RE: Wrong Precision with "N" ??

*To*: mathgroup at smc.vnet.net*Subject*: [mg24140] RE: [mg24108] Wrong Precision with "N" ??*From*: "David Park" <djmp at earthlink.net>*Date*: Wed, 28 Jun 2000 02:11:55 -0400 (EDT)*Sender*: owner-wri-mathgroup at wolfram.com

J.R., The documentation is a little misleading on the purpose of N, and its behavior has changed with later versions of Mathematica. The purpose of N is to change an exact number to an approximate number. If the precision asked for is equal to or less than the machine precision, then it produces a machine precision number as in your examples. If the precision specified is greater than the machine precision, then it produces an extended precision approximate number. Machine precision numbers in Version 4 are always displayed with 6 places, but dropping trailing zeros. This can be changed with the Option Inspector. Using N with the precision less than 6 will not change this. N should not be used to format numbers for output. If you want to do that, use NumberForm, or ScientificForm, EngineeringForm etc. If you wish to change the precision of a number, use SetPrecision. This topic is discussed at greater length in Sections 3.1.4 through 3.1.6 in the Mathematica Book. The following creates a new number with a precision of 3, and I believe that it will not be used as a machine precision number. shortpi = SetPrecision[Pi, 3] 3.14 Precision[shortpi] 3 If all you wish to do is format output, do this... NumberForm[N[Pi], 3] 3.14 Note that the precision of N[Pi,n] is machine precision unless n is greater than machine precision. Table[Precision[N[Pi, n]], {n, 5, 20}] {16, 16, 16, 16, 16, 16, 16, 16, 16, 16, 16, 16, 17, 18, 19, 20} David Park djmp at earthlink.net http://home.earthlink.net/~djmp/ > -----Original Message----- > From: J.R. Chaffer [mailto:jrchaff at mcn.net] To: mathgroup at smc.vnet.net > Will someone explain to this newbie why the advertised > behavior of the numerical evaluation function "N" to some > specific precision - flatly ignores the second argument? > > For example, on my machine, > > N[Pi,2] = N[Pi,3] = N[Pi,4] = N[Pi,10] = 3.14159 > > which is not at all what 'Help', and any number of books, > claim, with no explanation of why it just might not work > out that way (??) > > Thanks, > J.R. Chaffer >