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Re: Problem with Patterns and Integrate

  • To: mathgroup at smc.vnet.net
  • Subject: [mg123275] Re: Problem with Patterns and Integrate
  • From: Jack McInerney <ejmcinerney at gmail.com>
  • Date: Wed, 30 Nov 2011 03:19:44 -0500 (EST)
  • Delivered-to: l-mathgroup@mail-archive0.wolfram.com
  • References: <201111281053.FAA19741@smc.vnet.net> <jb2i4f$5ho$1@smc.vnet.net>

On Nov 29, 4:10 am, Bob Hanlon <hanlonr... at gmail.com> wrote:
> Compare Rule with RuleDelayed
>
> func -> Integrate[func, x]
>
> func -> func*x
>
> func :> Integrate[func, x]
>
> func :> Integrate[func, x]
>
> expr = D[f[x], x];
>
> expr /. func_ :> Integrate[func, x]
>
> f[x]
>
> Bob Hanlon
>
>
>
>
>
>
>
> On Mon, Nov 28, 2011 at 5:53 AM, Jack McInerney <ejmciner... at gmail.com> wrote:
> > There is something about using patterns that I don't understand, and
> > am hoping someone can shed some light.  Here is an example of my
> > problem.  Say I have a function f[x] and I take the derivative of it.
> > I can then integrate it and get f[x] back:
>
> > In[167]:= expr = D[ f[x], x ]
> > Out[167]= f=E2=80=B2[x]
>
> > In[168]:= Integrate[expr, x]
> > Out[168]= f[x]
>
> > If I use a ReplaceAll and a pattern to do the integration, the
> > Integrate function treats f'[x] as a constant and returns the wrong
> > answer:
>
> > In[169]:= expr /. func_ ->  Integrate[func,x]
> > Out[169]= x f=E2=80=B2[x]
>
> > Any thoughts as to what I am doing wrong?
>
> > Jack


Thanks everyone for the suggestions.  I have never been that clear on
when I should use RuleDelayed versus Rule.  The distinction is much
clearer now.

Jack



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