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MathGroup Archive 2004

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


Actually the problemis that you are using Rule instead of RuleDelayed. 
So when you evaluate:

s /. Rule[x_, y_] -> Rule[x, N[y]]

N[y] is evaluated to y, because y is not numeric and you simply end up 
with

  s /. Rule[x_, y_] -> Rule[x, y]

But


s = a -> 10/3;


s /. (x_ -> y_) :> x -> N[y]

a -> 3.3333333333333335

Andrzej Kozlowski




On 28 Dec 2004, at 20:30, 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
>


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