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From: Joe English on 21 Apr 2010 19:56
Glenn Jackman wrote:
> Donald G Porter wrote:
>> The [lsort -dictionary] command could also be the core of
>> a pairwise comparison function, but I'll leave it to other
>> to test whether the performance of such a thing is acceptable
>> and/or better than other alternatives.
>
> Here's an implementation
>
> proc string_comp_dictionary {s1 s2} {
> if {$s1 eq $s2} {
> return 0
> } else {
> set l [list $s1 $s2]
> if {$l == [lsort -dictionary $l]} {
> return -1
> } else {
> return 1
> }
> }
> }


Here's what I use when I need this:

proc dictionary_compare {a b} {
lindex [lsort -dictionary -index 0 [list [list $a -1] [list $b 1]]] 0 1
}

[ And then I always grumble about the unholy mess of options
flags and feechurs that is the [lsort] command and wish
that Tcl just provided [dictionary_compare] as a standalone
primitive instead. But that's a different problem. ]


> Is that the best way to examine list equality? I also considered
> examining them as strings:
>
> {"$s1 $s2" eq [join [lsort -dictionary [list $s1 $s2]]]}

I wouldn't really use either one of those.

"List equality" isn't really well-defined -- you need to
first decide "lists of *what*?" Comparing two lists
of numbers is a different function than comparing two
lists of strings, or two lists of dictionaries, or
two lists of....

It usually comes down to [all [zipWith $predicate $l1 $l2]],
where $predicate is a comparison function appropriate
to the type of the elements of the list.


--Joe English
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