Re: How to determine if double contains an integer value?
DeCaf ha scritto:
To complete this discussion, it would also be interesting to find out a
good way to figure out not only if the double (or other floating point
type) contains an integer, for which I think we've found a few working
ways, but also to find out if that value will fit in a specific integer
type.
Would the following test suffice for that?
bool is_int(double d)
{
return std::fmod(d, 1.0) == 0.0 &&
f >= (double)std::numeric_limits<int>::min() &&
f <= (double)std::numeric_limits<int>::max();
}
If we for the moment disregard any rounding errors. Obviously the
above would fail if double can not contain the max/min of the integer
type, but that doesn't really seem feasible, does it?
On a typical implementation with 32-bit integers and doubles with 53
bits of mantissa, ints are always represented exactly. However, if your
implementation has 64-bit integers, which can not be represented
exactly, your test may potentially succeed when it should fail or
viceversa. However, in that case the meaning of "a float that contains
an integer" is flawed in the first place (every number greater than 2^53
is represented with a fractional part equal to 0).
Ganesh
PS: I did a sanity check about my preference of using modf over fmod. A
simple benchmark on my machine (ix86-based, Windows, VC++ 7.1) has shown
that fmod is typically 50% slower than modf.
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