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Re: Inconsistencies in pattern matching.

  • To: mathgroup at
  • Subject: [mg12985] Re: Inconsistencies in pattern matching.
  • From: Tobias Oed <tobias at>
  • Date: Sun, 28 Jun 1998 02:52:08 -0400
  • Organization: Old Dominion University
  • References: <6mqf4r$>
  • Sender: owner-wri-mathgroup at

Sean Ross wrote:

> This is an example taken from Ken Wagners Power Programming book:
> In[1]:={{x1,y1},{x2,y2},{x3,y3}}/.{x_,y_}->{x,Log[y]}
> Out[1]:={{x1,Log[y1]},{x2,Log[y2]},{x3,Log[y3]}}
> In[2]:={{x1,y1}}/.{x_,y_}->{x,Log[y]}
> Out[2]:={{x1,Log[y1]}}
> As long as the list of {x,y} data points has one point or greater than
> two points, it transforms as one would expect.  If there are  two {x,y}
> data points, it transforms differently.
> In[3]:={{x1,y1},{x2,y2}}/.{x_,y_}->{x,Log[y]}
> Out[3]:={{x1,y1},{Log[x2],Log[y2]}}
> Now, if x_ is seen to match {x1,y1} in example 3, then why doesn't it
> also match it in example number 1 and 2?  This behavior can be fixed
> with a /;Head[x]=!=List  pattern restricting rule, but that is not the
> point.  This seems grossly inconsistent to me.  Can anyone explain why
> it does this and/or justify that this is a good thing?

  In the last example the pattern x_ matches is {x1,y1} and y_ matches
(x_ matches any mathematica expression) so the result is consistent.
Since Log
is Listable Log[{x2,xy}] gives {Log[x2],Log[y2]}. The following works:

In[12]= {{x1,y1},{x2,y2}}/.{x_?AtomQ,y_?AtomQ}->{x,Log[y]} Out[12]=
{{x1, Log[y1]}, {x2, Log[y2]}}

Hope this helps Tobias

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