diff options
Diffstat (limited to 'doc/tclvars.n')
| -rw-r--r-- | doc/tclvars.n | 33 |
1 files changed, 26 insertions, 7 deletions
diff --git a/doc/tclvars.n b/doc/tclvars.n index 32d4b08..45498c8 100644 --- a/doc/tclvars.n +++ b/doc/tclvars.n @@ -5,7 +5,7 @@ '\" See the file "license.terms" for information on usage and redistribution '\" of this file, and for a DISCLAIMER OF ALL WARRANTIES. '\" -'\" RCS: @(#) $Id: tclvars.n,v 1.43 2010/11/15 21:34:54 andreas_kupries Exp $ +'\" RCS: @(#) $Id: tclvars.n,v 1.44 2011/01/15 18:10:19 kennykb Exp $ '\" .so man.macros .TH tclvars n 8.0 Tcl "Tcl Built-In Commands" @@ -365,22 +365,41 @@ binary number. .RE .PP .RS -17 digits is +If \Btcl_precision\fB is not zero, then when Tcl converts a floating +point number, it creates a decimal representation of at most +\fBtcl_precision\fR significant digits; the result may be shorter if +the shorter result represents the original number exactly. If no +result of at most \fBtcl_precision\fR digits is an exact representation +of the original number, the one that is closest to the original +number is chosen. +If the original number lies precisely between two equally accurate +decimal representations, then the one with an even value for the least +significant digit is chosen; for instance, if tcl_precision is 3, then +0.3125 will convert to 0.312, not 0.313, while 0.6875 will convert to +0.688, not 0.687. Any string of trailing zeroes that remains is trimmed. +.RE +.PP +.RS +a \fBtcl_precision\fR value of 17 digits is .QW 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, -\fBexpr {1.4}\fR returns 1.3999999999999999 with \fBtcl_precision\fR -set to 17, vs. 1.4 if \fBtcl_precision\fR is 12. +binary with no loss of information. For this reason, you will often +see it as a value in legacy code that must run on Tcl versions before +8.5. It is no longer recommended; as noted above, a zero value is the +preferred method. .RE .PP .RS All interpreters in a thread share a single \fBtcl_precision\fR value: changing it in one interpreter will affect all other interpreters as -well. However, safe interpreters are not allowed to modify the +well. Safe interpreters are not allowed to modify the variable. .RE +.PP +.RS +Valid values for \Btcl_precision\fR range from 0 to 17. +.RE .TP \fBtcl_rcFileName\fR . |