Re: Replacement Rule

*To*: mathgroup at smc.vnet.net*Subject*: [mg32074] Re: [mg32064] Replacement Rule*From*: Ken Levasseur <Kenneth_Levasseur at uml.edu>*Date*: Sat, 22 Dec 2001 04:22:46 -0500 (EST)*References*: <200112210857.DAA25218@smc.vnet.net>*Sender*: owner-wri-mathgroup at wolfram.com

Brian: Pattern matching works from the top and proceeds downward. Since the whole expression {{3, 4}, {5, 6}} matches {x_,y_}, you're getting the first pair, {3, 4}. One way to fix this if you are working only with numbers is to use RuleDelayed: {x_,y_}:>x/;NumberQ[x] Ken Levasseur Math Sciences UMass Lowell Brian Higgins wrote: > I have the following list > > l1={{1,2},{3,4},{5,6}}; > > and then I operate on it with the following pattern/replacement rule > > In[3]:= l1 /. {x_, y_} -> x > > Out[3]= {1, 3, 5} > > This is what I would expect. Now consider the following list > > In[4]:= l2 = {{3, 4}, {5, 6}}; > > Then I use the same pattern/replacement rule > > In[5]:=l2 /. {x_, y_} -> x > > Out[5]= {3, 4} > > I was hoping to get > > {3,5} > > What am I missing? The FullForm of l2 is basically the same structure > as l1, as far as I can tell.... > > Brian

**References**:**Replacement Rule***From:*bghiggins@ucdavis.edu (Brian Higgins)