Re: Exercise of Programming with Mathematica

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• Subject: [mg131886] Re: Exercise of Programming with Mathematica
• From: mike.honeychurch at gmail.com
• Date: Wed, 23 Oct 2013 23:46:14 -0400 (EDT)
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```On Tuesday, October 22, 2013 3:54:30 PM UTC+11, Alexei Boulbitch wrote:
> 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.
>
>
>
>
>
> You are right, this is a nice exercise in Mathematica. Considering it as such, I would like to offer few solutions that were not yet mentioned. Some of them are not shorter. What's the matter, is it not the exercise?
>
>
>
> Here are two list to try with:
>
>
>
> lst1 = {{a, b}, {c, d}, {e, f}};
>
> lst2 = {{a, b}, {c, d}};
>
>
>
> 1. Mapping Permute onto a list:
>
>
>
> Map[Permute[#, Cycles[{{2, 1}}]] &, lst1]
>
> Map[Permute[#, Cycles[{{2, 1}}]] &, lst2]
>
>
>
> {{b, a}, {d, c}, {f, e}}
>
>
>
> {{b, a}, {d, c}}
>
>
>
> 2. Using Transpose + Rule:
>
>
>
> Transpose[lst] /. {x_, y_} -> {y, x} // Transpose
>
>
>
> {{b, a}, {d, c}}
>
>
>
> 3. Transpose and hands:
>
>
>
>
>
>
> {{b, a}, {d, c}}
>
>
>
> Or {Transpose[lst1][[2]], Transpose[lst1][[1]]} // Transpose
>
>
>
> 4. Mapping a rule onto a list:
>
>
>
> Map[ReplaceAll[#, {x_, y_} -> {y, x}] &, lst1]
>
> Map[ReplaceAll[#, {x_, y_} -> {y, x}] &, lst2]
>
>
>
> {{b, a}, {d, c}, {f, e}}
>
>
>
> {{b, a}, {d, c}}
>
>
>
>
>
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> Transpose[Thread[lst1] /. {x_, y_} -> {y, x}]
>
>
>
> {{b, a}, {d, c}, {f, e}}
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>
>
>
>
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> {{b, a}, {d, c}, {f, e}}
>
>
>
> Have fun, Alexei
>
>
>
> Alexei BOULBITCH, Dr., habil.
>
> IEE S.A.
>
> ZAE Weiergewan,
>
> 11, rue Edmond Reuter,
>
> L-5326 Contern, LUXEMBOURG
>
>
>
> Office phone :  +352-2454-2566
>
> Office fax:       +352-2454-3566
>
> mobile phone:  +49 151 52 40 66 44
>
>
>
> e-mail: alexei.boulbitch at iee.lu

While there are many ways to do this I usually use Part

lst1 = {{a, b}, {c, d}, {e, f}};

lst1[[All, {2, 1}]]

```

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