MathGroup Archive 2001

[Date Index] [Thread Index] [Author Index]

Search the Archive

Re: Replacement Rule


Brian Higgins wrote:

> I have the following list
>
> l1={{1,2},{3,4},{5,6}};
>
> and then I operate on it with the following pattern/replacement rule
>
> In[3]:= l1 /. {x_, y_} -> x
>
> Out[3]= {1, 3, 5}
>
> This is what I would expect. Now consider the following list
>
> In[4]:= l2 = {{3, 4}, {5, 6}};
>
> Then I use the same pattern/replacement rule
>
> In[5]:=l2 /. {x_, y_} -> x
>
> Out[5]= {3, 4}
>
> I was hoping to get
>
> {3,5}
>
> What am I missing? The FullForm of l2 is basically the same structure
> as l1, as far as I can tell....
>

The pattern matches from the outermost level, which in the second case
is the list of two elements {3,4} and {5,6}.  If you know that you
really want a list of integers then {x_Integer,y_Integer}->x works.

Ssezi



  • Prev by Date: Re: Solutions that are not solutions
  • Next by Date: Re: Displaying Mathematica's Global rules
  • Previous by thread: Re: Replacement Rule
  • Next by thread: Re: Replacement Rule