*NaN*is "Not a Number", a sort-of-valid floating point value from Tcl 8.5. Here are experiments:

(bin) 13 % set y [expr sqrt(-1)] -NaN (bin) 14 % expr {$y == $y} ;# the only value that is not equal to itself :^) 0 (bin) 15 % expr {$y eq $y} 1

male 2007-02-23: Sorry, but using tclkit(sh)? 8.5a4 ...

% expr {sqrt(-1)} domain error: argument not in valid rangeRS I used tclkit 8.5a4 as well, on Win XP. So the patchlevel doesn't yet tell the full story :( Do the Inf examples work for you?male yes, the Inf examples all work for me. But ... my tclkit(sh)? executables are bit older, some of the first built.LV Okay, so I use the base-tcl8.5-thread-solaris-sparc (a tclkit) from ActiveTcl 8.5 beta (Tcl 8.5a5), as well as built myself the latest CVS head for Tcl 8.5a6 on SPARC Solaris. In both cases, I get the domain error, not the -NaN.I also see the domain error using just standard tclsh from 8.5.4 or 8.6 cvs head..

RS 2007-09-28: With 8.5b1 on WinXP, NaN is gone too :(

% set n [expr sqrt(-1.)] domain error: argument not in valid rangePhilip Smolen It seems that NaN isn't completely gone. expr hides it. If you say ::tcl::mathfunc::sqrt -1 you still get "-NaN" as a result. TCL8.5But as everything is a string, we can still demonstrate the remarkable property

% set n NaN NaN % expr {$n == $n} 0

See also expr - Inf

AM (23 february 2009) While this works nicely from the Tcl side of things, you may run into trouble when the NaN comes from a C (compiled) extension that uses, say, the standard sprintf() function to pass the value to Tcl. While sprintf() is a standard function, the representation of NaN by the various compilers is not. Here are a few possibilities: NaN, nan, -1.#IND, 1.#QNANQuite a few others seem to exist as well - it is, in short, rather messy.DKF: Try this simple method. If you know a value

*should*be a number, try to parse it as such. If you fail, it's a NaN.AM Yes, that seems to be the simplest and most straightforward approach. See [1] for more gory details (as KBK pointed out to me).

AM (20 may 2010) Here is a small procedure to detect if a value is a "not-a-number" or not (that is, if it is a proper number):

proc isnan {x} { if { ![string is double $x] || $x != $x } { return 1 } else { return 0 } }(This works for Tcl 8.4 and previous as well as for Tcl 8.5 and later)