Date: Oct 19, 2013 3:46 AM
Author: Sseziwa Mukasa
Subject: Re: Exercise of Programming with Mathematica
I can't determine a pattern that represents a pair which does not also match {{a,b},{c,d}}, but I can think of two options. The first is not to use pattern matching at all, which may be more efficient anyway:

In[5]:= {{a, b}, {c, d}, {e, f}}[[All, {2, 1}]]

{{a, b}, {c, d}}[[All, {2, 1}]]

Out[5]= {{b, a}, {d, c}, {f, e}}

Out[6]= {{b, a}, {d, c}}

The other is to limit the pattern matching to the appropriate level:

In[21]:= Replace[{{a, b}, {c, d}, {e, f}}, {x_, y_} -> {y, x}, {1}]

Replace[{{a, b}, {c, d}}, {x_, y_} -> {y, x}, {1}]

Out[21]= {{b, a}, {d, c}, {f, e}}

Out[22]= {{b, a}, {d, c}}

Elegance as always is in the eye of the beholder,

Sseziwa

On Oct 18, 2013, at 4:45 AM, Zhenyi Zhang <infozyzhang@gmail.com> 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.

>