Services & Resources / Wolfram Forums
-----
 /
MathGroup Archive
2004
*January
*February
*March
*April
*May
*June
*July
*August
*September
*October
*November
*December
*Archive Index
*Ask about this page
*Print this page
*Give us feedback
*Sign up for the Wolfram Insider

MathGroup Archive 2004

[Date Index] [Thread Index] [Author Index]

Search the Archive

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


It's just the difference between Rule and RuleDelayed. Observe:

(inputs)
N[y]
Rule[x_,y_]->Rule[x,N[y]]

(outputs)
y
(x_ -> y_) -> x -> y

The right hand side of Rule is evaluated immediately, before it's applied to any expression, so N goes away.

Both of these approaches work, however:

(inputs)
s = a -> 10/3;
s /. Rule[x_, y_] :> Rule[x, N[y]]
N@s

(outputs)
a -> 3.33333
a -> 3.33333

Bobby

On Tue, 28 Dec 2004 06:30:32 -0500 (EST), yehuda ben-shimol <benshimo at bgu.ac.il> 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
>
>
>
>



-- 
DrBob at bigfoot.com
www.eclecticdreams.net


  • Prev by Date: Re: N[] does not work inside Replace[] ?
  • Next by Date: Re: N[] does not work inside Replace[] ?
  • Previous by thread: Re: N[] does not work inside Replace[] ?
  • Next by thread: Re: N[] does not work inside Replace[] ?