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RE: perplexed by blank sequence in pattern replacement


Blake,

It is because the rule is evaluated on the lhs so it becomes:

Times[mysequence__] -> {mysequence}
mysequence__ -> {mysequence}

mysequence__ matches a*b*c so a*b*c is put into brackets to give {a*b*c}.

This is not an uncommon problem and the solution is to use HoldPattern.

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

You could also use

a b c /. HoldPattern[Times[mysequence__]] -> {mysequence}
{a, b, c}

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

From: Blake [mailto:laing at ou.edu]
To: mathgroup at smc.vnet.net


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



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