[RS] 2007-02-23: ''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 range
======
[RS] 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..

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[RS] 2007-09-28: With 8.5b1 on WinXP, NaN is gone too :(
======
 % set n [expr sqrt(-1.)]
 domain error: argument not in valid range
======
But as [everything is a string], we can still demonstrate the remarkable property
======
 % set n NaN
 NaN
 % expr {$n == $n}
 0
======
----
See also [expr] - [Inf]

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[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.#QNAN

Quite 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 [http://wiki.apache.org/stdcxx/FloatingPoint]
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)

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