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MathGroup Archive 2005

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Adding two numbers of high precision results in a number of low precision??

  • To: mathgroup at smc.vnet.net
  • Subject: [mg56887] Adding two numbers of high precision results in a number of low precision??
  • From: Kees van Schaik <schaik at math.uni-frankfurt.de>
  • Date: Mon, 9 May 2005 01:46:09 -0400 (EDT)
  • Sender: owner-wri-mathgroup at wolfram.com

Hi,

I have the following question. In a pretty long code (so I won't send 
everything) at some point the following loop occurs:

> For[i=1,i<=3,i++,
>     bTemp[i,0,j]=bTemp[i,0,0]+Q[i,j];
>     Print["Precisie bTemp[",i,",0,0] = ",Precision[bTemp[i,0,0]]];
>     Print["Precisie Q[",i,",",j,"] = ",Precision[Q[i,j]]];
>     Print["Precisie bTemp[",i,",",0,",",j,"] = ",Precision[bTemp[i,0,j]]];
>  ]; (* einde For *)

("Precisie" is Dutch for "precision" ;)), where the j is a loop counter 
and the Q[.,.]'s and bTemp[.,0,0]'s are known numbers of a certain 
precision. Now two pieces of the output of this piece of code look like 
this:

> Precisie bTemp[2,0,0] = 397.142
> Precisie Q[2,1] = 397.172
> Precisie bTemp[2,0,1] = 395.193


and

> Precisie bTemp[3,0,0] = 389.685
> Precisie Q[3,1] = 390.729
> Precisie bTemp[3,0,1] =53.8232


Now the first one makes sense, but the last one, how is it possible that 
if I add two numbers of precision ca. 390 I get something of precision 
53 back? I hope somebody could explain, because there are numerical 
problems in my code that mess things up and I'm afraid the stuff above 
could have to do with it...

Thanks in advance & best wishes, Kees van Schaik

-- 

====================================================
   Kees van Schaik
   Frankfurt MathFinance Institute
   J.W. Goethe-Universitaet
   Frankfurt am Main
   Tel: +49 (0)69 79823453
   WWW: http://ismi.math.uni-frankfurt.de/vanSchaik/
====================================================


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