Re: Exercise of Programming with Mathematica

*To*: mathgroup at smc.vnet.net*Subject*: [mg131858] Re: Exercise of Programming with Mathematica*From*: "Harvey P. Dale" <hpd at hpdale.org>*Date*: Sat, 19 Oct 2013 04:10:16 -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>

{#[[2]],#[[1]]}&/@{{a,b},{c,d},{e,f}} Best, Harvey -----Original Message----- From: Zhenyi Zhang [mailto:infozyzhang at gmail.com] Sent: Friday, October 18, 2013 4:45 AM To: mathgroup at smc.vnet.net Subject: [mg131858] Exercise of Programming with Mathematica 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>