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comp.lang.c++

std:sort predicate function optional arguments

Ganesh

9/29/2008 10:02:00 AM

hi !

Can the predicate function used in std::sort take optional
arguments ? For instance. I have a class Point. I create a vector of
this and then want to compare the slopes of these points with respect
to another point of the same class (Graham's convex hull algorithm
anyone ?)

ganesh
5 Answers

Xiaobin.Huang

9/29/2008 10:51:00 AM

0

On 9?29?, ??6?01?, Ganesh <ganesh.i...@gmail.com> wrote:
> hi !
>
> Can the predicate function used in std::sort take optional
> arguments ? For instance. I have a class Point. I create a vector of
> this and then want to compare the slopes of these points with respect
> to another point of the same class (Graham's convex hull algorithm
> anyone ?)
>
> ganesh

you can provide your own predicate.

class cmp {
public:
cmp(...) {
// init thePoint_
}
bool operator() (point const& lh, point const& rh) {
// ...
}
private:
point thePoint_;
};

Juha Nieminen

9/29/2008 5:45:00 PM

0

Xiaobin Huang wrote:
> you can provide your own predicate.
>
> class cmp {
> public:
> cmp(...) {
> // init thePoint_
> }
> bool operator() (point const& lh, point const& rh) {
> // ...
> }
> private:
> point thePoint_;
> };

Nitpicking, but isn't that called a comparator? If is it *also* a
predicate?

Pete Becker

9/29/2008 7:51:00 PM

0

On 2008-09-29 13:45:24 -0400, Juha Nieminen <nospam@thanks.invalid> said:

> Xiaobin Huang wrote:
>> you can provide your own predicate.
>>
>> class cmp {
>> public:
>> cmp(...) {
>> // init thePoint_
>> }
>> bool operator() (point const& lh, point const& rh) {
>> // ...
>> }
>> private:
>> point thePoint_;
>> };
>
> Nitpicking, but isn't that called a comparator? If is it *also* a
> predicate?

There is a version of std::sort that takes a predicate. It doesn't care
whether you call it a comparator, a functoid, or fred. But when you're
talking about std::sort, if you refer to a predicate by some other
name, you run the risk of confusing your listener.

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

James Kanze

9/30/2008 9:17:00 AM

0

On Sep 29, 9:51 pm, Pete Becker <p...@versatilecoding.com> wrote:
> On 2008-09-29 13:45:24 -0400, Juha Nieminen <nos...@thanks.invalid> said:
> > Xiaobin Huang wrote:
> >> you can provide your own predicate.

> >> class cmp {
> >> public:
> >> cmp(...) {
> >> // init thePoint_
> >> }
> >> bool operator() (point const& lh, point const& rh) {
> >> // ...
> >> }
> >> private:
> >> point thePoint_;
> >> };

> > Nitpicking, but isn't that called a comparator? If is it
> > *also* a predicate?

> There is a version of std::sort that takes a predicate. It
> doesn't care whether you call it a comparator, a functoid, or
> fred. But when you're talking about std::sort, if you refer to
> a predicate by some other name, you run the risk of confusing
> your listener.

While it's obviously a predicate; any functional object which
returns bool is a predicate. The standard never uses the word
predicate for it, however, and calls it Compare. It also
imposes a number of additional constraints on it which don't
apply to predicates in general. In fact, it even says that "A
sequence is sorted with respect to a comparator comp if [...]",
using the very same word as Juha. So while it is a predicate,
I'd consider "comparator" a more precise word. (All comparators
are predicates, but not all predicates are comparators. And
here, we need a predicate which is a comparator.)

--
James Kanze (GABI Software) email:james.kanze@gmail.com
Conseils en informatique orientée objet/
Beratung in objektorientierter Datenverarbeitung
9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34

Pete Becker

9/30/2008 10:59:00 AM

0

On 2008-09-30 05:16:36 -0400, James Kanze <james.kanze@gmail.com> said:

> On Sep 29, 9:51 pm, Pete Becker <p...@versatilecoding.com> wrote:
>> On 2008-09-29 13:45:24 -0400, Juha Nieminen <nos...@thanks.invalid> said:
>>> Xiaobin Huang wrote:
>>>> you can provide your own predicate.
>
>>>> class cmp {
>>>> public:
>>>> cmp(...) {
>>>> // init thePoint_
>>>> }
>>>> bool operator() (point const& lh, point const& rh) {
>>>> // ...
>>>> }
>>>> private:
>>>> point thePoint_;
>>>> };
>
>>> Nitpicking, but isn't that called a comparator? If is it
>>> *also* a predicate?
>
>> There is a version of std::sort that takes a predicate. It
>> doesn't care whether you call it a comparator, a functoid, or
>> fred. But when you're talking about std::sort, if you refer to
>> a predicate by some other name, you run the risk of confusing
>> your listener.
>
> While it's obviously a predicate; any functional object which
> returns bool is a predicate. The standard never uses the word
> predicate for it, however, and calls it Compare. It also
> imposes a number of additional constraints on it which don't
> apply to predicates in general. In fact, it even says that "A
> sequence is sorted with respect to a comparator comp if [...]",
> using the very same word as Juha. So while it is a predicate,
> I'd consider "comparator" a more precise word. (All comparators
> are predicates, but not all predicates are comparators. And
> here, we need a predicate which is a comparator.)

You're right, of course. Sorting and partitioning rely on comparators
(which take two arguments), and most other algorithms rely on
predicates (which take one argument). I should know better than to post
while I'm in the middle of post-meeting editing.

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