Re: Exercise of Programming with Mathematica

*To*: mathgroup at smc.vnet.net*Subject*: [mg131869] Re: Exercise of Programming with Mathematica*From*: Ray Koopman <koopman at sfu.ca>*Date*: Sun, 20 Oct 2013 23:39:26 -0400 (EDT)*Delivered-to*: l-mathgroup@mail-archive0.wolfram.com*Delivered-to*: l-mathgroup@wolfram.com*Delivered-to*: mathgroup-outx@smc.vnet.net*Delivered-to*: mathgroup-newsendx@smc.vnet.net

Instead of using ReplaceAll (/.), use Replace and specify the level. In[1]:= Replace[{{a, b}, {c, d}}, {x_, y_} :> {y, x}, {1}] Out[1]= {{b, a}, {d, c}} In[2]:= Replace[{{a, b}, {c, d}}, {x_, y_} :> {y, x}, {0}] Out[2]= {{c, d}, {a, b}} ----- Zhenyi Zhang <infozyzhang at gmail.com> wrote: > Here is a rule designed to switch the order of each pair of expressions in a list. > It works fine on the first example, but fails on the second. > In[1]:= {{a, b}, {c, d}, {e, f}}/.{x_, y_} :> {y, x} > Out[1]= {{b, a}, {d, c}, {f, e}} > In[2]:= {{a, b}, {c, d}}/.{x_, y_} :> {y, x} > Out[2]= {{c, d}, {a, b}} > Explain what has gone wrong and rewrite this rule to correct the situation, > that is, so that the second example returns {{b, a}, {d, c}} > > My solution is the most stupid one. > > {{a, b}, {c, d}}/.{{x_, y_}, {w_, t_}} :> {{y, x}, {t, w}} > > May I ask any elegant solutions? > > Thanks.