       Re: Exercise of Programming with Mathematica

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• 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)
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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:= Replace[{{a, b}, {c, d}}, {x_, y_} :> {y, x}, {1}]
Out= {{b, a}, {d, c}}

In:= Replace[{{a, b}, {c, d}}, L : {_, _} :> Reverse[L], {1}]
Out= {{b, a}, {d, c}}

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:= {{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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