Floating point equality [was Re: What exactly is "exact" (was Clean Singleton Docstrings)]

[Chris Angelico]

On Wed, Jul 20, 2016 at 3:42 PM, Steven D'Aprano
<steve+comp.lang.python at pearwood.info> wrote:
> Arithmetic on integer-values (e.g. 1.0) is always exact, up to a limit of
> either 2**53 or approximately 1e53, I forget which. (That's why most Javascript
> programmers fail to notice that they don't have an integer type.)

It's 2**53, because 64-bit floats use a 53-bit mantissa (52-bits
stored and an implicit 1 at the beginning, although I can never
remember how denormals are represented). Works out to a bit under
1e16. AIUI asm.js offers a 32-bit integer type, which in fall-back
mode is represented with the native "Number" type; for values that
could be stored in a 32-bit integer, a 64-bit float is perfectly
accurate (just stupidly inefficient compared to a real integer type).

ChrisA