On Fri, Jan 23, 2015 at 5:41 PM, Guido van Rossum <guido@python.org> wrote:
Isn't an ulp just a base-2 way of specifying precision scaled so that 1 ulp is the low bit of the mantissa in IEEE fp?
Basically yes, but there are weird subtleties. E.g. 1 ulp remains the same absolute size between 1.0 and 2.0, so the same ulp threshold can vary by a factor of two in relative precision terms. And where you hit the boundary between exponents funny things happen: 2.0 +/- 1 ulp is [2.0 - 2.2e-16, 2.0 + 4.4e-16]. This can matter if you're looking for high precision -- if the value is supposed to be almost 2.0, then you don't want to get penalized for failing to get 2.0 + 2.2e-16, b/c there is no such number, but it might also be unacceptable to get 2 - 4.4e-16, which would be two values off. -n -- Nathaniel J. Smith Postdoctoral researcher - Informatics - University of Edinburgh http://vorpus.org