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Re: perplexed by blank sequence in pattern replacement


On 16 Aug 2006, at 09:36, Blake wrote:

> Dear MathGroup:
>
> I have been blithely using blank sequences in pattern matching for  
> some
> time. In persuit of a bug in a package of mine, I was quite alarmed to
> find that I don't really understand how to use the blank sequence, as
> expressed in the following simplified example:
>
> In[1]:=Replace[a*b*c,Times[mysequence__]:>{mysequence}]
>
> Out[1]={a b c}
>
> I expected Out[1]={a,b,c}, from a naieve reading of the full form of
> a*b*c
>
> In[2]:=FullForm[a*b*c]
>
> Out[2]//FullForm=Times[a,b,c]
>
> Will someone PLEASE tell me why In[1] does not yield the results I
> expected? (I can readily use a work-around, what I am concerned  
> with is
> a correct understanding of pattern matching).
>
> Blake Laing
> thesis slave
> University of Oklahoma
>

Just observe that:

Times[mysequence__]


mysequence__

hence the correct thing to do is:


Replace[Times[a,b,c],HoldPattern[Times[mysequence__]]:>{mysequence}]


{a,b,c}

Andrzej Kozlowski


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