std:sort predicate function optional arguments

[Ganesh]

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

[Xiaobin Huang]

Re: std:sort predicate function optional arguments

On 9ÔÂ29ÈÕ, ÏÂÎç6ʱ01·Ö, Ganesh <ganesh.i...@gm ail.comwrote:

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]

Re: std:sort predicate function optional arguments

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]

Re: std:sort predicate function optional arguments

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

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]

Re: std:sort predicate function optional arguments

On Sep 29, 9:51 pm, Pete Becker <p...@versatile coding.comwrote :

On 2008-09-29 13:45:24 -0400, Juha Nieminen <nos...@thanks. invalidsaid:

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.kan ze@gmail.com
Conseils en informatique orientée objet/
Beratung in objektorientier ter Datenverarbeitu ng
9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34

[Pete Becker]

Re: std:sort predicate function optional arguments

On 2008-09-30 05:16:36 -0400, James Kanze <james.kanze@gm ail.comsaid:

On Sep 29, 9:51 pm, Pete Becker <p...@versatile coding.comwrote :

>On 2008-09-29 13:45:24 -0400, Juha Nieminen <nos...@thanks. invalidsaid:

>>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)