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Re: Unexpected behavior of Floor and IntegerPart

  • To: mathgroup at smc.vnet.net
  • Subject: [mg97897] Re: [mg97872] Unexpected behavior of Floor and IntegerPart
  • From: Curtis Osterhoudt <cfo at lanl.gov>
  • Date: Wed, 25 Mar 2009 05:42:37 -0500 (EST)
  • Organization: LANL
  • References: <14546895.41213.1237896516060.JavaMail.hanlonr@127.0.0.1>
  • Reply-to: cfo at lanl.gov

  Thank you, Bob. I did end up using Rationalize in one of my functions to work around this. Luckily, that particular function doesn't need to be called often, and never on numbers which have a huge number of decimal places.

      Cheers, 
               C.O.



On Tuesday 24 March 2009 06:08:36 am Bob Hanlon wrote:
> The observed behavior is consistent. If you want the functions to behave 
> based on the apparent values rather than the machine numbers, then use 
> Rationalize or Round to whatever precision level that you want. This 
> will probably just shift the "unexpected behavior" to some other 
> condition.
> 
> myIntegerPart[x_] :=
>   IntegerPart[Rationalize[x]]
> 
> myFloor[x_] := Floor[Rationalize[x]]
> 
> myIntegerPart[0.29*100]
> 
> 29
> 
> myFloor[0.29*100]
> 
> 29
> 
> 
> 
> Bob Hanlon
> 
> 
> On Tue, Mar 24, 2009 at 8:07 AM , Curtis Osterhoudt wrote:
> 
> > Dear List,
> >    I'm sure this is a result of non-infinite-precision arithmetic, but 
> > it would be nice to have a more consistent behavior:
> >
> > In[26]:= IntegerPart[0.29*100]
> >
> > Out[26]= 28
> >
> > In[25]:= Floor[0.29*100]
> >
> > Out[25]= 28
> >
> > In[27]:= Floor[(29/100)*100]
> >
> > Out[27]= 29
> >
> > In[32]:= (Floor[#1*100] & ) /@ Range[0, 1, 0.01]
> >
> > Out[32]= {0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, \
> > 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 28, 30, 31, 32, 33, \
> > 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, \
> > 51, 52, 53, 54, 55, 56, 57, 57, 59, 60, 61, 62, 63, 64, 65, 66, 67, 
> > 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, \
> > 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 100}
> >
> > In[33]:= Differences[%]
> >
> > Out[33]= {1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 
> > \
> > 1, 1, 1, 1, 1, 1, 1, 1, 0, 2, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, \
> > 1,    1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 0, 2, 1, 1, 1, 1, 1, 1, 
> > 1, \
> > 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 
> > 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1}
> >
> >
> >
> > An interesting hint as to what may be happening is obtained by:
> >
> > ListPlot[Differences[(Floor[#1*100] & ) /@ Range[0, 100, 0.01]]]
> >
> >                      Regards,                                     C.O.
> >
> >
> >
> > -- 
> > ==========================================================
> > Curtis Osterhoudt
> > cfo at remove_this.lanl.and_this.gov
> > PGP Key ID: 0x4DCA2A10
> > Please avoid sending me Word or PowerPoint attachments
> > See http://www.gnu.org/philosophy/no-word-attachments.html
> > ==========================================================
> 



-- 
==========================================================
Curtis Osterhoudt
cfo at remove_this.lanl.and_this.gov
PGP Key ID: 0x4DCA2A10
Please avoid sending me Word or PowerPoint attachments
See http://www.gnu.org/philosophy/no-word-attachments.html
==========================================================


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