Re: explicitly defining table length + _ARG proposal
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- Subject: Re: explicitly defining table length + _ARG proposal
- From: David Manura <dm.lua@...>
- Date: 2010年6月26日 18:43:15 -0400
On Wed, Jun 23, 2010 at 2:50 PM, Florian Weimer <fw@deneb.enyo.de> wrote:
> * Juris Kalnins:
>> Main issue is that automatically maintaining the hidden "n" has a
>> runtime cost that is not acceptable for all users.
>
> The proposal linked to by David avoids that because the explicit
> length is never decreased automatically. This should be good enough
> in many cases (including {...}).
Expressed in Lua 5.1, it might look like this:
local lengths = setmetatable({}, {__mode='k'})
function table.len(t)
return lengths[t] or 0
end
function table.setlen(t, n)
lengths[t] = n ~= 0 and n or nil
-- maybe clear elements at index > n too, in which case
-- table.setlen(t, 0) might be an efficient way to clear the array
end
local oldinsert = table.insert
function table.insert(t, ...)
if select('#', ...) >= 2 then
oldinsert(t, ...)
else
local value = ...
t[table.len(t)+1] = value
end
table.setlen(t, table.len(t) + 1)
end
local oldremove = table.remove
function table.remove(t, pos)
pos = pos or table.len(t)
oldremove(t, pos)
table.setlen(t, table.len(t) - 1)
end
function table.pack(...)
local t = {...}
table.setlen(t, select('#', ...))
return t
end
-- example:
local function printtable(t)
io.stdout:write(tostring(table.len(t)), ": ")
for i=1,table.len(t) do io.stdout:write(tostring(t[i]), " ") end
io.stdout:write("\n")
end
-- example:
local function printtable(t)
io.stdout:write(tostring(table.len(t)), ": ")
for i=1,table.len(t) do io.stdout:write(tostring(t[i]), " ") end
io.stdout:write("\n")
end
t = table.pack(2,3,4,nil,nil) printtable(t)
table.insert(t,5) printtable(t)
t[6] = nil printtable(t)
table.remove(t) printtable(t)
table.remove(t,1) printtable(t)
table.setlen(t,1) printtable(t)
table.setlen(t,0) printtable(t)
--[[
5: 2 3 4 nil nil
6: 2 3 4 nil nil 5
6: 2 3 4 nil nil nil
5: 2 3 4 nil nil
4: 3 4 nil nil
1: 3
0:
]]