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    <p>Here is my approach: build one sub-list at a time, then append to the result before building the next one.</p> <pre><code>proc lnth2 {L n} { set listLen [llength $L] set subListLen [expr {$listLen / $n}] if {$listLen % $n != 0} { incr subListLen } set result {} for {set iteration 0} {$iteration &lt; $n} {incr iteration} { set subList {} for {set i $iteration} {$i &lt; $listLen} {incr i $n} { lappend subList [lindex $L $i] } lappend result $subList } return $result } </code></pre> <p>Let say that L = <code>{a 1 b 2 c 3}</code> and n = 2, then I will build the first sub-list <code>{a b c}</code> by picking out the 0th, 2nd, and 4th items from the original list, append that to the result and move on the the second sub-list. Likewise, the second sub-list will be the 1th, 3rd, and 5th items. </p> <h1>Update</h1> <p>After reviewing my solution, I still don't like the fact that I have to use <code>lindex</code>. I imagine <code>lindex</code> has to travese the list in order to find the list item, and my solution placed <code>lindex</code> right inside a loop; which means we traverse the same list several times. The next attempt is to traverse the list only once. This time, I mimic your algorithm, but avoid using the list functions such as <code>lrange</code>.</p> <pre><code>proc lnth3 {L n} { # Create a list of variables: subList0, subList1, subList2, ... for {set subListNumber 0} {$subListNumber &lt; $n} {incr subListNumber} { set subList$subListNumber {} } # Build the sub-lists set i 0 foreach item $L { set subListNumber [expr {$i % $n}] lappend subList$subListNumber $item incr i } # Build the result from all the sub-lists set result {} for {set subListNumber 0} {$subListNumber &lt; $n} {incr subListNumber} { lappend result [set subList$subListNumber] } return $result } </code></pre> <p>Sadly, this attempt performs worse than my first attempt. I still don't understand why.</p>
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    1. POLooking for an efficient implementation of the reverse of lzip or interleave in Tcl: Building sublists with every nth item
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    1. COThat is about 3x faster than my solution: `set L {a 1 b 2 c 3 d 4 e 5 f 6 g 7 h 8 j 9 i 10 k 11 l 12 m 13 n 14 o 15 p 16 q 17 r 18 s 19 t 20 u 21 v 22 w 23 x 24 y 25 z 26}` then `time {lnth $L 2} 1000` is 34us and for your `lnth2` it is 8.4us.
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    2. COAlso you deal more nicely with incomplete lists: My solution has an empty space between the last element of incomplete sublists and the closing brace `}` - it's functionally equivalent, but ugly nevertheless. Nice :-)
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    3. COKudos for sharing "Bad attempts"! I admit I was hoping for some O(N) magic like in some of the Tcl Wiki's list handling routines. `lset` performs decently, but I don't think building an empty list of lists upfront to then `lset` elements would be a better performer...
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