Re: questions on tables

[Doug Rogers <doug.rogers@...>]

spir wrote:
> David Given <dg@cowlark.com> wrote:
>   
>> - - bidirectional --- where the array part and the keyed part contain the
>> some data, but opposite ways around. So t["one"] == 1 and t[1] == "one".
>>     
> Waow, clever!
>   

I use this technique often. I call it a xref table. The code below sets
__newindex to automatically add the key to the array part if it doesn't
already exist. I rarely use the del() function.

One of the things that I like about this approach is that I can sort the
array part in different ways and print the values in the sorted order.
See the example below.

-- xref.lua:

-- Provides a table that contains:
--   o A list of keys in the array part.
--   o For each key, xref[key] = value.

local ipairs       = ipairs
local pairs        = pairs
local rawset       = rawset
local setmetatable = setmetatable
local table_remove = table.remove
local type         = type

module('xref')

local xref = {}

function add(t, k, v)
   -- If we had __usedindex then we could use this function when
   -- v is nil, too. But if the key already exists, then
   -- __newindex is not called. Hence xref.del().

   if t[k] == nil then
      -- If the key is not in the xref, add it.
      rawset(t, #t+1, k)
   end

   rawset(t, k, v)
end   -- add()

function del(t, k)
   for i, key in ipairs(t) do
      if key == k then
         table_remove(t, i)
         break
      end
   end

   rawset(t, k, nil)
end   -- del()

local mt = {
   __newindex = add
}

function new(t)
   t = t or {}

   if type(t) ~= 'table' then
      error('table expected, got ' .. type(t))
   end

   if #t > 0 then
      error('initialization table should not contain an array part')
   end

   for k, v in pairs(t) do
      t[#t+1] = k
   end

   return setmetatable(t, mt)
end

-- End of xref.lua.

-- Test run:
require'xref'

t = { name='Fred', last='Flintstone' }
t = xref.new(t)

print("After initialization:")
for i, k in ipairs(t) do
   print(i, k, t[k])
end

t.boss = 'Slate'

print("After adding boss:")
for i, k in ipairs(t) do
   print(i, k, t[k])
end

table.sort(t)

print("After sorting on key:")
for i, k in ipairs(t) do
   print(i, k, t[k])
end

table.sort(t, function (k1, k2) return t[k1] < t[k2] end)

print("After sorting on value:")
for i, k in ipairs(t) do
   print(i, k, t[k])
end

xref.del(t, 'last')

print("After deleting last:")
for i, k in ipairs(t) do
   print(i, k, t[k])
end

-- Output:
After initialization:
1    name    Fred
2    last    Flintstone
After adding boss:
1    name    Fred
2    last    Flintstone
3    boss    Slate
After sorting on key:
1    boss    Slate
2    last    Flintstone
3    name    Fred
After sorting on value:
1    last    Flintstone
2    name    Fred
3    boss    Slate
After deleting last:
1    name    Fred
2    boss    Slate




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