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Re: Re: Confused about precision vs accuracy

  • To: mathgroup at smc.vnet.net
  • Subject: [mg90434] Re: [mg90375] Re: Confused about precision vs accuracy
  • From: Murray Eisenberg <murray at math.umass.edu>
  • Date: Wed, 9 Jul 2008 04:54:21 -0400 (EDT)
  • Organization: Mathematics & Statistics, Univ. of Mass./Amherst
  • References: <g4smav$29$1@smc.vnet.net> <200807080623.CAA17949@smc.vnet.net>
  • Reply-to: murray at math.umass.edu

Of course the expression N[expr, digits] will not necessarily show you 
"digits" digits of expr when expr is not an exact quantity.  For example:

   N[Sin[1.], 12]
0.841471

despite:

   Precision[%]
MachinePrecision
   N[MachinePrecision]  (* on my machine *)
15.9546

But:

   NumberForm[N[Sin[1.]], 12]
0.841470984808

There's a difference in the value of an N expression and what is 
displayed in the Output cell.


Bob F wrote:
> 
> Just use the N[expr,digits] function, e.g., if you wanted to see
> several values of Pi you could:
> 
>    Table[N[Pi,i],{i,5,50,10}] would give
> 
> {3.1416, 3.14159265358979, 3.141592653589793238462643, \
> 3.1415926535897932384626433832795029, \
> 3.14159265358979323846264338327950288419716940}
> 
> So you could use any value for "expr" in the N[expr, digits] function
> call - not just for Pi or other constants.

-- 
Murray Eisenberg                     murray at math.umass.edu
Mathematics & Statistics Dept.
Lederle Graduate Research Tower      phone 413 549-1020 (H)
University of Massachusetts                413 545-2859 (W)
710 North Pleasant Street            fax   413 545-1801
Amherst, MA 01003-9305


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