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Topic: Pattern Matching with ReplaceRepated Behaving Unexpectedly
Replies: 1   Last Post: Dec 5, 2012 3:10 AM

 Bob Hanlon Posts: 906 Registered: 10/29/11
Re: Pattern Matching with ReplaceRepated Behaving Unexpectedly
Posted: Dec 5, 2012 3:10 AM

The second rule is never seen since the first result matches the first
rule. Note:

{u, a, a, v} //. {
{p___, x : Longest[(a_) ..], q___} :>
(Print["Rule 1 applied"];
{p, a^Length[{x}], q}),
{x___, a^2, y___} :>
(Print["Rule 2 applied"];
{x, e, y})}

Rule 1 applied

Rule 1 applied

{u, a^2, v}

MatchQ[%, {p___, x : Longest[(a_) ..], q___}]

True

You can just reverse the order of the rules

{u, a, a, v} //. {
{x___, a^2, y___} :>
(Print["Rule 1 applied"];
{x, e, y}),
{p___, x : Longest[(a_) ..], q___} :>
(Print["Rule 2 applied"];
{p, a^Length[{x}], q})}

Rule 2 applied

Rule 1 applied

Rule 2 applied

{u, e, v}

Bob Hanlon

On Tue, Dec 4, 2012 at 4:13 AM, Brentt <brenttnewman@gmail.com> wrote:
>
> I'm having trouble understanding how replace repeated works.
>
> I have this expression:
>
> In[1]:= {u,a, a,v} //. {
> {p___, x : Longest[(a_) ..], q___} :> {p, a^Length[{x}], q},
> {x___, a^2, y___} :> {x, e, y}
> }
>
>
> What I think should happen here is that {u,a,a,v} should become {u,a^2,v}
> and then, since ReplaceRepeated should make another pass, {u,a^2,v} should
> become {u,e,v}. And actually if I break up the replacement rules, and
> evaluate them one by one, this is precisely what happens. But when I try to
> put the replacement rules into a list of rules with replace repeated as
> above it just does the first rule and stops. I.e. it outputs
>
> Out[1]:= {u,a^2,v}
>
> Strangely if I do
>
> In[2]:= % //. {
> {p___, x : Longest[(a_) ..], q___} :> {p, a^Length[{x}], q},
> {x___, a^2, y___} :> {x, e, y}
> }
>
> I get the result I expected ReplaceRepeated to give me before
>
> Out[2]:= {u,e,v}
>
>
> I actually thought Replace Repated exists for this very reason, to keep on
> applying the rules until the rules do not change the output any longer, but
> it is failing to do that in this case for some reason even though there
> still seems to be patterns that should be changed?
>
> Can anyone tell me what I am missing here?
>
> Thank you, any help will be greatly appreciated.
>
>