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]

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]

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]

On 2008-09-29 13:45:24 -0400, Juha Nieminen <no****@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]

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:ja******* **@gmail.com
Conseils en informatique orientée objet/
Beratung in objektorientier ter Datenverarbeitu ng
9 place Sémard, 78210 St.-Cyr-l'École, France, [phone]

[Pete Becker]

On 2008-09-30 05:16:36 -0400, James Kanze <ja*********@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)