RE: Replacement Rule

*To*: mathgroup at smc.vnet.net*Subject*: [mg32084] RE: [mg32064] Replacement Rule*From*: "David Park" <djmp at earthlink.net>*Date*: Sat, 22 Dec 2001 04:23:02 -0500 (EST)*Sender*: owner-wri-mathgroup at wolfram.com

Brian, The second example is certainly correct because the rule picks the first of two items in a list of two lists. If the lists you are dealing with always contains numbers, you could use a more specific rule. l1 = {{1, 2}, {3, 4}, {5, 6}}; l2 = {{3, 4}, {5, 6}}; rule = {(x_)?NumberQ, (y_)?NumberQ} -> x; l1 /. rule l2 /. rule {1, 3, 5} {3, 5} If your lists contain expressions or symbols you could use a more general set of rules which handles the special case first. rules = {{{a_, b_}, {c_, d_}} -> {a, c}, {a_, b_} -> a} l1 /. rules l2 /. rules {1, 3, 5} {3, 5} {{a1, Sin[x]}, {b1, Cos[x]}, {c1, a + b}} /. rules {a1, b1, c1} {{a1, Sin[x]}, {b1, Cos[x]}} /. rules {a1, b1} David Park djmp at earthlink.net http://home.earthlink.net/~djmp/ > From: Brian Higgins [mailto:bghiggins at ucdavis.edu] To: mathgroup at smc.vnet.net > > 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 >