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MathGroup Archive 1993

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Re: Error in Mathematica?

  • To: mathgroup at yoda.physics.unc.edu
  • Subject: Re: Error in Mathematica?
  • From: jacobson at cello.hpl.hp.com
  • Date: Mon, 08 Nov 93 08:22:53 -0800

J Harris observes that

In[1]:= x=b
  Out[1]= b
  In[2]:= x^2
  Out[2]= b^2  (* as I would expect)
  In[3]:= x^2/.x->a
  Out[3]= a^2  
  In[4]:= x=2
  Out[4]= 2
  In[5]:= x^2/.x->7  
  Out[5]= 4   

What is happening is that the left side of the Rule operator (->) 
gets evaluated, so by the time Rule gets to work on it here is what it
is seeing


In[1]:= x=b
  Out[1]= b
  In[2]:= x^2				
  Out[2]= b^2  (* as I would expect)
  In[3]:= x^2/.x->a			b^2 /. b->a
  Out[3]= a^2  
  In[4]:= x=2				
  Out[4]= 2
  In[5]:= x^2/.x->7  			4 /. 2->7
  Out[5]= 4   

In item 3, the b gets replaced by a.  But in item 5, 2 does not match
4, so no replacement is done.

You can get the behavior you expect in item 3 by using Literal, as
follows


In[1]:= x=b

Out[1]= b

In[2]:= x^2 /. Literal[x]->a

         2
Out[2]= b

  -- David Jacobson





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