Re: Default value in pattern matching

*To*: mathgroup at smc.vnet.net*Subject*: [mg132255] Re: Default value in pattern matching*From*: Yi Wang <tririverwangyi at gmail.com>*Date*: Fri, 24 Jan 2014 04:20:18 -0500 (EST)*Delivered-to*: l-mathgroup@mail-archive0.wolfram.com*Delivered-to*: l-mathgroup@wolfram.com*Delivered-to*: mathgroup-outx@smc.vnet.net*Delivered-to*: mathgroup-newsendx@smc.vnet.net*References*: <20140118095946.3B4C36A11@smc.vnet.net> <lblc64$mic$1@smc.vnet.net>

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 at gmail.com> > > > To: mathgroup at 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. > > > > > >

**References**:**Default value in pattern matching***From:*Yi Wang <tririverwangyi@gmail.com>