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RE: Position on a List
 To: mathgroup at smc.vnet.net
 Subject: [mg12890] RE: [mg12845] Position on a List
 From: Ersek_Ted%PAX1A at mr.nawcad.navy.mil
 Date: Wed, 24 Jun 1998 03:44:14 0400
 Sender: ownerwrimathgroup at wolfram.com
Idoia Aguirre wrote:

 When I do: Position[{1/A, A}, A] 
 I obtain: {{1,1},{2}}

 But I would like to obtain only : {{2}} 
You need to give Position a levelspec as a third argument.
In[1]:=
?Position
"Position[expr, pattern] gives a list of the positions at which objects
\ matching pattern appear in expr. Position[expr, pattern, levspec]
finds only \
objects that appear on levels specified by levspec."
You need to specify levspec=1 as in the line below.
In[2]:=
Position[{1/A,A},A,1]
Out[2]=
{{2}}
_________________________________
Lets look at this a little closer with another example. With the default
levelspec we get the position of every A. In[4] shows us that the
default levlspec must be Infinity.
In[3]:=
Position[{1/(1+A), 1/A, A}, A]
Out[3]=
{{1,1,2},{2,1},{3}}
In[4]:=
Position[{1/(1+A), 1/A, A}, A, Infinity] Out[4]=
{{1,1,2},{2,1},{3}}
If we specify levelspec=2 we get all positions where A can be found from
level 1 through level 2.
In[5]:=
Position[{1/(1+A), 1/A, A}, A, 2]
Out[5]=
{{2,1},{3}}
If we specify levelspec=1 we get all positions where A can at Level 1.
In[6]:=
Position[{1/(1+A), 1/A, A}, A, 1]
Out[6]=
{{3}}
If we specify levelspec={2} we get all positions where A can be found at
level 2.
In[7]:=
Position[{1/(1+A), 1/A, A}, A, {2}]
Out[7]=
{{2,1}}
Ted Ersek
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