Re: erronneous typecasting

From:

Pete Becker <pete@versatilecoding.com>

Newsgroups:

comp.lang.c++

Date:

Sat, 08 Aug 2009 13:28:01 -0400

Message-ID:

<UNadnTEGw7o8KODXnZ2dnUVZ_qKdnZ2d@giganews.com>

Jonathan Lee wrote:

double remainder;
modf(dimnvalue, &remainder);
if (fabs(remainder) < eps)
// if (1.0 + fabs(remainder) == 1.0)
// if (remainder == 0.0)

The choice of if condition really depends on how much you want
to account for floating point error. You can choose a small eps
so that something "really close" to an integer is considered an
integer. Alternatively, the "add 1.0" version will force very
small floating point values to be wiped out. I don't really
recommend testing against 0.0, as in the third case, because
a floating point error could make this false when you expect it
to be true.

Well, an error in coding could cause the third case to produce incorrect
results. But "floating point error" is really no more meaningful than
"integer error". It occurs if the compiler generates incorrect code or
if something is wrong with the chip or if something gets hit by some
random gamma ray. Other than that, floating-point math is well defined
and correct. Users sometimes don't understand it, but that's user error.
Throwing in approximate tests is not a substitute for knowing what
you're doing.

--
Pete
Roundhouse Consulting, Ltd. (www.versatilecoding.com) Author of
"The Standard C++ Library Extensions: a Tutorial and Reference"
(www.petebecker.com/tr1book)