Re: using a float as the index in an STL map?

From:

James Kanze <james.kanze@gmail.com>

Newsgroups:

comp.lang.c++

Date:

24 Apr 2007 01:04:20 -0700

Message-ID:

<1177401859.993788.97150@s33g2000prh.googlegroups.com>

On Apr 23, 9:00 pm, Colander <colan...@gmail.com> wrote:

On Apr 21, 3:07 am, red floyd <no.s...@here.dude> wrote:

James Kanze wrote:

On Apr 21, 12:36 am, zeppethef...@gmail.com wrote:

I think a good alternative would be to manually define the comparison
operator "less" considering that the equality

(a == b)

on a map is given by

!less(a,b) && !less(b,a)

it should be sufficient to write less() as something like

less(a, b){
        if (a - b > threshold)
                return false;
        else
                return true;
}

with threshold a proper small number.

That's a nice way to get undefined behavior. Since such a
function doesn't define an ordering relationship (which must be
transitive), it doesn't meet the requirements for std::map or
std::set.

On the other hand, assuming a reasonable threshold (EPSILON), what's
wrong with:

less(float a, float b)
{
    if (fabs(a - b) > EPSILON) // difference big enough to be
           return a < b; // significant
    else
           return false; // elements are "equal"
}

What's wrong is that EPSILON is relative to a and b.

What's wrong is that it doesn't define a "strict weak ordering",
as defined in and required by the standard. Using a relative
epsilon won't change that.

--
James Kanze (GABI Software) email:james.kanze@gmail.com
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