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MathGroup Archive 1997

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Bug in pattern matching?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg8722] Bug in pattern matching?
  • From: "Peter Str÷mbeck" <pqst at celsiustech.se>
  • Date: Fri, 19 Sep 1997 02:48:02 -0400
  • Organization: CelsiusTech Electronics AB
  • Sender: owner-wri-mathgroup at wolfram.com

Hi, I've posted this problem to Wolfram support but received no answer so I
thought I might just post it here aswell.

I'm trying to "deconstruct" a list, perform an operation to a pert of the
list and then recombine the result. When I did this I found out that
multiplication in pattern matching performs differently than otherwise.

Check version:

In[1]:=
$Version
Out[1]=
"Microsoft Windows 3.0 (October 6, 1996)"

Define list:

In[2]:=
ls=Table[a[j], {j,0,5}]
Out[2]=
{a[0],a[1],a[2],a[3],a[4],a[5]}

I want to split this list into the following three sublists: 

{a[0],a[1],a[2],a[3],a[4],a[5]} -> {a[0]}, {a[1], a[2], a[3], a[4]}, {a[5]}

Use pattern matching:

In[3]:=
ls/.{first_, middle___, last_}\[Rule] {{first},{middle},{last}}
Out[3]=
{{a[0]},{a[1],a[2],a[3],a[4]},{a[5]}}

This works fine. Now I want to multiply the middle list by a factor C.
Ordinary multiplication works like this:

In[4]:=
C ls
Out[4]=
{C a[0],C a[1],C a[2],C a[3],C a[4],C a[5]}

Doing this in a pattern matching statement produces a totally different
result:

In[5]:=
ls/.{first_, middle___, last_}\[Rule] {{first},C {middle},{last}}
Out[5]=
{{a[0]},{C a[1] a[2] a[3] a[4]},{a[5]}}

The middle list now contains a completely different result than Out[4]!

Can anyone please explain this to me?

/Peter Strömbeck




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