Re: conditionnal rule

• To: mathgroup at smc.vnet.net
• Subject: [mg51515] Re: [mg51499] conditionnal rule
• From: Andrzej Kozlowski <akoz at mimuw.edu.pl>
• Date: Thu, 21 Oct 2004 22:20:18 -0400 (EDT)
• References: <200410200521.BAA08454@smc.vnet.net>
• Sender: owner-wri-mathgroup at wolfram.com

```On 20 Oct 2004, at 14:21, Wishmaster7 wrote:

>
> hi all !!
>
> I want to apply a rule under a certain condition. here is the example :
>
> myList = {{a, b}, {c, d}};
>
> myList //. {a,x_}->{a,d}
>
> apply this rule if Length[x]===1 (this means that x can not be a list)
>
> I tried something like :
>
> myList //. {a_,x_}->{c,d}/;Length[x]===1
>
> or
>
> myList //. {a_,x_?Length[x]===1}->{c,d}/
>
> but it does not work.
>
> can someone help me ?
>
>
Surely, you must have meant Length[x] == 0 not Length[x] == 1 ?

You do not need to use ReplaceRepeated here but you do need to use

myList = {{a, b}, {c, d}};

myList /. {a_, x_} :> {c, d} /; Length[x] == 0

{{c, d}, {c, d}}

Andrzej Kozlowski
Chiba, Japan
http://www.akikoz.net/~andrzej/
http://www.mimuw.edu.pl/~akoz/

```

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