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
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> first example, but fails on the second.
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> In[1]:= {{a, b}, {c, d}, {e, f}}/.{x_, y_} :> {y, x}
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> Out[1]= {{b, a}, {d, c}, {f, e}}
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> In[2]:= {{a, b}, {c, d}}/.{x_, y_} :> {y, x}
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> Out[2]= {{c, d}, {a, b}}
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> Explain what has gone wrong and rewrite this rule to correct the situation, that is, so that the second
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> example returns {{b, a}, {d, c}}
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> My solution is the most stupid one.
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> {{a, b}, {c, d}}/.{{x_, y_}, {w_, t_}} :> {{y, x}, {t, w}}
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> May I ask any elegant solutions?
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> Thanks.
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> 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?
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> Here are two list to try with:
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>
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> lst1 = {{a, b}, {c, d}, {e, f}};
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> lst2 = {{a, b}, {c, d}};
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> 1. Mapping Permute onto a list:
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> Map[Permute[#, Cycles[{{2, 1}}]] &, lst1]
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> Map[Permute[#, Cycles[{{2, 1}}]] &, lst2]
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> {{b, a}, {d, c}, {f, e}}
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> {{b, a}, {d, c}}
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> 2. Using Transpose + Rule:
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> Transpose[lst] /. {x_, y_} -> {y, x} // Transpose
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> {{b, a}, {d, c}}
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> 3. Transpose and hands:
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> {Transpose[lst1][[2]], Transpose[lst1][[1]]} // Thread
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> {{b, a}, {d, c}}
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> Or {Transpose[lst1][[2]], Transpose[lst1][[1]]} // Transpose
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> 4. Mapping a rule onto a list:
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> Map[ReplaceAll[#, {x_, y_} -> {y, x}] &, lst1]
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> Map[ReplaceAll[#, {x_, y_} -> {y, x}] &, lst2]
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> {{b, a}, {d, c}, {f, e}}
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> {{b, a}, {d, c}}
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> 5. Using Thread+Rule+Thread or Transpose
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> Transpose[Thread[lst1] /. {x_, y_} -> {y, x}]
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> {{b, a}, {d, c}, {f, e}}
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> Thread[Thread[lst1] /. {x_, y_} -> {y, x}]
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> {{b, a}, {d, c}, {f, e}}
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> Have fun, Alexei
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>
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> Alexei BOULBITCH, Dr., habil.
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> IEE S.A.
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> ZAE Weiergewan,
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> 11, rue Edmond Reuter,
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> L-5326 Contern, LUXEMBOURG
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>
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> Office phone : +352-2454-2566
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> Office fax: +352-2454-3566
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> mobile phone: +49 151 52 40 66 44
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>
>
> 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}]]