Re: Curiosity concerning transformation rules for List

*To*: mathgroup at smc.vnet.net*Subject*: [mg70899] Re: [mg70860] Curiosity concerning transformation rules for List*From*: János <janos.lobb at yale.edu>*Date*: Wed, 1 Nov 2006 03:55:11 -0500 (EST)*References*: <200610301032.FAA13307@smc.vnet.net>

On Oct 30, 2006, at 5:32 AM, Andrew Moylan wrote: > Since the List symbol is locked, I am curious about the possibility > (or > otherwise) of giving definitions for which the left-hand-side of the > transformation rule contains only the List symbol. Here's an > arbitrary, > explicit example: > > Is it possible to make a definition such that: any list of two > identical elements evaluates to the empty list? E.g. {x_, x_} -> {}. > > I can't see any way this transformation rule can be added. It's not > possible to modify the DownValues for List; and there are no > first-level symbols to which an UpValue can be added. Does anyone have > any ideas? Here is a newbie approach: In[11]:= lst = {{2, 2}, {1, 5}, {5, 6}, {8, 3}, {1, 5}, {5, 5}} Out[11]= {{2, 2}, {1, 5}, {5, 6}, {8, 3}, {1, 5}, {5, 5}} In[12]:= lst /. {x_, x_} -> {} Out[12]= {{}, {1, 5}, {5, 6}, {8, 3}, {1, 5}, {}} Unless I misunderstood... János ---------------------------------------------- Trying to argue with a politician is like lifting up the head of a corpse. (S. Lem: His Master Voice)