Re: Re: List Help Needed

*To*: mathgroup at smc.vnet.net*Subject*: [mg68895] Re: [mg68863] Re: List Help Needed*From*: "Bharat Bhole" <bbhole at gmail.com>*Date*: Tue, 22 Aug 2006 05:20:43 -0400 (EDT)*References*: <ecbo4l$r5j$1@smc.vnet.net> <200608211033.GAA10513@smc.vnet.net> <a3224ef10608210825t56f3d97u4449f36f88e8a9f6@mail.gmail.com> <22d35c5a0608210903s2497b22ds5ab3d4e2f99fdcbc@mail.gmail.com>*Sender*: owner-wri-mathgroup at wolfram.com

Thanks Jean-Marc. The original question was not by me by the way. Thanks very much anyway for the clarification. Bharat. On 8/21/06, Jean-Marc Gulliet <jeanmarc.gulliet at gmail.com> wrote: > > On 8/21/06, Bharat Bhole <bbhole at gmail.com> wrote: > > The solution given by you works when lst has three or more pairs. But it > > does not work with two pairs, i.e., with {{a,b},{c,d}}. Is there a > simple > > replacement rule which gives the result desired here regardless of the > > number of elements in lst. This behavior of replacement rule seems > > arbitrary. > > Hi Bharat, > > you are right: the first solution I gave you, using replacement rules, > does not yield the expected result. > > lst= { { a,b}, { c,d}}; lst/. { x_,y_}-> { x, f[y]} returns {{a, b}, > f[{c, d}]} rather than the expected list {{a, f[b]}, {c, f[d]}}. The > returned result is the correct one, indeed, since the replacement > operator /. looks for the most general patterns first and then goes > down to more and more specialised/restricted matches. Thus, applied to > a list of two lists only, the rule matches the list itself before the > individual sublists: > > MatchQ[{{a, b}, {c, d}}, {x_, y_}] --> True > MatchQ[{{a, b}, {c, d}, {e, f}}, {x_, y_}] --> False > > One way to avoid this pitfall is to add a test on one or more heads of > the elements of the sublists, assuming of course that they have the > regular and definite structure. For instance, say that the first > element is always an atom, > > lst/. { x_?AtomQ,y_}-> { x, f[y]} > --> {{a, f[b]}, {c, f[d]}} > > Another way is to use Replace [1] with a level specification [2]. In > the following example, we force Mathematica to start looking from the > lowest level of the expression: > > Replace[ lst, { x_,y_}-> { x, f[y]}, -1] > --> {{a, f[b]}, {c, f[d]}} > > Regards, > Jean-Marc > > [1] http://documents.wolfram.com/mathematica/functions/Replace > > [2] "Level Specifications", > http://documents.wolfram.com/mathematica/book/section-A.3.6 > -- ---------------------------------------------------------- "No problem can withstand the assault of sustained thinking." Voltaire ----------------------------------------------------------

**References**:**Re: List Help Needed***From:*Jean-Marc Gulliet <jeanmarc.gulliet@gmail.com>