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MathGroup Archive 1999

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RE: Mathematica 3.0 - 4.0 discrepancies.

  • To: mathgroup at smc.vnet.net
  • Subject: [mg20403] RE: [mg20377] Mathematica 3.0 - 4.0 discrepancies.
  • From: "Ersek, Ted R" <ErsekTR at navair.navy.mil>
  • Date: Tue, 26 Oct 1999 00:32:55 -0400
  • Sender: owner-wri-mathgroup at wolfram.com

Jordi Igns indicated Version 3 and Version 4 gave very different results
when certain calculations were done under Win9x or Linux, but not under
MacOS.

I found the source of the problem and simplified it to the examples 
below.

Using Version 3 under Windows 98 we get the following:
-------------------------------------

In[1]:=
reflected[air] ={{1,2},{3,4}}.{E[air][1],E[air][2]}

Out[1]=
{E[air][1] + 2*E[air][2], 3*E[air][1] + 4*E[air][2]}


In[2]:=
{{1.0,2.0},{3.0,4.0}}.{E[air][1],E[air][2]}

Out[2]=
{1.*E[air][1] + 2.*E[air][2], 3.*E[air][1] + 4.*E[air][2]}


In[3]:=
reflected[air] ={{1.0,2.0},{3.0,4.0}}.{E[air][1],E[air][2]}

Out[3]=
{1.*E[air][1] + 2.*E[air][2], 3.*E[air][1] + 4.*E[air][2]}


In[4]:=
{{1.0,2},{3.0,4}}.{E[air][1],E[air][2]}

Out[4]=
{1.*E[air][1] + 2.*E[air][2], 3.*E[air][1] + 4.*E[air][2]}

-------------------------
Using Version 4 under Windows 98 we get the same result from
In[1], In[2], but not from In[3], In[4] !!
Instead Version 4 gives the result below.

In[4]:=
reflected[air] ={{1.0,2.0},{3.0,4.0}}.{E[air][1],E[air][2]}

Out[4]=
{8.15485, 19.028}


In[5]:=
{{1.0,2},{3.0,4}}.{E[air][1],E[air][2]}

Out[5]=
{8.15485, 19.028}


Apparently the kernel gives E[air][1], E[air][2] the numeric value of E in
the lines above.  I would say this is a bug in Version 4.0 under certain
platforms.

However, it's a bad practice to use symbols such as (E, I, Pi, D) for
variables because the built-in meaning might mess things up.  As we see
above, your code might work with the present version of Mathematica, but not
in a future version.

It seems Jordi was using E, B to represent electric, magnetic fields, and
that sounds like a logical choice.  Now that we know this can cause problems
I think a better choice would be \[ScriptCapitalE], \[ScriptCapitalB].  


--------------------
Regards,
Ted Ersek

For Mathematica tips, tricks see
http://www.dot.net.au/~elisha/ersek/Tricks.html


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