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Topic: Exercise of Programming with Mathematica
Replies: 8   Last Post: Oct 23, 2013 11:33 PM

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 Harvey P. Dale Posts: 12 Registered: 10/24/12
Re: Exercise of Programming with Mathematica
Posted: Oct 19, 2013 4:05 AM

{#[[2]],#[[1]]}&/@{{a,b},{c,d},{e,f}}

Best,

Harvey

-----Original Message-----
From: Zhenyi Zhang [mailto:infozyzhang@gmail.com]
Sent: Friday, October 18, 2013 4:45 AM
Subject: 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[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.

Date Subject Author
10/19/13 Sseziwa Mukasa
10/19/13 Harvey P. Dale