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MathGroup Archive 1999

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Re: Position and Real Numbers

  • To: mathgroup at
  • Subject: [mg18907] Re: Position and Real Numbers
  • From: Paul Abbott <paul at>
  • Date: Mon, 26 Jul 1999 14:27:47 -0400
  • Organization: University of Western Australia
  • References: <7n694d$>
  • Sender: owner-wri-mathgroup at

Jason Gill wrote:

> Is there an easy way to make Position work as expected for real numbers, or more
> accurately make Range work as
> expected ???

Both work "as expected". With

In[1]:= x=Range[-3,2,.05];

since the elements are distinct, to locate -0.05 you could use a test such as

In[2]:= Position[x,_?(-0.06<#<-0.04&)]

Out[2]= {{60}}

or you could Rationalize the entries

In[3]:= Position[Rationalize[x],Rationalize[-1/20]]

Out[3]= {{60}}

You can make Range give a set of exact values if you give it exact input:

In[4]:= x=Range[-3,2,Rationalize[0.05]];

In[5]:= Position[x,Rationalize[-1/20]]

Out[5]= {{60}}

Moral: If you give Mathematica exact input and it will generally give you exact
ouput. With approximate input you generally get approximate output.

Paul Abbott                                   Phone: +61-8-9380-2734
Department of Physics                           Fax: +61-8-9380-1014
The University of Western Australia
Nedlands WA  6907                     mailto:paul at

            God IS a weakly left-handed dice player

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