Re: Pattern gremlins.

• To: mathgroup at smc.vnet.net
• Subject: [mg55128] Re: Pattern gremlins.
• From: Josef Karthauser <joe at tao.org.uk>
• Date: Sun, 13 Mar 2005 04:57:47 -0500 (EST)
• References: <d0u6kr\$kl6\$1@smc.vnet.net> <005401c5271f\$45b00480\$6402a8c0@home>
• Sender: owner-wri-mathgroup at wolfram.com

```On Sat, Mar 12, 2005 at 05:19:31PM +0100, wouter meeussen wrote:
> you're close, real close.
>
> In[3]:=a b c d e f g /. d f :> x
> Out[3]=a b c e g x
> multiplication commutes, all possible pairs are tested, so d*f is found t=
o match. But you knew that
> already. But why use ":>" delayed if "->" suffices? The RHS is not up for=
immediate interpretation
> before the LHS is filled, now is it? So:
> In[4]:=a b c d e f g /. d f -> x
> Out[4]=a b c e g x
>
> Now, mind your precedences & brackets !!
> In[5]:=a b c d e f g /. x : d f :> u
> Out[5]=a b c d e f g
>
> boo, try again:
> In[6]:=a b c d e f g /. (x : d) f :> u
> Out[6]=a b c e g u
> mmm, better, nah?
>
> and, vide supra:
> In[7]:=a b c d e f g /. (x : d) f -> u
> Out[7]=a b c e g u
>
> Cool software, I say!
>
> W.

And so the point is I guess that,

In[1]:= a b c d e f g /. z:(d f) :> FOO[z]

doesn't match, so instead I must do:

In[1]:= a b c d e f g /. (x:d) (y:f) :> FOO[x y]

So by attaching a name, z, to z:(d f), this means that it would only
find Times[d, f], and not any other times with other arguments also?

Joe
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```

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