Re: Sorting part of a list

[David Manura <dm.lua@...>]

On Wed, Apr 18, 2012 at 6:57 AM, Dirk Laurie <dirk.laurie@gmail.com> wrote:
> Op 18 april 2012 12:18 heeft Daniel Slaney het volgende geschreven:
>> I've recently tried to implement an algorithm that requires sorting just
>> a section of an array at a time.
> You could make a proxy table for the subarray.[...
> it will not work] for the routines in the table library since
> "For performance reasons, all table accesses (get/set)
> performed by these functions are raw. "

The following actually does work in Lua 5.2:

$ lua52
Lua 5.2.0  Copyright (C) 1994-2011 Lua.org, PUC-Rio
> t = {10,9,8,7,6,5,4,3,2,1}
> setmetatable(t, {__len = function() return 5 end})
> table.sort(t)
> for _,v in ipairs(t) do print(v) end
6 7 8 9 10 5 4 3 2 1

but it is only able to set the upper bound, not the lower bound, on
the sort, so it's of limited use.

Observe that the table.* functions do honor the __len operator in 5.2.
 This change between 5.1 and 5.2 was for reasons that remain unclear
to me [1-3].  I'm not aware of a serious argument or use case for it,
so I'm a bit puzzled by it.  I would be interested if anyone relies on
that behavior in production.

BTW, you can reimpement the table.sort function in pure Lua.
Performance can actually be quite good [4].  I recently used a form of
[4] to sort two arrays in parallel, which is another case that
table.sort itself doesn't support well.

[1] http://lua-users.org/lists/lua-l/2010-12/msg00653.html
[2] http://lua-users.org/lists/lua-l/2011-11/msg01003.html
[3] http://lua-users.org/lists/lua-l/2011-11/msg00992.html
[4] http://lua-users.org/wiki/LuaSorting