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Topic: Default value in pattern matching
Replies: 2   Last Post: Jan 24, 2014 4:07 AM

 Messages: [ Previous | Next ]
 Yi Wang Posts: 33 Registered: 3/30/12
Re: Default value in pattern matching
Posted: Jan 24, 2014 4:07 AM

Hi, Larry,

Thanks a lot for your reply! This works for this example, but not exactly in the way I wanted. Because eventually my purpose is not using myPlus to re-implement Plus. But instead I would need myPlus to be some other functions.

With your code, when tracing the evaluation, we have

Trace[a /. {a + myPlus[b_.] -> "matched"}]

{{{{{myPlus[b_.], b_.}, a + (b_.)}, a + (b_.) -> matched, a + (b_.) -> \
matched}, {a + (b_.) -> matched}}, a /. {a + (b_.) -> matched}, \
matched}

Thus first a + myPlus[b_.] is transformed into a+b_., and then comes the "magic" of default value in Plus (which is exactly the thing I want to understand).

If we use HoldPattern, then the above no longer match:

a /. {a + HoldPattern[myPlus[b_.]] -> "matched"}

Out[] = a

To compare, the default value of Plus itself works even for HoldPattern:

a /. {a + HoldPattern[b_.] -> "matched"}

Out[] = "matched"

Best,
Yi

> Don't you need to define myPlus[b_.]?
>
>
>
> In[173]:= myPlus[b_.] = b;
>
> Default[myPlus] = 0;
>
> myPlus[0] := 0;
>
> a /. {a + myPlus[b_.] -> "matched"}
>
>
>
> Out[176]= "matched"
>
>
>
>
>
> Larry
>
>
>
> ----- Original Message -----
>

> > From: "Yi Wang" <tririverwangyi@gmail.com>
>
> > To: mathgroup@smc.vnet.net
>
> > Sent: Saturday, January 18, 2014 3:59:46 AM
>
> > Subject: Default value in pattern matching
>
> >
>
> > Hello,
>
> >
>
> > I was investigating the usage of the "a_." pattern. I understand that f=
or
>
> > some built-in functions with default value, we can do
>
> >
>
> > In[] := a /. {a + b_. -> "matched"}
>
> >
>
> > Out[] = "matched"
>
> >
>
> > However, if I define a function myself with
>
> >
>
> > In[] := Default[myPlus] = 0;
>
> > myPlus[0] := 0;
>
> > a /. {a + myPlus[b_.] -> "matched"}
>
> >
>
> > Out[] = a
>
> >
>
> > In other words, my default value is not used in the "b_." pattern. Is t=
here a
>
> > way to get a /. {a + myPlus[b_.] -> "matched"} match in the same way as=
a /.
>
> > {a + b_. -> "matched"}?
>
> >
>
> > Otherwise I have to write separately as
>
> >
>
> > In[] := a /. {a -> "matched", a + myPlus[b_] -> "matched"}
>
> >
>
> > This is a lot of additional work for complicated cases.
>
> >
>
> >

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