Re: Problem with base 2 logs and Floor

*To*: mathgroup at smc.vnet.net*Subject*: [mg73022] Re: Problem with base 2 logs and Floor*From*: "dimitris" <dimmechan at yahoo.com>*Date*: Mon, 29 Jan 2007 04:43:39 -0500 (EST)*References*: <ephdk1$s2c$1@smc.vnet.net>

Something like? Block[{Message}, r - Floor[Log[2, r]] - 1 /. r -> 4] FullSimplify[%] 3 - Floor[Log[4]/Log[2]] 1 Block[{Message}, r - Ceiling[Log[2, r]] - 1 /. r -> 4] FullSimplify[%] 3 - Ceiling[Log[4]/Log[2]] 1 Block[{Message}, r - Floor[Log[2, r]] - 1 /. r -> 8] FullSimplify[%] 7 - Floor[Log[8]/Log[2]] 4 Block[{Message}, r - Ceiling[Log[2, r]] - 1 /. r -> 8] FullSimplify[%] 7 - Ceiling[Log[8]/Log[2]] 4 Dimitris On Jan 28, 7:50 am, neillcl... at msn.com wrote: > Hi, > > When I use an expression like this: > > r - Floor[Log[2, r]] - 1 /. r -> 4 > > I get precision errors in Mathematica 5.2. If I use an expression like > this: > > N[r - Floor[Log[2, r]] - 1] /. r -> 4 > > I get a correct result of 1 and no errors. If I use an expression like > this I get the same result: > > N[r - Ceiling[Log[2, r]] - 1] /. r -> 4 > > This is to be expected as Floor[Log[2, r]] = Ceiling[Log[2, r]] when > r is a power of two. > Unfortunatly the expessions diverge for r=8: > > N[r - Floor[Log[2, r]] - 1] /. r -> 8 gives 5 > N[r - Ceiling[Log[2, r]] - 1] /. r -> 8 gives 4 > > How can I get exacts results for expessions like this? > Thanks. > Neill.