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MathGroup Archive 2002

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Re: How To Change A Rule

  • To: mathgroup at smc.vnet.net
  • Subject: [mg32350] Re: How To Change A Rule
  • From: Detlef Mueller <dmueller at mathematik.uni-kassel.de>
  • Date: Wed, 16 Jan 2002 03:30:01 -0500 (EST)
  • Organization: University of Kassel - Germany
  • References: <a20mon$4hf$1@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

Ariel Fligler wrote:
> 
> Greetings,
> 
> I'm quite newby to Mathematica programming and will thank you if you could
> help me solve the following:
> 
> I have a list of rules of the form:
> {s1->-11, s2->12}
> And would like to change every rule in the list that it's RHS is negative to
> 0 so that I will get:
> {s1->0, s2->12}
> I tried to find an access function to manipulate rules but could not.
> 

You can apply rules on a list of rules:

{s1->-11, s2->12}/.{x_/;x<0->0}

does the Job 
(x_ stands for any Pattern and names it with "x", 
the condition /;x<0 ensures, that only the negative
RHS are replaced).

{s1 -> -11, s2 -> 12, s3 -> -PI, s4 -> 3} /. {x_ /; x < 0 -> 0}
leads to
{s1 -> 0, s2 -> 12, s3 -> 0, s4 -> 3} as desired.

But
{-5 -> u, s1 -> -11, s2 -> 12, s3 -> -PI, s4 -> 3} 
/. {x_ /; x < 0 -> 0}
yields:
{0 -> u, s1 -> 0, s2 -> 12, s3 -> 0, s4 -> 3}

So we specify the pattern more detailed:
{-5 -> u, s1 -> -11, s2 -> 12, s3 -> -PI, s4 -> 3} 
/.{(x_ -> y_ /; y < 0) -> (x -> 0)}

resulting in
{-5 -> u, s1 -> 0, s2 -> 12, s3 -> -PI, s4 -> 3}

Why -PI isn't changed here, but in the first
version? 

I don't know.

Greetings
  Detlef


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