
Boost : 
From: Pavel Vozenilek (pavel_vozenilek_at_[hidden])
Date: 20050803 07:08:10
"Rob Stewart" wrote:
>> if (more_than_half_of(a) >= more_than_half_of(b)) ...
>> doesn't make sense.
>
> It does to me.
>
> Suppose a is the set of female students and b the male students.
> Further suppose that >= compares grades. Then the expression
> asks whether more than half of the girls got grades at least as
> high as more than half of the boys.
>
You are right. Seems my deductive thinking
isn't as strong as I always thought.
________________________________________________________
>> >> if ( is_true_that(all_of(a), a_binary_functor, one_of(b)) ) ...
>
> If you allow that, then symmetry cannot be guarranteed. There's
> already a question as to whether the operators must be
> symmetrical. I've posited that asymmetry will lead to confusion,
> but many comparisons sound asymmetrical. Consider these two
> expressions:
>
> none_of(a) == any_of(b) // 1
>
> any_of(a) == none_of(b) // 2
>
> 1. Sounds like it should be true if none of the values in a equal
> any value in b.
>
> 2. Sounds like it should be true if any of the values in a equals
> none of the values in b.
>
No, the (2) should be true if any value from 'a'
cannot be equaled to some single value from 'b'.
This would make it symetrical to (1).
Say:
a = green, blue, blue
b = blue, red
none_of(a) == any_of(b)
is false because there is blue (even 2 of them)
from 'a' that match something in 'b'
any_of(a) == none_of(b)
is false because there are two different cases
when blue from 'a' is in 'b'.
Not because 'b' lacks 2 blues.
IOW any_of() should care about cardinality.
For the
any_of(s1) == one_of(s2)
one_of(s2) == any_of(s1)
I think the "one_of" should read as
"exactly_one_and_only_one_of"
and "any_of" as
"any_individual_item_of"
This should restore the symetry.
This is my understanding how these qualifiers should
be implemented and thought about. The other
interpretation didn't occured to me but perhaps I am
still missing the 'aha!' moment.
I didn't study the implementation deep enough
(I would need to step through) but it should be
symetrical.
> Perhaps we should consider a functional style:
>
> if (compare(all_of(a), one_of(b), _1 >= _2))
>
That would be
if (compare(_1 >= _2, all_of(a), one_of(b)))
woudn't it?
________________________________________________________
>> > if (evaluate(all_of(a).where(filter1)
>> > , _1 >= _2
>> > , one_of(b).where(filter2)))
>> > ...
>> >
> That may be true, but I'm concerned about duplicating
> functionality and, thus, limiting flexibility. I guess we just
> need to be sure that this library interoperates easily with
> filters created using the facilities of the other libraries.
>
IMO syntax should decide what to use.
If filter iterator is as easy to use as hypothetical
embedded filter than I am all for for it.
Expression that could be written on single line
it is much prefered to alternatives:
vector<int> a;
set<int> b;
if ( is_true_that(any_of(a), _1 * 2 >= _2, all_of(b)).where(_1 >= 0 && _2 >
10) ) {
is still somehow readable (as long as the filter is simple).
/Pavel
Boost list run by bdawes at acm.org, gregod at cs.rpi.edu, cpdaniel at pacbell.net, john at johnmaddock.co.uk