Victor Bazarov wrote:
> On 3/23/2012 11:03 AM, Rui Maciel wrote:
>> Victor Bazarov wrote:
>>
>>> The usual way to find the minimum is to initialize the value from the
>>> first element, and then start comparing from the second element. An
>>> empty set is a special case for which the calculation of the "minimum"
>>> should just throw an exception. A set of one element is also a special
>>> case: there is no need to compare anything. Two elements could be
>>> made into a special case by use of std::min.
>>
>> I was hoping to use a single loop.
>
> Are you concerned with less typing, and not with implementing it
> correctly *logically*? Do you consider "a single loop" better or more
> efficient in some way?
You either failed to understand what I wrote or you are intentionally trying
to misrepresenting what I said. No one claimed that it is better to use
code which is logically incorrect if it provides a way to save on typing. I
don't know where you came up with that nonsense.
> > Relying on a separate initialization
>> block feels a bit like a crudeish hack.
>
> "Crudeish"? Really?
Really.
> <shrug> Using an infinity value in that manner is
> crudish, IMNSHO.
What happened to logical correctness? And do you also believe that, if the
objective was to get the largest nonnegative number, initializing it to
zero or even any negative number would also be crudish?
> It suggests that (a) infinity is not a valid value for
> any set element to be associated with (which might be true in your
> model, but doesn't necessarily sound right in all cases),
Zero is also not valid in a considerable number of cases, and yet variables
are still set by default as zero.
> and (b) that
> the maximum value from the elements of an empty set is infinity, which
> is a number (if you divide by it, you get 0).
As a side note, and nitpicking a bit, this isn't true. Infinity isn't a
number, and k/infinity is meaningless. The k/infinity = 0 is only valid
because it was a specific indeterminate form which is often defined as
lim{x>infinity} k/x.
Similarly, division by zero has also been defined as k/0 = infinity, but
this doesn't mean it's a good idea to hold this as true. For a start, this
would mean that infinity*0 = k.
> I'd probably use NaN for
> that, although by definition of "seeking a maximum associated floating
> point number" should *not* be allowed for an empty set, such search
> shouldn't return a value.
>
>>> As for infinity (unrelated to searching through a set of numbers), there
>>> is 'std::numeric_limits<double>::infinity()', which you could call if
>>> 'std::numeric_limits<double>::has_infinity' is 'true'.
>>
>> Yes, I was using that, and according to the standard
>> std::numeric_limits<T>::has_infinity is true for T = float and double, so
>> no
>> test is necessary. The only problem I have with it is that it doesn't
>> feel
>> quite right to handle infinity values like this. At least I never saw
>> this being done anywhere else.
>
> <another shrug> I have. But it's still not right. You can use any
> other designated value that can never be found in your set. And if you
> don't have any identifiable value to use, don't. Use *logic*.
Why is it "not right"? Is there actually a valid technical reason behind
your assertion?
> Essentially you're trying to have a mapping of yourtype values to
> double/float values without
>
> std::map<double, yourtype const*> yourmap;
>
> . And you're trying to figure out a hack to get
> (*yourmap.rbegin()).first without checking whether the 'yourmap' is
> empty or not. <third shrug>
Again, you either failed to understand what I wrote or you are intentionally
trying to misrepresent what I said. No one claimed that the set in question
could be empty, and somehow you felt the need to attribute that claim, which
you invented, to someone else.
So, to avoid any more misconceptions or any attempts to misrepresent
anything, here is a clear description of this case.
 there is a nonempty set of data.
 there is a set of operators which map each element of that set to a
floating point number.
 the objective is to evaluate which is the minimum value of the codomain of
a particular operator.
I suggested the following approach:
<pseudoish code>
float minimum = std::numeric_limits<float>::infinity();
for(auto element: element_list)
{
if( operator(element) < minimum)
minimum = operator(element);
}
</pseudoish code>
Then, I asked if it was a good idea to do this. In other words, if there
was any reason that would made it a bad idea. Until now, no reason has been
given.
I also asked if there was a better way to get the minimum value.
Simple as that.
Rui Maciel
