Re: Pure Functions in rules
- To: mathgroup at smc.vnet.net
- Subject: [mg16001] Re: [mg15932] Pure Functions in rules
- From: BobHanlon at aol.com
- Date: Fri, 19 Feb 1999 03:27:12 -0500
- Sender: owner-wri-mathgroup at wolfram.com
In a message dated 2/18/99 5:39:37 AM, wself at viking.emcmt.edu writes:
>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:
>
[deleted examples]
>
>Now try this:
>
>In[80]:= {1,2,3}/.(m_List->(2*#& /@ m))
>Out[80]= {1,2,3}
>
Will,
You need to use RuleDelayed vice Rule.
{1,2,3}/.(m_List:>(2*#& /@ m))
{2,4,6}
Bob Hanlon