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Re: (2/3)[[1]]

  • To: mathgroup at smc.vnet.net
  • Subject: [mg96402] Re: (2/3)[[1]]
  • From: Jean-Marc Gulliet <jeanmarc.gulliet at gmail.com>
  • Date: Fri, 13 Feb 2009 03:40:05 -0500 (EST)
  • Organization: The Open University, Milton Keynes, UK
  • References: <gn11e8$7vn$1@smc.vnet.net>

In article <gn11e8$7vn$1 at smc.vnet.net>, obott0 at gmail.com wrote:

> I'm not sure if this has been asked before, but why can't Part be used
> with Rational?
> 
> i.e. (2/3)[[1]]

Because even if rational numbers look like non-atomic expressions (see, 
for instance, their full or tree forms), they are deemed by Mathematica 
as *atomic* expressions (test with *AtomQ[]*). 

In[1]:= (2/3)[[1]]

During evaluation of In[1]:= Part::partd: Part specification (2/3)[[1]] 
is longer than depth of object. >>

Out[1]= 
 2
(-)[[1]]
 3

In[2]:= FullForm[2/3]

Out[2]//FullForm= Rational[2, 3]

In[3]:= %[[1]]

During evaluation of In[3]:= Part::partd: Part specification (2/3)[[1]] 
is longer than depth of object. >>

Out[3]= 
 2
(-)[[1]]
 3

In[4]:= TreeForm[2/3]

Out[4]//TreeForm= -Graphics-

In[5]:= AtomQ[2/3]

Out[5]= True

Regards,
--Jean-Marc


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