[Richard Suchenwirth] 2002-09-03 - The [lsort] command allows many
useful ways of sorting, but sometimes one needs other ways. For
instance, to sort strings according to their length; or alphabetically,
but reversed (whole dictionaries are produced for this, e.g for finding
rhymes). [lsort] takes a ''-command'' option, but see that page for why
it is "a pig in performance".

The recommended way is rather to produce from the original input a list
of pairs consisting of sorting key and original element; sort using -index
according to the key; and
extract the original elements from the result. Here is a little wrapper for
this approach, which takes as first argument a sorting function name (possibly
multi-word like ''string length'') and then the usual arguments of [lsort], the
last of which is the list to be sorted:

 proc lsortby {sortf args} {
    set list [lindex $args end] ;# list ot be sorted is last
    set args [lrange $args 0 end-1]
    set t {}
    foreach element $list {
        lappend t [list [eval $sortf [list $element]] $element]
    }
    set res {}
    foreach i [eval lsort $args -index 0 [list $t]] {
        lappend res [lindex $i 1]
    }
    set res
 }
 #----------- testing (and usage examples):
 puts [lsortby {string length} -int {long longer short shorter {very long} x}]
 proc reverse s {
    set i [string length $s]
    set res {}
    while {$i} {append res [string index $s [incr i -1]]}
    set res
 }
 puts [lsortby reverse -unique {this is the rest of the best test beast}]
if 0 {results in:
 x long short longer shorter {very long}
 the of is this beast best rest test
----
[Arts and crafts of Tcl-Tk programming]