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MathGroup Archive 2007

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Re: Transformation rules - explain please

  • To: mathgroup at smc.vnet.net
  • Subject: [mg74047] Re: [mg74023] Transformation rules - explain please
  • From: bsyehuda at gmail.com
  • Date: Wed, 7 Mar 2007 03:15:48 -0500 (EST)

Hi,
In the second example you used = (which stand for Set) in place of == (which
stands for Equal)
after fixing that the code works just fine

lat = {banana, ice, cream, smeared, chocolate, topping};
MySubst[new_, old1_, old2_, {x___, old_, r___} /; Or[old == old1, old
==  old2]] := {x, new, r};
MySubst[strawberry, chocolate, cream, lat]

regards
yehuda

On 3/6/07, wooks <wookiz at hotmail.com> wrote:
>
> This is supposed to substitute the first occurrence of old1 or old2
> with new.
>
> Why does this work
>
> Clear[old1, old, old2, lat, MySubst]
> lat = {banana, ice, cream, smeared, chocolate, topping};
> MySubst[new_, old1_, old2_, {x___, old_, r___}] := {x, new, r} /;
> Or[old == old1, old == old2];
> MySubst[strawberry, chocolate, cream, lat]
>
> but not this (with the conditional on the lhs).
>
> Clear[old1, old, old2, lat, MySubst]
> lat = {banana, ice, cream, smeared, chocolate, topping};
> MySubst[new_, old1_, old2_, {x___, old_, r___} /; Or[old = old1, old
> =  old2]] := {x, new, r};
> MySubst[strawberry, chocolate, cream, lat]
>
> I like the idea of using conditional rules like this but have found it
> very hit and miss. Often times things don't work so I am seeking
> further enlightment.
>
> I'd be interested to see what would need to be done to make the
> conditional work on the lhs.
>
>
>


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