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Re: replace by value in expr

• To: mathgroup at yoda.ncsa.uiuc.edu (Mathematica)
• Subject: Re: replace by value in expr
• From: jacobson at cello.hpl.hp.com
• Date: Tue, 18 Dec 90 11:35:57 PST

Charles Herrick writes questioning the operation of substitution.  The
problem is that whenever x is defined *all* the instances of x,
incuding those on the left of "->" get replaced by the value of x.
I've annotated his list by what (I think) is happening at the time
substitution takes place

In[n]:= x = .;
In[n+1]:= 1 + x^2 /. x -> 1+a		(1+x^2)/.x->(1+a)
Out[n+1]:= 1 + (1 + a)^2
In[n+2]:= x = 3;
In[n+3]:= 1 + x^2 /. x -> 1+a		10/.3->(1+a)
Out[n+3]:= 10
In[n+4]:= x = 5;
In[n+5]:= 5 + x /. x -> 3		10/.(5->3)
Out[n+5]:= 10
In[n+6]:= 5 + (x /. x -> 3)		5 + (5/.5->3)
Out[n+6]:= 8
In[n+7]:= x = 3;
In[n+8]:= 1 + (x^2 /. x -> 1+a)		1+(9/.3->1+a)
Out[n+8]:= 10

So, its a feature.  If you want the x on the left of "->" to not be
replaced by the value of x *before substition* use ":>" instead.
(Actually an x on the right won't get evaluated either, until after
the substition.)

  -- David Jacobson