Re: Passing arguments and pattern matching

*To*: mathgroup at smc.vnet.net*Subject*: [mg33836] Re: [mg33792] Passing arguments and pattern matching*From*: "Martin Jenkins" <lamarth at optushome.com.au>*Date*: Fri, 19 Apr 2002 02:28:25 -0400 (EDT)*References*: <200204160750.DAA20097@smc.vnet.net>*Sender*: owner-wri-mathgroup at wolfram.com

Mark, myPattern[lista_, listb_] := Flatten[Map[Position[lista, #] &, listb], 1] appears to do what you have requested. The Flatten is so that it comes out with the level of nesting in your desired output. In[10]:= myPattern[a, {x, Log[x]}] Out[10]= {{1}, {2, 1}, {4, 1}, {2}} I'm sure you can apply a Sort of some kind if you wish it. I would recommend not requiring the inputs to be of type "List" unless it does not make sense to have something else as the head (in my own project I have found that most built-in Mathematica functions are like this, and have cursed the ones that are not). Certainly, Position works with something as a head other than List. Martin Jenkins ----- Original Message ----- From: "Coleman, Mark" <mark.coleman at dri-wefa.com> To: mathgroup at smc.vnet.net Subject: [mg33836] [mg33792] Passing arguments and pattern matching > Greetings, > > I have a question on pattern matching that I have been struggling with. First I'll describe the issue broadly, and then attempt to illustrate it with a specific example. My intution is that is a relatively simple answer, but I have been unable to find it. > > I would like to write a general function that accepts these two lists as arguments, i.e., myPattern[a_List,b_List]. The function would act something like the built-in Mathematica Postion[] pattern function and return the positions of any elements of b that occur in List a. This function will be called by other functions, so the user will not know in advance the values of lists a or b. In my attempts to build this function I have used the Position[] function. The difficulty is that Position treats the arguments I pass it as 'literal', and not as references to what the arguement represents. > > For instance, let's say I define two lists as follows: > > a = {x, Log[x], y, x y, Log[y] }; > b = {x}; > > Now say I wish to compute the position of x or various functions of x in a, then of course > > > In[24]:= Position[a,x] > > Out[24]= {{1},{2,1},{4,1}} > > In[26]:= Position[a,Log[x]] > > Out[26]= {{2}} > > and so on. But now assume I wish to define a function > > myPattern[lista_List,listb_List] := Position[lista,listsb] > > If I call myPatten[a,b], this is the same as executing > > > In[27]:=Position[a,b] > > Out[27]= {} > > My question is, how do I have Mathematica treat the second argument not as a literal ("b"), but as representing a the value of the underlying argument? For instance > > myPattern[a,{Log[x]}] would return {{2}} > > Thanks! > >

**References**:**Passing arguments and pattern matching***From:*"Coleman, Mark" <mark.coleman@dri-wefa.com>