sorting question

[John]

I have a class

class Point2D {

double x[2];

public:
.....

}

I want to implement a function that can take in as input

point2Dsort( double * pdA, int number_of_point s)

and sort it in lexicographic order. (i.e. if p1.x() p2.x() then p1 >
p2 ...

Is there a way to define operator< on the point class and implement
point2Dsort using c++ sort() function?

Thanks,
--j

[Mark P]

Re: sorting question

John wrote:

I have a class
>
class Point2D {
>
double x[2];
>
public:
....
>
}
>
I want to implement a function that can take in as input
>
point2Dsort( double * pdA, int number_of_point s)
>
and sort it in lexicographic order. (i.e. if p1.x() p2.x() then p1 >
p2 ...
>
Is there a way to define operator< on the point class and implement
point2Dsort using c++ sort() function?
>
Thanks,
--j
>

You can give Point2D a public member function:

bool operator<( const Point2D& rhs) const;

Then you can use std::sort.

[David Harmon]

Re: sorting question

On 20 Sep 2006 12:25:58 -0700 in comp.lang.c++, "John"
<weekender_ny@y ahoo.comwrote,

>I want to implement a function that can take in as input
>
>point2Dsort( double * pdA, int number_of_point s)
>
>and sort it in lexicographic order. (i.e. if p1.x() p2.x() then p1 >
>p2 ...
>
>Is there a way to define operator< on the point class and implement
>point2Dsort using c++ sort() function?

Yes.

bool operator<(Point 2D l, Point2D r)
{
// return true or false here
}

However, it's not obvious how you are going to get from a pointer to
double, to anything having to do with Point2D, in point2Dsort().
That "point" may need some rethinking.

[Kai-Uwe Bux]

Re: sorting question

John wrote:

I have a class
>
class Point2D {
>
double x[2];
>
public:
....
>
}
>
I want to implement a function that can take in as input
>
point2Dsort( double * pdA, int number_of_point s)
>
and sort it in lexicographic order. (i.e. if p1.x() p2.x() then p1 >
p2 ...
>
Is there a way to define operator< on the point class and implement
point2Dsort using c++ sort() function?

Yes, you have several options:

a) define a member function

class Point2D {
...
bool operator< ( Point2D const & rhs ) const {
}

}

b) define a free-standing friend operator:

bool operator< ( Point2D const & lhs, Point2D const & rhs ) {
}

c) add a specialization to std::less<>:

namespace std {

template <>
struct less< Point2D : binary_predicat e< Point2D {

bool operator() ( Point2D const & lhs, Point2D const & rhs ) {
}

};

d) define a free standing friend predicate

bool is_less ( Point2D const & lhs, Point2D const & rhs ) {
}

and pass this as a parameter to std::sort and similar critters.

The third and fourth alternatives convey the understanding that the ordering
is not natural. The third is more convenient since std::less will be
automatically used by std::sort and its friends.


Best

Kai-Uwe Bux

[Mark P]

Re: sorting question

Kai-Uwe Bux wrote:

John wrote:
>

>I have a class
>>
>class Point2D {
>>
>double x[2];
>>
>public:
>....
>>
>}
>>
>I want to implement a function that can take in as input
>>
>point2Dsort( double * pdA, int number_of_point s)
>>
>and sort it in lexicographic order. (i.e. if p1.x() p2.x() then p1 >
>p2 ...
>>
>Is there a way to define operator< on the point class and implement
>point2Dsort using c++ sort() function?

>
Yes, you have several options:
>
>
c) add a specialization to std::less<>:
>
namespace std {
>
template <>
struct less< Point2D : binary_predicat e< Point2D {
>
bool operator() ( Point2D const & lhs, Point2D const & rhs ) {
}
>
};
>

Is one "allowed" to add stuff to namespace std?

[Thomas Tutone]

Re: sorting question


Mark P wrote:

Kai-Uwe Bux wrote:

c) add a specialization to std::less<>:

namespace std {

template <>
struct less< Point2D : binary_predicat e< Point2D {

bool operator() ( Point2D const & lhs, Point2D const & rhs ) {
}

};

>
Is one "allowed" to add stuff to namespace std?

Yes, if it's a specialization of a template already in namespace std,
as in this example.

Best regards,

Tom