Re: Bug in pattern matching?

*To*: mathgroup at smc.vnet.net*Subject*: [mg8739] Re: [mg8722] Bug in pattern matching?*From*: "C. Woll" <carlw at u.washington.edu>*Date*: Sat, 20 Sep 1997 22:28:06 -0400*Sender*: owner-wri-mathgroup at wolfram.com

Hi Peter, The problem you are having is due to using Rule instead of RuleDelayed. Try ls /. {f_,m___,l_}:>{{f},c {m},{l}} instead. If you use Rule, Mma first converts the rule from {f_,m___,l_}->{{f},c {m},{l}} to {f_,m___,l_}->{{f},{c m},{l}} and then applies this rule, which produces your unexpected behavior. Carl Woll Dept of Physics U of Washington On Fri, 19 Sep 1997, Peter Str=F6mbeck wrote: > Hi, I've posted this problem to Wolfram support but received no answer so= I > thought I might just post it here aswell. >=20 > 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. >=20 > Check version: >=20 > In[1]:=3D > $Version > Out[1]=3D > "Microsoft Windows 3.0 (October 6, 1996)" >=20 > Define list: >=20 > In[2]:=3D > ls=3DTable[a[j], {j,0,5}] > Out[2]=3D > {a[0],a[1],a[2],a[3],a[4],a[5]} >=20 > I want to split this list into the following three sublists:=20 >=20 > {a[0],a[1],a[2],a[3],a[4],a[5]} -> {a[0]}, {a[1], a[2], a[3], a[4]}, {a[5= ]} >=20 > Use pattern matching: >=20 > In[3]:=3D > ls/.{first_, middle___, last_}\[Rule] {{first},{middle},{last}} > Out[3]=3D > {{a[0]},{a[1],a[2],a[3],a[4]},{a[5]}} >=20 > This works fine. Now I want to multiply the middle list by a factor C. > Ordinary multiplication works like this: >=20 > In[4]:=3D > C ls > Out[4]=3D > {C a[0],C a[1],C a[2],C a[3],C a[4],C a[5]} >=20 > Doing this in a pattern matching statement produces a totally different > result: >=20 > In[5]:=3D > ls/.{first_, middle___, last_}\[Rule] {{first},C {middle},{last}} > Out[5]=3D > {{a[0]},{C a[1] a[2] a[3] a[4]},{a[5]}} >=20 > The middle list now contains a completely different result than Out[4]! >=20 > Can anyone please explain this to me? >=20 > /Peter Str=F6mbeck >=20 >=20 >=20 >=20