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Re: Bug in pattern parsing?

• To: mathgroup at smc.vnet.net
• Subject: [mg62014] Re: [mg61996] Bug in pattern parsing?
• From: "David Park" <djmp at earthlink.net>
• Date: Wed, 9 Nov 2005 03:45:28 -0500 (EST)
• Sender: owner-wri-mathgroup at wolfram.com

Kristjan,

Look at the FullForm of each of the rules and you will see the problem.

It is not c_. that is causing the problem, but the X_. that is taken as
ANOTHER optional pattern. Rule1 combines all three factors into a single dot
product. Leave a space before the dot.

rule3 = c_. X_ .Y_ -> X.Y

a.b /. rule3
a.b

David Park
djmp at earthlink.net
http://home.earthlink.net/~djmp/

From: Kristjan Kannike [mailto:kkannike at physic.ut.ee]
To: mathgroup at smc.vnet.net

Hello,

I may have discovered a bug in pattern parsing or applying transformation
rules.

The following two rules should be equivalent:

rule1 = c_.*X_.Y_ -> X.Y

with c an optional variable, and

rule2 = c_.*Dot[X_, Y_] -> X.Y

Yet applying rule1 on a.b as

a.b/.rule1

gives

1.a.b

(the same result obtains for the optional factor actually present as in
const a.b), but

a.b/.rule2

gives

a.b

as it should.

I think that it has to do with the dot in c_., but curiously I get the
same result when writing the LHS of rule1 as Optional[c]*Y.Z and that
confuses me...

Any thoughts?

Kristjan Kannike
<http://www.physic.ut.ee/~kkannike/english/>