Re: Exercise of Programming with Mathematica

*To*: mathgroup at smc.vnet.net*Subject*: [mg131857] Re: Exercise of Programming with Mathematica*From*: Sseziwa Mukasa <mukasa at gmail.com>*Date*: Sat, 19 Oct 2013 04:09:56 -0400 (EDT)*Delivered-to*: l-mathgroup@mail-archive0.wolfram.com*Delivered-to*: l-mathgroup@wolfram.com*Delivered-to*: mathgroup-outx@smc.vnet.net*Delivered-to*: mathgroup-newsendx@smc.vnet.net*References*: <20131018084528.A030C6A21@smc.vnet.net>

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 at 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. >

**References**:**Exercise of Programming with Mathematica***From:*Zhenyi Zhang <infozyzhang@gmail.com>