Re: Tcl precision [was: Re: [Plplot-devel] Java changes to support plsurf3d()]

[Maurice L. <mj...@ga...>]

Joao Cardoso writes:
 > On Tuesday 15 October 2002 23:07, Maurice LeBrun wrote:
 > > | The story becomes more interesting.  I believe the two representations
 > > | (variable or tclMatrix) would've given the same result in pre-Tcl8.0. 
 > > | The reason being that in the old Tcl, "everything's a string".  But no
 > > | longer -- the variable that you create with "set" is "dual-ported",
 > > | meaning it has both a binary and string representation.  The binary one
 > > | is used in expression evaluations and so is more accurate.
 > 
 > But this means that it is single precision, doesn't it? Tcl is build with 
 > floats or doubles? So, if the default is float, as I supose, we shouldn't 
 > compare plots made with the tcl front-ends with plots made from frontends 
 > compiled with doubles. It is not the device driver that matters, but the 
 > origin of the data -- floats in, say, x08.tcl with doubles in x08c. And I 
 > think to remember from Alan's e-mail (that I dont have at the moment) that he 
 > was using plplot compiled with doubles.

Tcl uses double internally.  But the precision when printing them out defaults
to 12.  From the tclvars man page:

       tcl_precision
              This  variable  controls the number of digits to generate when |
              converting floating-point values to strings.  It  defaults  to |
              12.   17 digits is ``perfect'' for IEEE floating-point in that |
              it allows double-precision values to be converted  to  strings |
              and  back  to  binary  with  no loss of information.  However, |
              using 17 digits prevents any rounding, which produces  longer, |
              less   intuitive  results.   For  example,  expr  1.4  returns |
              1.3999999999999999 with tcl_precision set to 17,  vs.  1.4  if |
              tcl_precision is 12.                                           |

-- 
Maurice LeBrun    mj...@ga...
Research Organization for Information Science and Technology of Japan (RIST)