[Date Index] [Thread Index] [Author Index]

Re: Serious Bug in Mathematica 7 and 8.0.4.0 (latest version)

• To: mathgroup at smc.vnet.net
• Subject: [mg126185] Re: Serious Bug in Mathematica 7 and 8.0.4.0 (latest version)
• From: David Bailey <dave at removedbailey.co.uk>
• Date: Tue, 24 Apr 2012 05:30:49 -0400 (EDT)
• Delivered-to: l-mathgroup@mail-archive0.wolfram.com
• References: <jn387r$g80$1@smc.vnet.net>

On 23/04/2012 10:43, Alexander Mayer wrote:
> In[1]:= (* This notebook produced by Mathematica 8.0.4.0 *)
> (* Platform: OSX 10.6.8 on MacBook Pro *)
>
> In[1]:= b = 6371.01 (* We set the value of b to this real number. *)
>
> Out[1]= 6371.01
>
> In[2]:= SetPrecision[a = b, 20] (* We test the actual value of b. *)
>
> Out[2]= 6371.0100000000002183
>
> In[3]:= (* Mathematica has added a small constant! *)
>
> In[4]:= (* Adding zeros to the 16th place fixes the problem. *)
>
> In[5]:= c = 6371.0100000000000000
>
> Out[5]= 6371.0100000000000000
>
> In[6]:= SetPrecision[a = c, 20]
>
> Out[6]= 6371.0100000000000000
>
Your original number is a machine precision number - that means it 
stored as a binary floating point number. Such a number will not in 
general be an exact decimal number - hence the small additional constant.

If you take a low precision number, there is no way for the system to 
conjure the extra precision information from nowhere!

David Bailey
http://www.dbaileyconsultancy.co.uk