Re: Pattern Matching

*To*: mathgroup at smc.vnet.net*Subject*: [mg23854] Re: Pattern Matching*From*: "Allan Hayes" <hay at haystack.demon.co.uk>*Date*: Mon, 12 Jun 2000 01:17:46 -0400 (EDT)*References*: <8hsu3g$dh4@smc.vnet.net>*Sender*: owner-wri-mathgroup at wolfram.com

Johannes, Re your first question. We get x[1] b[1] + x[2] b[2] /. Plus[Times[x[_], b[_]] ..] -> z b[1] x[1] + b[2] x[2] possibly because, before the matching and replacement, the left side of the rule is evaluated thus: Plus[Times[x[_], b[_]] ..] b[_] x[_] .. So we end up with x[1] b[1] + x[2] b[2] /. (*1*) (x[_] b[_]) .. -> z b[1] x[1] + b[2] x[2] We can avoid this by using HoldPattern: x[1] b[1] + x[2] b[2] /. HoldPattern[Plus[Times[x[_], b[_]] ..]] -> z z However it does seem odd that, in spite of (*1*), we get x * b[1] + x* b[2] /. (x* b[_]) .. -> z 2 z -- 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 "Johannes Ludsteck" <ludsteck at zew.de> wrote in message news:8hsu3g$dh4 at smc.vnet.net... > Dear Group Members, > I would like to "find" and replace expressions with the simple > structure x[1] b[1]+x[2] b2]+...+x[n] b[n] > I tried to use the following replacement rule > In[27]:= x[1] b[1] + x[2] b[2] /. Plus[Times[x[_], b[_]] ..] -> z > > Out[27]= b[1] x[1] + b[2] x[2] + b[3] x[3] > Which didn't work (Out[27] should be z). > Why? > The following FullForm seems to give exactly the structure I used > in my replacement rule. > > In[17]:= > FullForm[x[1] b[1] + x[2] b[2] + x[3] b[3]] > Out[17]//FullForm= > Plus[Times[b[1], x[1]], Times[b[2], x[2]], Times[b[3], x[3]]] > > Even if this worked, my pattern wouldn't account for equal indices, > i.e. it would match x[1] b[500]+x[12] b[3], even if it shouldn't. > > Any suggestions? > Thanks, > Johannes Ludsteck > > > Johannes Ludsteck > Centre for European Economic Research (ZEW) > Department of Labour Economics, > Human Resources and Social Policy > Phone (+49)(0)621/1235-157 > Fax (+49)(0)621/1235-225 > > P.O.Box 103443 > D-68034 Mannheim > GERMANY > > Email: ludsteck at zew.de >