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Topic: Exercise of Programming with Mathematica
Replies: 3   Last Post: Nov 6, 2013 12:21 AM

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Zhenyi Zhang

Posts: 1
Registered: 10/18/13
Exercise of Programming with Mathematica
Posted: Oct 18, 2013 4:32 AM
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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?


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