% lstride {a b c d e f g h i j k l} 4 {a b c d} {e f g h} {i j k l} % lstride {a b c d e f g h i j k l} 3 {a b c} {d e f} {g h i} {j k l} % lstride {a b c d e f g h i j k l} 2 {a b} {c d} {e f} {g h} {i j} {k l}So here's how I would write a list stride:

proc lstride {L n} { set t [list]; set res [list] foreach i $L { lappend t $i if {[llength $t]==$n} { lappend res $t set t [list] } } if [llength $t] {lappend res $t] ;# maybe keep the rest set res }I follow some simple rules:

- foreach is better than for (because it's simpler)
- short variable names (L is uppercase to distinguish from digit One)
- empty lists are slightly more efficiently initialized by [list] - {} is still a string, albeit empty!
- Finally returning a result looks nicer with "set res"

Here's a brain-twisting variation, which generates numeric variable names with "iota" (e.g. iota 5 => {0 1 2 3 4}) as first

*foreach*argument, and retrieves them by

*eval*-ling a transformed

*list $0 $1 $2 $3 $4*:

proc iota n { for {set i 0;set res [list]} {$i<$n} {incr i} { lappend res $i } set res } proc lstride2 {L n} { set vars [iota $n] set cmd "list \$[join $vars { $}]" set res [list] foreach $vars $L {lappend res [eval $cmd]} set res } ;# RSLars H: Here's a version that's twisted a bit further:

proc lstride2b {L n} { set vars [iota $n] set res [list] foreach $vars $L "lappend res \[list \$[join $vars { $}]\]" set res }

RS in Feb. 2005 prefers this simpler version:

proc lstride {list n} { set res {} while {[llength $list]} { lappend res [lrange $list 0 [expr {$n-1}]] set list [lrange $list $n end] } set res } % lstride {a b c d e f g h i} 3 {a b c} {d e f} {g h i}Lars H: Note however that it runs in quadratic time. The other versions above manage to run in linear time. - RS: Time? You mean because partial lists are copied on every turn? - Lars H: Rather because the tail of the list is copied on every turn. The following runs in linear time:

proc lstride {list n} { set res {} for {set i 0; set j [expr {$n-1}]} {$i < [llength $list]} {incr i $n; incr j $n} { lappend res [lrange $list $i $j] } return $res }

Andreas Kupries - People without fear of using a C extension can use

**listx**[1] and write:

% ::listx split -group {a b c d e f} 2 {a b} {c d} {e f} % ::listx split -group {a b c d e} 2 {a b} {c d} {e {}}Or

% ::listx split {a b c d e f} 2 {a c e} {b d f}

Arts and crafts of Tcl-Tk programming Additional list functions