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Re: Pure Functions in rules
*To*: mathgroup at smc.vnet.net
*Subject*: [mg15975] Re: Pure Functions in rules
*From*: gaylord at ux1.cso.uiuc.edu (richard j. gaylord)
*Date*: Fri, 19 Feb 1999 03:26:59 -0500
*Organization*: university of illinois
*References*: <7ag34l$aie@smc.vnet.net>
*Sender*: owner-wri-mathgroup at wolfram.com
In article <7ag34l$aie at smc.vnet.net>, wself at viking.emcmt.edu (Will Self) wrote:
> It appears that I cannot depend on using a pure function
> in a pattern-matching rule.
> 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?
i'll comment:
i think you have a problem with premature evaluation [not with the use of
anonymous functions in a rule per se ].look at this
In[7]:=
Map[(2*#)&, m]
Out[7]=
m
In[9]:=
Trace[Map[ (2*#)&, m]]
Out[9]=
{(2 #1&)/@m,m}
the problem is that in use lhs -> rhs, the rhs of the rule is evaluated
before its used for substition[and it evaluates to the global variable m
which you introduced in the rule
look at
Trace[{1,2,3} /. m_List -> Map[ (2*#)&, m] ]
and at
m = 5;
Trace[{1,2,3} /. m_List -> Map[ (2*#)&, m] ]
[i'd put the output here but i'm having a problem copy-pasting from Mma
to my newswatcher app]
if you use :> instead of -> so that the rhs of the rule is not evaluated
before its applied to the evaluated expression [in this case {1,2, 3} it
works fine.
{1,2,3}/.(m_List:>(2*#& /@ m))
note: - i suppose there's some reason you mapping an anonymous function
for doubling the elements of a list onto a list rather than just
multiplying the list by two.
--
"I would say life is pretty pointless, wouldn't you, without the movies?"
Vincent Gallo as Johnny Tempi in The Funeral (1996)
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