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MathGroup Archive 1996

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Re: patterns

  • To: mathgroup at smc.vnet.net
  • Subject: [mg3734] Re: patterns
  • From: ianc (Ian Collier)
  • Date: Sun, 14 Apr 1996 02:59:46 -0400
  • Organization: Wolfram Research, Inc.
  • Sender: owner-wri-mathgroup at wolfram.com

In article <4ki9uv$9dm at dragonfly.wolfram.com>, Susan Rempe
<rempe at euclid.chem.washington.edu> wrote:

> I have a list of numbers and symbols.
> How can I collect all the members of the list
> which have a common denominator?
> 
> ex.  list={Sin[theta]/r, Cos[theta]/(r*s), 2*Csc[theta]/(2*r)}
> 
>  Say I want to collect all members of the list with r in the denominator;
> 
>  i.e.  answer=Sin[theta]/r;
> 


If you know exactly what the denominator you want to select is then the
folling will work:


In[19]:=
    list={Sin[theta]/r, Cos[theta]/(r*s), 2*Csc[theta]/(2*r)}
Out[19]=
     Sin[theta]  Cos[theta]  Csc[theta]
    {----------, ----------, ----------}
         r          r s          r

In[20]:=
    ?Select

    Select[list, crit] picks out all elements ei of list for
       which crit[ei] is True. Select[list, crit, n] picks out
       the first n elements for which crit[ei] is True.

In[21]:=
    Select[ list, Denominator[ # ]==r&]
Out[21]=
     Sin[theta]  Csc[theta]
    {----------, ----------}
         r           r

You can use more sophisticated selction criteria if you wish.

I hope this helps.

--Ian

-----------------------------------------------------------
Ian Collier
Wolfram Research, Inc.
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tel:(217) 398-0700   fax:(217) 398-0747    ianc at wolfram.com
Wolfram Research Home Page:         http://www.wolfram.com/
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