       Exercise of Programming with Mathematica

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• 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:= {{a, b}, {c, d}, {e, f}}/.{x_, y_} :> {y, x}
Out= {{b, a}, {d, c}, {f, e}}
In:= {{a, b}, {c, d}}/.{x_, y_} :> {y, x}
Out= {{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.

```

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