Re: Pure Functions in rules
- To: mathgroup at smc.vnet.net
- Subject: [mg16014] Re: [mg15932] Pure Functions in rules
- From: Clemens Frey <btml01 at uni-bayreuth.de>
- Date: Sat, 20 Feb 1999 02:52:05 -0500
- Sender: owner-wri-mathgroup at wolfram.com
Hi Will!
The problem in your In/Out[80] is that you employ Rule instead
of RuleDelayed; but you need evaluation after pattern matching, so
just analogously to using SetDelayed instead of Set when defining a
function you have to use RuleDelayed, and it will work as intended:
In[81] := {1,2,3}/.(m_List:>(2*#& /@ m))
Out[80] = {2,4,6}
Hope this helped
Clemens
------------------------------------------------------------
Clemens Frey
Doctoral Student at the
Department of Mathematics/BITOEK
University of Bayreuth(Germany)
clemens.frey at uni-bayreuth.de
http://www.bitoek.uni-bayreuth.de/~Clemens.Frey
------------------------------------------------------------
On Wed, 17 Feb 1999, Will Self wrote:
> It appears that I cannot depend on using a pure function
> in a pattern-matching rule.
>
> Here I am trying to convince reluctant students that they're
> better off learning to use Mathematica than doing things
> by hand, and we run across something like this, and in a
> much more complicated situation where the trouble was
> hard to isolate.
>
> I am quite frankly incensed by the behavior shown in
> In/Out 80, below. Look at these examples:
>
> In[73]:= {1,2,3}/.(m_List->7)
> Out[73]= 7
>
> In[74]:= {1,2,3}/.(m_List->(2*m))
> Out[74]= {2,4,6}
>
> In[75]:= 2*#& /@ {1,2,3}
> Out[75]= {2,4,6}
>
> In[77]:= f[m_List]:=2*#& /@ m
>
> In[78]:= f[{1,2,3}]
> Out[78]= {2,4,6}
>
> In[79]:= {1,2,3}/.m_List->f[m]
> Out[79]= {2,4,6}
>
> Now try this:
>
> In[80]:= {1,2,3}/.(m_List->(2*#& /@ m))
> Out[80]= {1,2,3}
>
> Does anyone (say, at WRI for example) care to comment on
> this?
>
> Will Self
>