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

*To*: mathgroup at smc.vnet.net*Subject*: [mg53196] Re: [mg53185] N[] does not work inside Replace[] ?*From*: Bob Hanlon <hanlonr at cox.net>*Date*: Tue, 28 Dec 2004 23:12:14 -0500 (EST)*Reply-to*: hanlonr at cox.net*Sender*: owner-wri-mathgroup at wolfram.com

Use RuleDelayed s=Rule[a,10/3]; s//N a -> 3.3333333333333335 s /. Rule[x_,y_] :> Rule[x,N[y]] a -> 3.3333333333333335 b = Range[5]/3 {1/3, 2/3, 1, 4/3, 5/3} b // N {0.333333,0.666667,1.,1.33333,1.66667} b /. x_Rational :> N[x] {0.3333333333333333, 0.6666666666666666, 1, 1.3333333333333333, 1.6666666666666667} Bob Hanlon > > From: yehuda ben-shimol <benshimo at bgu.ac.il> To: mathgroup at smc.vnet.net > Date: 2004/12/28 Tue AM 06:30:32 EST > To: mathgroup at smc.vnet.net > Subject: [mg53196] [mg53185] N[] does not work inside Replace[] ? > > 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 > >