Re: Table compare
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- Subject: Re: Table compare
- From: "Vaughan McAlley" <ockegheim@...>
- Date: 2008年12月31日 00:05:10 +1100
> You have two table traversals here, that kind of defeats the table.len() check purpose.
If table.len returns false there is no need to do the traversals.
Otherwise it's pretty unavoidable.
This code uses metatables to calculate table lengths as items are
added and removed:
----------------------
local uniqueKey = {}
local tableLength = uniqueKey -- for naming clarity
local metaT = {
__newindex = function(t, k, v)
local realTable = ShowRealTable(t)
if v == nil then
if realTable[k] ~= nil then
realTable[tableLength] = realTable[tableLength] - 1
realTable[k] = nil
end
else
if realTable[k] == nil then
realTable[tableLength] = realTable[tableLength] + 1
end
realTable[k] = v
end
end ,
__index = function(t, k)
return ShowRealTable(t)[k]
end
}
function CreateTable()
local intermediateTable = {}
local realTable = {}
intermediateTable[uniqueKey] = realTable
realTable[tableLength] = 0
setmetatable(intermediateTable, metaT)
return intermediateTable
end
-- instead of pairs(t), use pairs(ShowRealTable(t))
function ShowRealTable(t)
return t[uniqueKey]
end
function ShowLength(t)
return ShowRealTable(t)[tableLength]
end
function TableCompare(a, b)
if ShowLength(a) ~= ShowLength(b) then
return false
end
local a1 = ShowRealTable(a)
local b1 = ShowRealTable(b)
for key, value in pairs(a1) do
if b1[key] ~= value then
return false
end
end
for key, value in pairs(b1) do
if a1[key] ~= value then
return false
end
end
return true
end
local t1 = CreateTable()
local t2 = CreateTable()
t1.x = 45
t1[34] = "blah"
t1[false] = TableCompare
t2[34] = "blah"
t2[false] = TableCompare
t1.x = nil
print(ShowLength(t1), TableCompare(t1, t2)) --> 2, true
------------------
It could be possible using metatables to have a two-dimensional array
of differences between tables built up as tables are created and
modified. That would save the traversals when comparing but would add
a memory and perhaps performance burden.
Vaughan