RE: perplexed by blank sequence in pattern replacement

*To*: mathgroup at smc.vnet.net*Subject*: [mg68694] RE: [mg68674] perplexed by blank sequence in pattern replacement*From*: "David Park" <djmp at earthlink.net>*Date*: Thu, 17 Aug 2006 04:17:52 -0400 (EDT)*Sender*: owner-wri-mathgroup at wolfram.com

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