'''abs''' is one of several extended math commands available in [Tclx].  It makes the [expr] abs function available as a command, and takes expressions as arguments. ([Tcl 8.5] has its own copy that it puts in the ::tcl::mathfunc [namespace].)

If you don't have that, it can easily be emulated:

 proc abs x {expr {abs($x)}}

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[AMG]: In Tcl 8.4.7, `expr {abs(-0.0)}` returns `-0.0`.  In Tcl 8.6b1, it returns `0.0`.  This is on an Intel Xeon platform running CentOS 4.2 ([RedHat]).  Look for `ExprAbsFunc` in the 8.4.7 source code: [http://core.tcl.tk/tcl/artifact?name=eb251b0543e6b6562a6ae6bd36c5e6bb7f369810]:

======
        d = valuePtr->internalRep.doubleValue;
        if (d < 0.0) {
            dResult = -d;
        } else {
            dResult = d;
        }
======

If `d` is -0.0, it's not less than 0.0, so it gets copied directly to `dResult`.  This may be SF bug #2954959 [http://sourceforge.net/tracker/index.php?func=detail&aid=2954959&group_id=10894&atid=110894] and/or SF bug #1893815 [http://sourceforge.net/tracker/index.php?func=detail&aid=1893815&group_id=10894&atid=110894].

[AMG]: I found another problem with Tcl 8.4.7 abs.  It seems I somehow convinced Tcl that "0" is a double, so "`set x 0; expr {abs($x)}`" returns `0.0`.  (This is actually fatal to my program.)  My guess is that I used a "0"-valued Tcl_Obj somewhere else in a double-precision context, so Tcl imbued it with a double internal [representation].  When I later set a variable to "0", Tcl grabs the existing Tcl_Obj from its cache, and the intrep comes along for the ride.  When this value is passed to abs, it sees it has a double intrep (never mind the fact that its string representation is ''not'' that of a double), so it returns a double.

Running the same program on the same machine in Tcl 8.6b1 doesn't trigger this problem.  Either it's been fixed, or the problem is better hidden. ;^)

By the way, if I change from "`set x 0`" to "`set x [[expr 0]]`", my program works.

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See also:

   * [Importing expr functions]
   * [tcl::mathfunc]

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