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;
>
>

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.
-----------------------------------------------------------
tel:(217) 398-0700   fax:(217) 398-0747    ianc at wolfram.com