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MathGroup Archive 2002

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Re: Passing arguments and pattern matching

  • To: mathgroup at smc.vnet.net
  • Subject: [mg33821] Re: [mg33792] Passing arguments and pattern matching
  • From: Andrzej Kozlowski <andrzej at tuins.ac.jp>
  • Date: Fri, 19 Apr 2002 02:27:41 -0400 (EDT)
  • Sender: owner-wri-mathgroup at wolfram.com

The answer to your question seems indeed rather simple but it may be 
that I have just missed yur point . Anyway it seems to me that the 
following does what you are asking for(?):

In[1]:=
a = {x, Log[x], y, x y, Log[y] };

In[2]:=
  b = {x};

In[3]:=
    myPattern[lista_List,listb_List] := Position[lista,#]&/@listb

In[4]:=
myPattern[a,b]

Out[4]=
{{{1},{2,1},{4,1}}}

In[5]:=
myPattern[a,{Log[x]}]

Out[5]=
{{{2}}}

Andrzej Kozlowski
Toyama International University
JAPAN
http://platon.c.u-tokyo.ac.jp/andrzej/


On Tuesday, April 16, 2002, at 04:50  PM, Coleman, Mark wrote:

> 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!
>
>
>
>



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