Re: Why doesn't this rule work?
- To: mathgroup at smc.vnet.net
- Subject: [mg32478] Re: Why doesn't this rule work?
- From: "Allan Hayes" <hay at haystack.demon.co.uk>
- Date: Wed, 23 Jan 2002 00:59:52 -0500 (EST)
- References: <email@example.com>
- Sender: owner-wri-mathgroup at wolfram.com
Andrzej, I agree that RuleCondition is very useful, and I have queried why it is not a regular function with full documentation.. However, Condition works as well in this case: c - a - b /. (n1_.)*c + (n2_.)*a :> n1*b /; n1 == -n2 0 The difference between Condition and RuleCondition shows up below: Condition replaces with 1+1; RuleCondition replaces with its value, 2. Hold[a] /. a :> Condition[1 + 1,True] Hold[1+1] Hold[a] /. a :> RuleCondition[1 + 1, True] Hold -- Allan --------------------- Allan Hayes Mathematica Training and Consulting Leicester UK www.haystack.demon.co.uk hay at haystack.demon.co.uk Voice: +44 (0)116 271 4198 Fax: +44 (0)870 164 0565 "Andrzej Kozlowski" <andrzej at tuins.ac.jp> wrote in message news:a2j7ra$717$1 at smc.vnet.net... > I think the best way is to use RuleCondition instead of Rule and > Condition as you did. (Unfortunately RuleCondition, one of the most > useful Mathematica functions in this sort of situations, is > undocumented...) > > In:= > c-a-b/.n1_. c+n2_. a:>RuleCondition[n1 b,n1==-n2] > > Out= > 0 > > Andrzej Kozlowski > Toyama International University > JAPAN > http://platon.c.u-tokyo.ac.jp/andrzej/ > > > > On Monday, January 21, 2002, at 04:54 PM, James Jennings wrote: > > > Suppose I want to simplify expressions linear in a, b, and c where a+b=c > > -- something of this sort. > > > > In:= a+b /. a+b -> c > > > > Out= c > > > > In:= a+b+c /. a+b -> c > > > > Out= 2 c > > > > With more complicated coefficients I can use: > > > > In:= -2 a - 2 b /. n_. a+n_. b -> n c > > > > Out= -2 c > > > > In:= -a-b+c /. n_. a+n_. b -> n c > > > > Out= 0 > > > > I also want to apply rules based on c-a=b > > > > In:= c-a /. n1_. c+n2_. a /; n1==-n2 -> n1 b > > > > Out= b > > > > The problem comes with I apply the above rule to longer expressions. > > > > In:= c-a-b /. n1_. c+n2_. a /; n1==-n2 -> n1 b > > > > Out= -a-b+c > > > > Why didn't that work? I had thought that since Plus is Orderless, my > > rule ought to be applied to all pieces of my expression, but it doesn't > > appear to be. > > > > --- > > > > Before someone suggests that I should just set c = a+b and be done with > > it, my actual problem involved 12 objects like a, b, and c, where 16 > > distinct triplets add up like a+b=c. I'm looking for rules that will > > keep my expressions as short as possible, even if they aren't unique. > > > > Thanks. > > James > > > > > > > >