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Transformation rules - explain please

• To: mathgroup at smc.vnet.net
• Subject: [mg74023] Transformation rules - explain please
• From: "wooks" <wookiz at hotmail.com>
• Date: Tue, 6 Mar 2007 05:32:02 -0500 (EST)

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.

• Follow-Ups:
  • Re: Transformation rules - explain please
    • From: Andrzej Kozlowski <akoz@mimuw.edu.pl>
  • Re: Transformation rules - explain please
    • From: "Chris Chiasson" <chris@chiasson.name>