Re: Replacement Rule

• To: mathgroup at smc.vnet.net
• Subject: [mg32076] Re: [mg32064] Replacement Rule
• From: BobHanlon at aol.com
• Date: Sat, 22 Dec 2001 04:22:49 -0500 (EST)
• Sender: owner-wri-mathgroup at wolfram.com

```In a message dated 12/21/01 4:17:57 AM, bghiggins at ucdavis.edu writes:

>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....

In your first case the outer list has three elements and does not match
the pattern so the rule is applied to each of the individual element lists.
In the second case, the outer list has two elements and matches the
pattern.  Consequently, the replacement rule is applied to the outer list.

One solution is to restrict the elements of the list in the replacement
rule to being numeric

l1 = {{1,2},{3,4},{5,6}};
l2 = {{3,4},{5,6}};
rr1 = {x_?NumericQ,y_?NumericQ}->x;

l1 /. rr1

{1, 3, 5}

l2 /. rr1

{3, 5}

Bob Hanlon
Chantilly, VA  USA

```

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