4 ms·
> Your claim is fallacious, because a floating-point value denotes a range of the real number line, whereas you're insisting that, no, it stands for its literal
by acidflask 11y ago
> Your claim is fallacious, because a floating-point value denotes a range of the real number line, whereas you're insisting that, no, it stands for its literal value: the absolutely precise rational number that lies at the center of that range
See Misconception #2 of http://lipforge.ens-lyon.fr/www/crlibm/documents/cern.pdf http://lipforge.ens-lyon.fr/www/crlibm/documents/cern.pdf.
Floating point numbers are _not_ intervals. If you read carefully any formal definition of floating point numbers (The IEEE standards, TAoCP Chapter 4, or Higham's _Accuracy and Stability of Numerical Algorithms_, to name just three possible references), you will see that floating point numbers by definition form an exact rational subset of the extended real line.
> All numbers in that range alias to the same double, yet differ wildly in their decimal digits beyond the fifteenth. That "long tail" of digits is a meaningless residue arising from the arbitrary difference between the chosen center-of-range point and the actual number it approximates.
See Misconceptions #1 and #2 on Kahan's list (https://www.cs.berkeley.edu/~wkahan/JAVAhurt.pdf https://www.cs.berkeley.edu/~wkahan/JAVAhurt.pdf).
You are not being clear about the set of real numbers a user may input and their ultimate representation as a floating point number. While it's true that many real numbers round to the same floating point number _f_, that's not the same as then saying that _f_ carries that interval around with it. The latter is false, since the intervals do _not_ propagate in floating point arithmetic. It's also false that the floating point number _f_ is the midpoint of the set of numbers that round to _f_; the precise set depends on the rounding mode and the granularity of the set of floating point numbers around _f_.
- kazinator 11y agoI didn't say they were intervals (I'm well aware of interval representations for numbers, which single point rationals are not). > that floating point numbers by definition form an exact rational subset of the extended real line. Sure, that's what they form; it's not what they (usually) denote. > It's also false that the floating point number _f_ is the midpoint of the set of numbers that round to _f_; It is close enough to being the case for my purpose in the grandparent article. Whether the cluster of numbers is lopsided one way or the other doesn't really detract from my point.
- StefanKarpinski 11y agoAre they specific values or are they intervals? You can't have it both ways.