[Keith Vetter] 2003-09-29 -- I love tcl's [foreach] command. It's a
beautiful command that lets you express elegantly some otherwise
cumbersome constructs. Compared to similar constructs in other languages,
tcl's foreach command outshines them all.

People often complain that tcl's ''simple'' syntax can be annoying--it's
prolix and confusing to the initiate: when is a comment a comment? (even
[JO] got that one wrong), always having to use [expr] to do any math,
the whole {expand} debate [http://mini.net/tcl/9468], etc.

But the foreach command is one of tcl's strengths.

Most of the common uses of foreach such as walking a list, swapping
two variables
   foreach {a b} [list $b $a] break
, handling multiple return values from a function (see
[foreach] for more details) have similar constructs in other
languages, often times more elegantly expressed.

But once you get past these simple uses of foreach, other languages
can't compare. Take even the simple example of traipsing down a
list--in tcl you can take bigger steps. Try doing this in perl (without
destroying the data list):

 foreach {x y} $dataSet { ... }

Another simple (but I admit dubious) use of foreach is setting 
multiple variables at once:

 foreach x [$w canvasx $x] y [$w canvasy $y] break


But advanced use of foreach that many people don't realize comes
when you traipse over a list of variable '''names'''. Below is
an example from I program I wrote that cleans up user supplied data:

 foreach var {north south east west altitude} {
     set $var [::Data::Fix [set $var]]
 }

Here's a more complicated example:

 # Get canvas dimensions shrunk by some amount
 foreach who {x0 y0 x1 y1} val [.c cget -scrollregion] d {30 20 -30 -20} {
     set $who [expr {$val + $d}]
 }


Now that I've admitted my love of the foreach command, I confess I 
have two complaints. First, I'd love some kind of iteration counter. But 
the obvious way of doing that is via a special variable and that not the
''tcl way''. Second, I'd love foreach to understand [streams], but that's
a whole other discussion.