Re: Non-deterministic numerical inaccuracies in Mathematica 7
- To: mathgroup at smc.vnet.net
- Subject: [mg94995] Re: Non-deterministic numerical inaccuracies in Mathematica 7
- From: David Bailey <dave at removedbailey.co.uk>
- Date: Thu, 1 Jan 2009 20:30:57 -0500 (EST)
- References: <gjcuio$cnf$1@smc.vnet.net> <gjicrh$c5s$1@smc.vnet.net>
Luis Rademacher wrote: > What makes you think that this is a bug? Is there really a guarantee > that the execution of Mathematica code is deterministic (especially, > involving machine-precision)? I am no expert in floating point numbers, > but I am aware that dealing with them can easily become tricky. > Have a look at the following quotations and their sources: > >> We shall discuss the following myths, among others: [...] Arithmetic >> operations are deterministic; that is, if I do z=x+y in two places in >> the same program and my program never touches x and y in the >> meantime, then the results should be the same. First, the bug manifests itself as a variation at the same place in the code - the loop goes round and the results vary - so I don't think this caveat applies. Furthermore, I think the reason that z=x+y could generate slightly different results at two different points in a C or Fortran program, would be as a result of some sort of reordering of the code by an optimiser, and would not apply here. I certainly think this is a bug, and I can even guess what may have happened - see my reply to a later post on this issue! David Bailey http://www.dbaileyconsultancy.co.uk