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POLooking for an efficient implementation of the reverse of lzip or interleave in Tcl: Building sublists with every nth item
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<a href="http://wiki.tcl.tk/12187" rel="nofollow">This</a> describes an <code>interleave</code> function that can <code>lzip</code> data: <pre><code>% interleave {a b c} {1 2 3} a 1 b 2 c 3 </code></pre> I am looking for the reverse operation. Also I would like to specify into how many sublists the input shall be split. For example: <pre><code>% lnth {a 1 b 2 c 3} 1 {a 1 b 2 c 3} % lnth {a 1 b 2 c 3} 2 {a b c} {1 2 3} % lnth {a 1 b 2 c 3} 3 {a 2} {1 c} {b 3} % lnth {a 1 b 2 c 3} 6 {a} {1} {b} {2} {c} {3} </code></pre> For uneven splits, the missing elements shall be just omitted. If you feel like it you could provide a default argument to be filled in, but that's not required. Also I don't mind the exact quotation of the two corner cases where <code>n==1</code> or <code>n==[llength $L]</code>. Thanks Hai Vu for pointing this out in your earlier answer. It would be good to have some notion of complexity in time and memory. I'm on Tcl8.4 (this cannot be changed). Update For these kind of benchmark question its always good to have a central summary. All tests ran on the same machine, on the (rather small) example list <code>$L</code> as shown below. It's all highly un-scientific.Good code comes from the answers below, errors are mine. Test code: <pre><code>#!/usr/bin/tclsh proc build_list {len} { incr len while {[incr len -1]} { lappend res {} } set res } proc lnth3_prebuild_no_modulo {L n} { # Build empty 2D list to hold result set iterations [expr {int(ceil(double([llength $L]) / $n))}] set one [build_list $iterations] set res [list] set cnt [expr {$n+1}] while {[incr cnt -1]} { lappend res $one } # Fill in original/real values set iteration 0 set subListNumber 0 foreach item $L { lset res $subListNumber $iteration $item if {[incr subListNumber] == $n} { set subListNumber 0 incr iteration } } set res } proc lnth3_no_modulo {L n} { # Create a list of variables: subList0, subList1, subList2, ... for {set subListNumber 0} {$subListNumber < $n} {incr subListNumber} { set subList$subListNumber {} } # Build the sub-lists set subListNumber 0 foreach item $L { lappend subList$subListNumber $item if {[incr subListNumber] == $n} { set subListNumber 0 } } # Build the result from all the sub-lists set result {} for {set subListNumber 0} {$subListNumber < $n} {incr subListNumber} { lappend result [set subList$subListNumber] } return $result } proc lnth {L n} { set listvars "" for {set cnt 0} {$cnt < $n} {incr cnt} { lappend listvars "L$cnt" } set iterations [expr {ceil(double([llength $L]) / $n)}] for {set cnt 0} {$cnt < $iterations} {incr cnt} { foreach listvar $listvars el [lrange $L [expr {$cnt*$n}] [expr {($cnt+1)*$n-1}] ] { lappend $listvar $el } } set res [list] foreach listvar $listvars { lappend res [eval "join \$$listvar"] } set res } proc lnth_prebuild {L n} { set iterations [expr {int(ceil(double([llength $L]) / $n))}] set one [build_list $iterations] set listvars "" for {set cnt 0} {$cnt < $n} {incr cnt} { lappend listvars L$cnt set L$cnt $one } for {set cnt 0} {$cnt < $iterations} {incr cnt} { foreach listvar $listvars el [lrange $L [expr {$cnt*$n}] [expr {($cnt+1)*$n-1}] ] { lset $listvar $cnt $el } } set res [list] foreach listvar $listvars { lappend res [eval "join \$$listvar"] } set res } 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 < $n} {incr iteration} { set subList {} for {set i $iteration} {$i < $listLen} {incr i $n} { lappend subList [lindex $L $i] } lappend result $subList } return $result } proc lnth3 {L n} { # Create a list of variables: subList0, subList1, subList2, ... for {set subListNumber 0} {$subListNumber < $n} {incr subListNumber} { set subList$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 < $n} {incr subListNumber} { lappend result [set subList$subListNumber] } return $result } # stuff subcommands in a namespace namespace eval ::unlzip {} proc unlzip {L n} { # check if we have the proc already set name [format "::unlzip::arity%dunlzip" $n] if {[llength [info commands $name]]} { return [$name $L] } else { # create it proc $name {V} [::unlzip::createBody $n] return [$name $L] } } proc ::unlzip::createBody {n} { for {set i 0} {$i < $n} {incr i} { lappend names v$i lappend lnames lv$i } set lbody "" set ret { return [list } foreach lname $lnames name $names { append lbody [format { lappend %s $%s} $lname $name] append ret "\$$lname " } append ret {]} return [format {foreach {%s} $V { %s } %s} $names $lbody $ret] } ### Tests set proc_reference lnth set procs {lnth_prebuild lnth2 lnth3 unlzip lnth3_no_modulo lnth3_prebuild_no_modulo} 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} set Ns {1 2 3 4 5 6 7 8 9 10 13 26} # Functional verification foreach n $Ns { set expected [$proc_reference $L $n] foreach p $procs { set result [$p $L $n] if {$expected ne $result} { puts "Wrong result for proc $p, N=$n." puts " Expected: $expected" puts " Got: $result" } } } # Table header puts -nonewline [format "%30s" {proc_name\N}] foreach n $Ns { puts -nonewline [format " %7d" $n] } puts "" # Run benchmarks foreach proc_name [concat $proc_reference $procs] { puts -nonewline [format "%30s" $proc_name] foreach n $Ns { puts -nonewline [format " %7.2f" [lindex [time "$proc_name \$L $n" 10000] 0]] } puts "" } </code></pre> The results: <pre><code> proc_name\N 1 2 3 4 5 6 7 8 9 10 13 26 lnth 33.34 23.73 21.88 20.51 21.33 21.33 22.41 23.07 23.36 25.59 26.09 38.39 lnth_prebuild 41.14 31.00 28.88 27.24 28.48 29.06 30.45 31.46 31.43 34.65 34.45 49.10 lnth2 8.56 8.08 8.35 8.78 9.12 9.29 9.66 9.98 10.29 10.61 11.22 14.94 lnth3 17.15 18.35 18.91 19.55 20.55 21.42 22.24 23.54 23.71 24.27 25.79 33.78 unlzip 5.36 5.25 5.03 4.97 5.27 5.42 5.52 5.43 5.42 5.96 5.51 6.83 lnth3_no_modulo 14.88 16.56 17.20 17.97 18.63 19.42 19.78 20.74 21.53 21.84 23.60 31.29 lnth3_prebuild_no_modulo 14.44 13.30 12.83 12.51 12.51 12.43 12.36 12.41 12.41 12.83 12.70 14.09 </code></pre>
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Looking for an efficient implementation of the reverse of lzip or interleave in Tcl: Building sublists with every nth item
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