Re: Sum pattern

*To*: mathgroup at smc.vnet.net*Subject*: [mg128303] Re: Sum pattern*From*: "Dave Snead" <dsnead6 at charter.net>*Date*: Sat, 6 Oct 2012 01:47:31 -0400 (EDT)*Delivered-to*: l-mathgroup@mail-archive0.wolfram.com*Delivered-to*: l-mathgroup@wolfram.com*Delivered-to*: mathgroup-newout@smc.vnet.net*Delivered-to*: mathgroup-newsend@smc.vnet.net*References*: <20121005064827.40BDA684C@smc.vnet.net>

Thanks everyone, I've concluded that the shortest way to do this (in terms of the number of characters I need to type) which works is: In[1]:= x = f[a1, s] + f[a2, s] + f[a3, s] Out[1]= f[a1, s] + f[a2, s] + f[a3, s] In[3]:= z = x /. (p : Plus[_f, __f]) :> f[First /@ p, s] Out[3]= f[a1 + a2 + a3, s] -- Dave Snead -----Original Message----- From: Fred Simons Sent: Thursday, October 04, 2012 11:48 PM To: mathgroup at smc.vnet.net Subject: [mg128303] Re: Sum pattern On Oct 3, 12:12 am, "Dave Snead"<dsne... at charter.net> wrote: > Hi, > > I'm trying to put together a rule whose left hand side is a sum of > arbitrary > length whose elements all have the same head f. > > For example: > > In[4]:= x = f[a1, s] + f[a2, s] + f[a3, s] > > Out[4]= f[a1, s] + f[a2, s] + f[a3, s] > > In[6]:= y = f[First /@ x, s] > > Out[6]= f[a1 + a2 + a3, s] > > which is what I want. > > However when I turn this into a rule > > In[7]:= z = x /. (p : Plus[__f]) -> f[First /@ p, s] > > Out[7]= f[f[a1, s], s] + f[f[a2, s], s] + f[f[a3, s], s] > > Why isn't z equal to y? > How can I make this rule work? > > Thanks in advance, > Dave Snead Your solution is almost correct, but fails since you did not take into account that both sides of your rule are evaluated before it is applied. In[1]:= (p:Plus[__f])->f[First/@p,s] Out[1]= p__f->f[p,s] Your rule wraps every expression with head f in another f, as you see in your output. To prevent the evaluation, use RuleDelayed for keeping the right-hand side and HoldPattern for the left-hand side: In[2]:= x=f[a1,s]+f[a2,s]+f[a3,s] x /. (p:HoldPattern[Plus[__f]]):>f[First/@p,s] Out[2]= f[a1,s]+f[a2,s]+f[a3,s] Out[3]= f[a1+a2+a3,s] Fred Simons Eindhoven University of Technology

**References**:**Re: Sum pattern***From:*Fred Simons <f.h.simons@tue.nl>