RE: Replacement Rule

• To: mathgroup at smc.vnet.net
• Subject: [mg32094] RE: [mg32064] Replacement Rule
• From: Tomas Garza <tgarza01 at prodigy.net.mx>
• Date: Sat, 22 Dec 2001 04:23:27 -0500 (EST)
• Sender: owner-wri-mathgroup at wolfram.com

```It's the kind of problem one finds when working with patterns. The thing is,
the pattern {x_, y_} matches each element of your list, but the list itself
as well: the list has two elements! Try this

In[1]:=
l2 = {{3, 4}, {5, 6}};
In[2]:=
l2 /. {x_?NumericQ, y_} -> x
Out[2]=
{3, 5}

This checks for patterns which match lists with numeric elements, i.e., not
lists themselves.

Tomas Garza
Mexico City

> -----Original Message-----
> From: Brian Higgins [mailto:bghiggins at ucdavis.edu]
To: mathgroup at smc.vnet.net
> Sent: Friday, December 21, 2001 12:58 AM
> To: mathgroup at smc.vnet.net
> Subject: [mg32094] [mg32064] Replacement Rule
>
>
> 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....
>
> Brian
>

```

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