TCL(Tool Command Language) Scripting :: Simple maths question


	Simple maths question Hello, I was demonstrating how easy Tcl is to use (or at least to learn to use) when I was immediately flummoxed. On a Windows XP machine I created a trivial proc: (bin) 289 % proc subtract {a b} { return [expr {$a - $b}] } (bin) 290 % subtract 3 2.2 0.7999999999999998 Eh? This was using Tcl 8.5a4. It works properly in 8.4.11. Is this a known bug (sorry if it is)? Thanks, Frem. On 10 mei, 22:32, Frem <freminl@gmail.com> wrote: - Hide quoted text - > Hello, > I was demonstrating how easy Tcl is to use (or at least to learn to use) > when I was immediately flummoxed. > On a Windows XP machine I created a trivial proc: > (bin) 289 % proc subtract {a b} { > return [expr {$a - $b}] > } > (bin) 290 % subtract 3 2.2 > 0.7999999999999998 > Eh? This was using Tcl 8.5a4. It works properly in 8.4.11. Is this a > known bug (sorry if it is)? > Thanks, > Frem. It is working properly in 8.5 as well. The difference between 8.4 and 8.5 is that the tcl_precision global is set differently by default. In 8.5 the two lines: set tcl_precision 12 subtract 3 2.2 will give the same result as in 8.4. The real reason you get this result however is that floating point math is not precise on a computer (0.2 is a repeating binary fraction). This is not a limitation of Tcl, but a limitation of doing decimal floating point math in binary. See http://wiki.tcl.tk/879 for a more elaborate explanation. Also see http://www.python.org/doc/current/tut/node16.html for another explanation which also shows this is a general problem with floating point math not limited to Tcl Mark On May 11, 5:27 am, Mark Janssen <mpc.jans@gmail.com> wrote: - Hide quoted text - > On 10 mei, 22:32, Frem <freminl@gmail.com> wrote: > > Hello, > > I was demonstrating how easy Tcl is to use (or at least to learn to use) > > when I was immediately flummoxed. > > On a Windows XP machine I created a trivial proc: > > (bin) 289 % proc subtract {a b} { > > return [expr {$a - $b}] > > } > > (bin) 290 % subtract 3 2.2 > > 0.7999999999999998 > > Eh? This was using Tcl 8.5a4. It works properly in 8.4.11. Is this a > > known bug (sorry if it is)? > > Thanks, > > Frem. > It is working properly in 8.5 as well. The difference between 8.4 and > 8.5 is that the tcl_precision global is set differently by default. > In 8.5 the two lines: > set tcl_precision 12 > subtract 3 2.2 > will give the same result as in 8.4. > The real reason you get this result however is that floating point > math is not precise on a computer (0.2 is a repeating binary > fraction). People should be reminded with floating point, any fraction that doesn't end in a '5' is imprecise. Just like with regular pen-on-paper decimal math, 1/3 is imprecise (interestingly if you do your math in base 3, 1/3 is not a repeating fraction and can be precisely represented). For perfect precision with rational numbers, you can treat them as pairs of integer {numerator denominator}. See "Playing with rationals", http://wiki.tcl.tk/13877