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Re: How to use Rules on Sequences?

  • To: mathgroup at
  • Subject: [mg29478] Re: How to use Rules on Sequences?
  • From: "Orestis Vantzos" <atelesforos at>
  • Date: Thu, 21 Jun 2001 01:56:52 -0400 (EDT)
  • Organization: National Technical University of Athens, Greece
  • References: <9gpo0n$hd8$>
  • Sender: owner-wri-mathgroup at

Say you want to replace the pattern x_y_ with x+y...then you use the rule
{L___,x_y_,R___}:>{L, x+y, R}probably repeatedly:
myList//.{L___,x_y_,R___}:>{L, x+y, R}
I am not sure if this the most efficient method, but it is how you would do
it with patterns.

"Detlef Mueller" <dmueller at> wrote in message
news:9gpo0n$hd8$1 at
> Hello,
> I am faced to the following Problem:
> given a List of Pairs of the Form
> L = {{a1,m1},{a2,m2}, ... ,{ai,mi}}
> and a Number a, I want to replace the Pair
> {a, mj}, if it appears in the List by
> {a, mj-1}.
> This Job is done by
> L /. {{a,_m}->{a,m-1}}.
> Now I want to remove an Item of the Form
> {a,0} entirely from The List.
> Can this be done by applying rules (without
> leaving Null-Entrys), or is "Select" the only
> Way?
> (By the way: L is sorted with respect to the first Elements
> of the Sublists, this first elements are unic identifiers.
> Another Question is to replace an {a,_m} in L by {a,m+1},
> if it exists or else insert {a,1} in the propper
> Place - the inverse Operation of the above desribed
> one. Suerly not difficult to realize - but what might
> be the most efficient Method?)
> More general, I wonder, how to replace
> a Pattern in a Sequence - for example to change
> every occurence of "x_, y_?positive, x_" in a Sequence
> by "x,0,x".
> Say
> Emil[1,2,3,2,0,a,b,c]/.R ---> (Replacing ... 2,3,2 ... )
> Emil[1,2,0,2,0,a,b,c]/.R ---> (Replacing ... 0,2,0 ... )
> Emil[1,2,0,0,0,a,b,c]/.R      (No more changes).
> I Fail to write down a syntactically correct rule R to
> do this Job.
> Greetings
>   Detlef Mueller

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