Re: N does not work inside Replace ?
- To: mathgroup at smc.vnet.net
- Subject: [mg53197] Re: [mg53185] N does not work inside Replace ?
- From: "David Park" <djmp at earthlink.net>
- Date: Tue, 28 Dec 2004 23:12:16 -0500 (EST)
- Sender: owner-wri-mathgroup at wolfram.com
Yehuda, 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 earthlink.net http://home.earthlink.net/~djmp/ From: yehuda ben-shimol [mailto:benshimo at bgu.ac.il] To: mathgroup at smc.vnet.net 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/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