
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. > >

