Math.log2 applied to powers of 2
kpreid at google.com
Thu Sep 18 11:12:42 PDT 2014
On Thu, Sep 18, 2014 at 10:56 AM, Tab Atkins Jr. <jackalmage at gmail.com>
> On Thu, Sep 18, 2014 at 3:17 AM, Claude Pache <claude.pache at gmail.com>
> > Question: Should Math.log2 give exact results for powers of 2?
> > The same issue holds for Math.log10 (might be applicable for nonnegative
> > powers only): Math.log10(1e15) != 15 in Chrome.
> I have no idea of the computation complexity underlying a log
> implementation, so given that: yes, it should totally give exact
> results for powers of 2, and log10 should do the same for (positive)
> powers of 10. (Negative powers of 10 can't actually be represented by
> a JS number, so there's no need to talk about them.)
It would also be useful, though perhaps not feasible, if they are
guaranteed to be monotonic everywhere and strictly monotonic near those
exact values; that is,
for all x < 2^k, log2(x) < k
for all x > 2^k, log2(x) > k
and similarly for log10. If this property held, then naïve "number of
digits" tests expressed using logarithms would always give the right
answers. (This probably conflicts with generally desirable rounding
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