
Re: Exercise of Programming with Mathematica
Posted:
Oct 20, 2013 11:19 PM


On Fri, Oct 18, 2013 at 04:45:28AM 0400, Zhenyi Zhang 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}}
Nothing has gone wrong. :) Seriouly, You're entirely input is an ordered pair with x > {a,b} and y > {c, d}. The reason the first example works is that it has 3 pairs, so the binding isn't possible. Remember, ReplaceAll (/.) always looks for the larges subexpression it can match first.
> 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?
With this pattern, x and y cannot themselves be lists:
In[115]:= {{a, b}, {c, d}} /. {x : Except[_List], y : Except[_List]} > {y, x}
Out[115]= {{b, a}, {d, c}}
 Itai Seggev Mathematica Algorithms R&D 2173980700

