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From: Andreas Leitgeb <avl@logic.at>
Newsgroups: comp.lang.tcl
Subject: regarding lrange and stride ...
Date: Mon, 1 Jul 2024 19:34:54 -0000 (UTC)
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Strided lists are a somewhat common feature in Tcl commands,
starting with  foreach/lmap, lsort, ...

Recently it occurred to me that [lrange $list $first $last]
would be extensible to become more helpful for strided lists.

In essence, an optional 4th argument would be the $stride:

set list {a A 1 b B 2 c C 3 d D 4}
lrange $list 1 2 3
->  {b B 2 c C 3}

If not already obvious, the extra stride argument "pretends" that
there is an extra list level of each 3 elements,
 {{a A 1} {b B 2} {c C 3} {d D 4}} on which the usual lrange
happens, and then the extra level "disappears".

In the simple "numeric" cases it would be essentially like:
   proc lrange_s {l f t s} {
      lrange $l [expr {$f*$s}] [expr {($t+1)*$s-1}]
   }
....plus extra code for nonnumeric indices "end", "end-42", "40+2",...
and some special cases where [llength $list] is not an integral multiple
of the stride.