What is a concise way to test if floating point value is integral?

[Jonathan M Davis]

On Thursday, August 29, 2013 10:07:31 Paul Jurczak wrote:
> On Thursday, 29 August 2013 at 07:51:40 UTC, Jonathan M Davis
> wrote:
> [..]
> 
> > as any integral value in a float will fit in an
> > int.
> 
> [..]
> 
> Will it? Most of them will not fit

Sure, they will. float has 32 bits, just like int, so it can't possibly hold a 
value larger than an int can hold. This code passes with flying colors

    foreach(i; int.min .. int.max)
        assert(cast(float)i == i);

> but cast to int produces
> nonsensical value anyway as in this example:
> 
> cast(int)float.max

So what? All you care about here is whether the value in the float is an 
integral value. float.max isn't an integral value, so it really doesn't matter 
if you get overflow when converting. It would have to convert back to the exact 
same value when converting it back to a float, and it won't.

assert(cast(int)float.max == float.max);

will fail. It might make more sense to use to!int if you want to use the 
resulting int for something, but all you're doing with it is comparing it 
against the original float to see if it's the same value. If anything, the fact 
that to!int throws would be a serious problem for what you're trying to do, 
because if what you're testing for is whether a float holds an integral value, 
throwing on overflow when converting to int would just get in your way. It 
should just result in false in that case (because it's not an integral value, 
or it wouldn't overflow), and the cast will result in false, unlike to!int.

- Jonathan M Davis