Re: Position and Real Numbers

• To: mathgroup at smc.vnet.net
• Subject: [mg18907] Re: Position and Real Numbers
• From: Paul Abbott <paul at physics.uwa.edu.au>
• Date: Mon, 26 Jul 1999 14:27:47 -0400
• Organization: University of Western Australia
• References: <7n694d\$h5o@smc.vnet.net>
• Sender: owner-wri-mathgroup at wolfram.com

```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 physics.uwa.edu.au
AUSTRALIA                            http://physics.uwa.edu.au/~paul

God IS a weakly left-handed dice player
____________________________________________________________________

```

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