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MathGroup Archive 2004

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

  • To: mathgroup at
  • Subject: [mg53197] Re: [mg53185] N[] does not work inside Replace[] ?
  • From: "David Park" <djmp at>
  • Date: Tue, 28 Dec 2004 23:12:16 -0500 (EST)
  • Sender: owner-wri-mathgroup at


I don't understand why you don't just use the following...

s = Rule[a, 10/3]
% // N
a -> 10/3
a -> 3.33333

But if you want a Rule to change a Rule you have to use a delayed Rule.

s /. (x_ -> y_) :> x -> N[y]
a -> 3.33333

David Park
djmp at 

From: yehuda ben-shimol [mailto:benshimo at]
To: mathgroup at

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

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

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 

I tried then a simpler example
and then tried the same replacement idea
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

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