Re: Exercise of Programming with Mathematica

*To*: mathgroup at smc.vnet.net*Subject*: [mg131866] Re: Exercise of Programming with Mathematica*From*: Adriano Pascoletti <adriano.pascoletti at uniud.it>*Date*: Sun, 20 Oct 2013 23:38:26 -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*References*: <20131018084528.A030C6A21@smc.vnet.net>

In the second example the whole expression is matched by /. (ReplaceAll): x is the first pair, y the second one. Use Replace and specify the level In[4]:= Replace[{{a, b}, {c, d}}, {x_, y_} :> {y, x}, {1}] Out[4]= {{b, a}, {d, c}} In[5]:= Replace[{{a, b}, {c, d}}, L : {_, _} :> Reverse[L], {1}] Out[5]= {{b, a}, {d, c}} Adriano Pascoletti 2013/10/18 Zhenyi Zhang <infozyzhang at gmail.com> > 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. >

**References**:**Exercise of Programming with Mathematica***From:*Zhenyi Zhang <infozyzhang@gmail.com>