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MathGroup Archive 2005

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Re: rules and lists

  • To: mathgroup at smc.vnet.net
  • Subject: [mg54246] Re: rules and lists
  • From: "Wolf, Hartmut" <Hartmut.Wolf at t-systems.com>
  • Date: Mon, 14 Feb 2005 21:50:38 -0500 (EST)
  • Sender: owner-wri-mathgroup at wolfram.com

>-----Original Message-----
>From: Erich Neuwirth [mailto:erich.neuwirth at univie.ac.at] 
To: mathgroup at smc.vnet.net
>Sent: Monday, February 14, 2005 4:17 AM
>To: mathgroup at smc.vnet.net
>Subject: [mg54246]  rules and lists
>
>I would like to replace sublists of a list to be replaced by 
>another list, such that for example the rule {1,2}->{1,2,3} 
>wout tranform {1,2,3,3,1,2} into {1,2,3,3,3,1,2,3} Simply 
>gibbin the transformation rule in the form 
>{x___,1,2,y___}->{x,1,2,3,y} does not work.
>What is the idiom to do what I want?
>
>

Eric,

In[1]:=
{1, 2, 3, 3, 1, 2} /. {x___, 1, 2, y___} -> {x, 1, 2, 3, y}
Out[1]=
{1, 2, 3, 3, 3, 1, 2}

doesn't do, because ReplaceAll just stops at the first match and
replacement.


ReplaceRepeated doesn't help in your case, because the replacement
contains the search pattern, and thus will loop indefinitely:

In[2]:=
{1, 2, 3, 3, 1, 2} //. {x___, 1, 2, y___} -> {x, 1, 2, 3, y}
Out[2]=
$Aborted


To avoid that, make your replacements in two steps, first replace the
searched sequence 1,2 by a symbol

In[3]:=
{1, 2, 3, 3, 1, 2} //. {x___, 1, 2, y___} -> {x, $, y}
Out[3]=
{$, 3, 3, $}

and then replace that symbol by the substitution sequence 1,2,3:


In[5]:=
{1, 2, 3, 3, 1, 2} //. {x___, 1, 2, y___} -> {x, $, y} //. 
     {x___, $, y___} -> {x, 1, 2, 3, y}
Out[5]=
{1, 2, 3, 3, 3, 1, 2, 3}


--
Hartmut Wolf


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