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Exercise of Programming with Mathematica

• To: mathgroup at smc.vnet.net
• Subject: [mg131855] Exercise of Programming with Mathematica
• From: Zhenyi Zhang <infozyzhang at gmail.com>
• Date: Fri, 18 Oct 2013 04:45:28 -0400 (EDT)
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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.

• Follow-Ups:
  • Re: Exercise of Programming with Mathematica
    • From: Itai Seggev <itais@wolfram.com>
  • Re: Exercise of Programming with Mathematica
    • From: Adriano Pascoletti <adriano.pascoletti@uniud.it>
  • Re: Exercise of Programming with Mathematica
    • From: "Harvey P. Dale" <hpd@hpdale.org>
  • Re: Exercise of Programming with Mathematica
    • From: Sseziwa Mukasa <mukasa@gmail.com>