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

From:

"Victor Bazarov" <v.Abazarov@comAcast.net>

Newsgroups:

comp.lang.c++

Date:

Fri, 20 Apr 2007 17:33:05 -0400

Message-ID:

<f0bbij$svd$1@news.datemas.de>

JDT wrote:

Victor Bazarov wrote:

JDT wrote:

It seems that using floats as the first tupelo for an STL map (shown
below) can cause problems but I don't remember what the problems
were.

The only thing I can think of is that if you expect two different
results of a calculation to lead to the same number (and ultimately
to the same stored in the map value), you may be in for a surprise.
Due to rounding errors in the FPU the mathematically equivalent
calculations can lead to different numbers internally.

Example, given that 'a' is 1.f and 'b' is 2.f, the expressions
(a + 2.f/3) and (b - 1.f/3) _can_ give you different results.

Is it common practice to use a structure like below? I would
appreciate if you can share your experience using such a structure.
Thanks.
std::map<float, int> m;

I _never_ saw a 'map' where 'float' would be the Key type. I cannot
claim to have seen all code in the world, and even a significant part
of it, so I cannot attest to "commonality" of the practice.

V

Hi Victor:

In the following scenario, I think using float as the key to a map
makes sense. For example, the following table needs to be sorted in
the ascending order of the 1st tuple (i.e. Values). So I can simply
insert the pairs into a map and then get a list of sorted pairs
automatically. Do people often have similar needs or are there other
better ways to accomplish this purpose? Any further help is much
appreciated.
Values # of items
3.5 5
4.7 9
9.3 7
.....

That's fine. Another way is to have a vector<pair<float,int> >, stuff
it with all the pairs you encounter, then sort them based on the pair's
'.first' member. You'd have better space economy that way. Of course,
in that case if you need to extract the correct order at any moment,
you'd have to sort right there (whereas 'std::map' always keeps them
sorted).

V
--
Please remove capital 'A's when replying by e-mail
I do not respond to top-posted replies, please don't ask