Next Metamethod For Index Table

wiki

Rewriting next() for when __index is a Table

To make a next() function that matches this indexing method (including the shadowing of higher level keys by lower level ones) you can use the following code...

function tnext(t,o)

  local i,v



  if o then

    -- 'o' is not nil (it is a real existing key).

    -- Locate the key's table.

    local r = t

    while not rawget(r,o) do

      local m = getmetatable(r)

      r = m and m.__index

      assert(type(r)=="table", "Key not in table")

    end



    -- Grab the next non-shadowed index

    local s

    i = o -- Start with the current index.

    repeat

      -- Get next real (non-nil) index.

      i,v = next(r,i)

      while (i==nil) do

        local m = getmetatable(r)

        r = m and m.__index

        if (r==nil) then return nil,nil end -- None left.

        assert(type(r)=="table", "__index must be table or nil")

        i,v = next(r)

      end

      -- Find the next index's level.

      s = t

      while not rawget(s,i) do

        local m = getmetatable(s)

        s = m and m.__index

      end

      -- If match then not shadowed, else repeat.

    until (r==s)

    -- Return it.

    return i,v



  else

    -- 'o' is nil, so want the first real key. Scan each table in

    -- turn until we find one (or give up if all are empty).

    while t do

      i,v = next(t)

      if i then break end

      local m = getmetatable(t)

      t = m and m.__index

      assert(t==nil or type(t)=="table", "__index must be table or nil")

    end

    return i,v

  end

end

Example usage

t = {a=111,  b=222, c=333}

u = {a=123, d=444}

setmetatable(t, {__index=u})

for i,v in tnext,t do print(i,v) end

a  111

b  222

c  333

d  444

-- PeterHill

See Also

• GeneralizedPairsAndIpairs - alternate approach using new metamethods.