Re: Equality of floating-point numbers (special case)



Robert Latest wrote On 04/19/06 10:06,:
Hi guys,

I'm sure this has been beaten to death on this newsgroup, but I can't
find it in the CLC FAQ.

Consider the following code:

--------

double x = some_value_from_somewhere;
double y = x;

if ((x == y) && ((x - y) == 0)) {
puts("This is what I want");
}

--------

Will the condition in the if statement always, unconditionally, and
portably evaluate to a true value?

No. The Standard permits floating-point implementations
that support the notion of "not a number," or NaN. In IEEE
implementations, NaN is unequal to all floating-point values,
including itself (so `x == y' is false) and most ordinary
arithmetic operations with NaN operands yield a NaN result
(so `x - y' is NaN and `x - y == 0' is false).

--
Eric.Sosman@xxxxxxx

.