precision

*To*: mathgroup at christensen.cybernetics.net*Subject*: [mg445] precision*From*: "Ed Boss" <boss at wln.com>*Date*: Fri, 3 Feb 95 11:44:43 -1000

Jonathan Rich writes: ------------------------ Is there any way to specify, at the start of a session, or at the start of a Mathematica script or program, that one would like MMA to use a given precision (i.e. something other than $MachinePrecision) thruout the calculations? It doesn't appear to me that "PrecisionGoal" is what I want as that is an option for specific numerical functions. In other words, what I want is something like "SetPrecision[]", but not for a specific expression but rather a global default value which tells MMA to assume such a precision for all expressions and numerical calculations, unless specified otherwise. Then when one typed "x=3.", x would automatically have whatever precision one had globally set at the beginning of the session. I guess this would correspond to use of C or Fortran compiler flags like "-r8" that instruct the compiler that all variables are to be double precision (for example). Is there a way of doing such a thing in mathematica? Thanks for your help and advice. Jonathan Rich --------------------------- I believe waht you want to do is set the global variable $Post. From Nancy Blachman's book, Mathematica: A Practical Approach, page 28: In: nThirty[x_] := N[x,30] In: nThirty[5/7] Out: 0.71428571428571428571428571429 In: $Post = nThirty Out: nThirty In: Sqrt[3] Out: 1.73205080756887729352744634151 In: Precision[%] Out: 30. To return to the default precesion In: $Post =. Regards, Ed Ed Boss .^ * NOAA/Pacific Marine Environmental Laboratory / / / | / \ ___.---~ `*------ ...