Re: Help w/ Floats needed!

From:

"Daniel T." <daniel_t@earthlink.net>

Newsgroups:

comp.lang.c++

Date:

Sun, 18 May 2008 10:07:51 -0400

Message-ID:

<daniel_t-2672BF.10075118052008@earthlink.vsrv-sjc.supernews.net>

"Somebody" <somebody@cox.net> wrote:

I have a point "int x", and a width "int cx". In order to support scaling, I
store this as "double ratio = x / cx" which invariably led to round off
errors when I tried to re-calculate the new position from the ratio as in:

x = ratio * cx;

I tried to solve this by multiplying the numerator by 10,000 to gain more
precision... recent testing showed that I had not solved the problem in all
cases. Thinking about it, no matter how many times I multiply the numerator
of a fraction, I'll never be able to recover 100 from 1/3 (0.333333....) for
example.

I came up with this idea: instead of storing "double ratio = x / cx", I will
store int numerator=x, int denominator = cx;

That way, in my case off 1/3, I'll be able to fully recover the original
value. Ie...:

original pos = 33
original width = 100

num = 33;
denom = 100;

new width = num * 100 / denom = 33;

Does this sound like a reasonable solution (with ABSOLUTELY ZERO LOSS OF
PRECISION)? Or is there a better way?

There is no such thing as zero loss of precision. Your solution works as
well as any other would. If you are on a system where floating point
operations aren't too expensive, then use a double to represent the
ratio (keep in mind Jim Langston's advice though.) Otherwise use the
idea you had. If you find you are doing this sort of thing a lot, then
it might behoove you to make a class that stores the numerator and
denominator and knows how to handle all the math.