <mu...@gm...> wrote:

>I'm using a vector of a structure pointer (I use dynamic
>allocation), like

 I'm in a good mood today, so I wrote you up a short example using 
a functor predicate. I'm sure it can be solved using an overloaded 
operator<() too, that's just now how I'd do it.

#include <iostream>
#include <string>
#include <vector>

using namespace std;

template <typename T>
class on_age
{
public:
    bool operator() (const T& v1, const  T& v2) const
    {
        return v2->age_ > v1->age_;
    }
};


struct t_person
{
    t_person(const string& name, int age) : name_(name), age_(age) 
{ };

    string   name_;
    int      age_;
};


int main(int argc, char *argv[])
{
  vector<t_person*> people;
  vector<t_person*>::const_iterator it;

  people.push_back( new t_person("Gordon", 31) );
  people.push_back( new t_person("Tiny", 1) );
  people.push_back( new t_person("Alice", 16) );
  people.push_back( new t_person("Second", 2) );
  people.push_back( new t_person("Grandpa", 101) );

  sort(people.begin(), people.end(), on_age<t_person*>() );

  for( it = people.begin() ; it != people.end() ; ++it ) 
    cout << (*(it))->name_ << ", " << (*(it))->age_ << endl;

  // yeah, yeah, "leak". Whatever
  return 0;
}

-- 
"Reading this line brought you three seconds closer to death."
Eddy L O Jansson     |     http://gazonk.org/~eloj

RE: [Dev-C++] STL sort() problem

[Ioannis V. <no...@ho...>]

> -----Original Message-----
> From: dev...@li... 
> [mailto:dev...@li...] On Behalf 
> Of Abhijit Shylanath
> Sent: Sunday, August 18, 2002 11:47 AM
> To: Dev-C++ Mailing List
> Subject: [Dev-C++] STL sort() problem
> 
> 
> Greets.
> 
> I'm using a vector of a structure pointer (I use dynamic 
> allocation), like
> so:
> 
>     vector<PlsLine *> lines;
> 
> and PlsLine is a structure:
> 
> typedef struct _PlsLine
> {
>     char    *fn;            // Full filename (c:\foo\bar.mp3)
>     char    *filename;      // Filename part only (bar.mp3)
>     int     size;           // Filesize
> } PlsLine;
> 
> I need to sort this, and I call sort() like this:
> 
>     sort(lines.begin(), lines.end());
> 
> However, sort() uses the comparison operators (< and >), and 
> I can't define
> them for pointers... The compiler flags this:
> 
>     bool operator <(const PlsLine *&left, const PlsLine *&right)
> 
> as an error. It says it should be a class or enumerated type. 


Yes you can not have a reference to a pointer with exception to
templates.



> I tried doing
> this:
> 
>     bool operator <(const PlsLine &left, const PlsLine &right)
> 
> and the compile worked, but it doesn't seem to sort.
> Any work-around?


Yes, you probably made some logical error in the definition of
operator<(). Send us its definition.



Ioannis

* Ioannis Vranos
* Programming pages: http://www.noicys.cjb.net
* Alternative URL: http://run.to/noicys

Re: [Dev-C++] STL sort() problem

[Abhijit S. <mu...@gm...>]

> > I tried doing
> > this:
> >
> >     bool operator <(const PlsLine &left, const PlsLine &right)
> >
> > and the compile worked, but it doesn't seem to sort.
> > Any work-around?
>
>
> Yes, you probably made some logical error in the definition of
> operator<(). Send us its definition.
>

The mistake was that the vector was still instantiated with PlsLine *, not
with PlsLine. It's working now, thanks. Just for the record, the other form
of sort() which takes a comparison function also works, as Eddy said.

Of course it works... it's meant to :).

___________________________________________________________

Abhijit Shylanath

E-mail:   mu...@gm... || ibr...@bi...
Web-site: http://mudeth.tripod.com/

RE: [Dev-C++] STL sort() problem

[Ioannis V. <no...@ho...>]

> -----Original Message-----
> From: Abhijit Shylanath [mailto:mu...@gm...] 
> Sent: Monday, August 19, 2002 11:24 PM
> To: Ioannis Vranos; Dev-C++ Mailing List
> Subject: Re: [Dev-C++] STL sort() problem
> 
> 
> > > I tried doing
> > > this:
> > >
> > >     bool operator <(const PlsLine &left, const PlsLine &right)
> > >
> > > and the compile worked, but it doesn't seem to sort.
> > > Any work-around?
> >
> >
> > Yes, you probably made some logical error in the definition of
> > operator<(). Send us its definition.
> >
> 
> The mistake was that the vector was still instantiated with 
> PlsLine *, not
> with PlsLine. It's working now, thanks.


Good.


> Just for the record, 
> the other form
> of sort() which takes a comparison function also works, as Eddy said.


It is the same sort(). It uses < as default. If the default works do not
use a comparison function or function object. The default is usually
more optimised. Also try to make the operator<() inline (either a member
function or a regular function).



Ioannis

* Ioannis Vranos
* Programming pages: http://www.noicys.cjb.net
* Alternative URL: http://run.to/noicys