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MathGroup Archive 2004

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Re: N[] does not work inside Replace[] ?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg53189] Re: N[] does not work inside Replace[] ?
  • From: David Bailey <dave at Remove_Thisdbailey.co.uk>
  • Date: Tue, 28 Dec 2004 23:12:05 -0500 (EST)
  • References: <200412241059.FAA05903@smc.vnet.net> <200412271141.GAA12681@smc.vnet.net> <cqrh2u$qi5$1@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

yehuda ben-shimol wrote:
> Hi,
> I just found a small but annoying problem with operating N[] inside a 
> replace rule
> s was actually part of an answer I got from Solve[] and I wanted to 
> leave the Rule, but change the right hand side to a machine real number.
> A reduced form of the problem is given below. I'm using version 5.0.0 
> (ver 5.1 is on the way ... )
> 
> s = Rule[a, 10/3]
> and the output is
> a->10/3
> 
> case 1:
> now use a replace rule with N[]
> s /. Rule[x_, y_] -> Rule[x, N[y]]
> and the result is still a->10/3
> 
> case 2:
> using some other header in place of N[]
> s /. Rule[x_, y_] -> Rule[x, g[y]]
> and the results is as expected
> a->g[10/3]
> 
> Trying to convert the second argument to a machine real number in 
> different way
> 
> case 3:
> s /. Rule[x_, y_] -> Rule[x, 1. y]
> results with
> a -> 3.33333
> as expected.
> 
> 
> Using Trace[] for the three cases reveals that N[y] is replaced by y 
> very early in the evaluation process, so it explains this unexpected 
> behavior.
> 
> I tried then a simpler example
> b=Range[5]/3
> and then tried the same replacement idea
> b/.x_Rational->N[x]
> and still the results leave the rational numbers unchanged
> 
> but with the b/.x_Rational->g[x] or b/.x_Rational->1.0 * x Mathematica 
> works as expected.
> 
> So, again, is this a bug? If it is so in version 5.0.0, does it still 
> exists in version 5.1?
> 
> Thanks in advanced
> yehuda
> 
Hi,

There is no bug here - if you use -> (Rule) an evaluation happens before 
the rule is applied - so N gets applied to y and leaves y. Obviously, an 
arbitrary function g without a definition stays as g[y].

You should use :> (RuleDelayed)

{x -> 10/3} /. Rule[a_,b_] :> Rule[a,N[b]]

      {x -> 3.33333}

Although there are a few obscure bugs in Mathematica, it is wise to 
assume the fault is yours until you know the system very well!

David Bailey
dbaileyconsultancy.co.uk


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