- From: C. M. Sperberg-McQueen <cmsmcq@acm.org>
- Date: Tue, 22 Jan 2002 07:02:20 -0700
- To: W3C XML Schema Comments list <www-xml-schema-comments@w3.org>
- Cc: Dave Peterson <davep@acm.org>, Ashok Malhotra <ashokma@microsoft.com>, "Biron,Paul V" <Paul.V.Biron@kp.org>

Some time ago, it was suggested that the XML Schema spec should specify clearly the range of numbers representable by our float type, and make explicit when numbers round to the largest finite value and when they round to infinity. A piece of paper I've just found on my desk gives Dave Peterson's answer to the first of these questions. I'm sending this mail to the XML Schema comments list so as to ensure that this topic gets into the queue of candidate changes for XML Schema, and to get this piece of paper off my desk and off my conscience. The largest finite float, if I understand the notes correctly, is m * 2**e where ** means exponentiation, m is the largest number representable in the mantissa, and e is the largest number representable as an exponent Since we have m = 2 ** 24 - 1 e = 127 it follows that m * 2**e = (2**24) * (2**127) - 2**127 = 2**151 - 2**127 The decimal expansion of this is approximately 2.85449517E+45, or exactly 2854495215270736301647340207211686556881387520. With commas, that's 2,854,495,215,270,736,301,647,340,207,211,686,556,881,387,520. (Calculation courtesy of Regina, an implementation of the Rexx programming language which supports sufficient precision to make this an exact calculation.) Some things are, unfortunately, not so clear to me: (a) what the next largest float would be if we had one (b) where the watershed point is between infinity and 2.85...E45 (c) whether the negative numbers are exactly the same as these plus a minus sign, or divergent in some way I don't believe there has been any confusion over the watershed between zero and the smallest representable float. Dave, if you can confirm that I have correctly interpreted your notes, I'd be grateful. Ditto if anyone can shed light on questions (a), (b), (c) above. -C. M. Sperberg-McQueen

Received on Tuesday, 22 January 2002 10:29:21 UTC