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Re: Exact real numbers

  • To: mathgroup at
  • Subject: [mg50548] Re: [mg50501] Exact real numbers
  • From: Murray Eisenberg <murray at>
  • Date: Thu, 9 Sep 2004 05:19:31 -0400 (EDT)
  • Organization: Mathematics & Statistics, Univ. of Mass./Amherst
  • References: <>
  • Reply-to: murray at
  • Sender: owner-wri-mathgroup at

Is this the sort of thing you want?

   roundReal[x_, n_] := 10.^(-n) Round[10^n x]

   roundReal[1.56012309843, 3]

    NumberForm[%, 20]

Ross Sean Civ AFRL/DELO wrote:
> I have been having an on-going problem with real numbers in Mathematica that
> has come up again.  In short, I can find no way of "rounding" a number in
> Mathematica.  To me, when I say, round 1.56012309843 to three decimal
> places, I want the result to be 1.560 exactly.  No hanging 00000000001's .
> The reason for this is that I need to do 3-D logic involving the location of
> points in space.  I don't like getting faked out by numbers that ought to be
> the same but are slighly unequal due to rounding errors. 
> Round only applies to integers.  Multiplying the results of Round by machine
> precision reals results in all the hanging digits I want to get rid of.
> SetPrecision and SetAccuracy don't seem to do what I want.
> SetPrecision[1.5000287965,5] returns a result with more than 5 digits to the
> right of the decimal as does SetAccuracy.
> RealDigits is great in that I can actually get a list of all digits and
> manipulate them, however, its "inverse" FromDigits returns an F**ing
> fraction.  I don't want a fraction.  I want an exact decimal.  I also prefer
> function pairs that are exact inverses to one another.
> One strange assymetry of Mathematica is that I can type in the exact decimal
> number 1.50027` and Mathematica knows that is an exact decimal.  FullForm
> reveals that the only decimal digits in that number are what I specified.
> However, I know of no way to create an exact decimal from a machine
> precision one.
> Does anyone know of a way to do true decimal rounding in Mathematica?
> Please respond to sean.ross at  I no longer subscribe to
> mathgroup.
> Dr. Sean Ross
> 3550 Aberdeen Ave SE, Building 761
> Kirtland AFB, NM 87117
> phone: 505-846-9148
> fax: 505-853-0485

Murray Eisenberg                     murray at
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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