       Re: Exercise of Programming with Mathematica

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• 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)
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```	{#[],#[]}&/@{{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:= {{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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