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MathGroup Archive 2001

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Re: Replacement Rule

  • To: mathgroup at smc.vnet.net
  • Subject: [mg32083] Re: [mg32064] Replacement Rule
  • From: Sseziwa Mukasa <mukasa at jeol.com>
  • Date: Sat, 22 Dec 2001 04:23:00 -0500 (EST)
  • References: <200112210857.DAA25218@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

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



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