When you're writing a sort routine, passing in "a" and "b" to be compared, how do you access other data that's needed in association with a and b? If - for example - a and b are keys to a hash or dictionary or associative array, and you want to sort based on the values in that array? You could make your hash global (or rely on the global default in Perl or Lua) but that is a poor solution - messy and unmaintainable.

Lua uses the concept of a closure - in Lua, variables that are local to a function are also available in functions that are defined within that function i.e. within nested definitions, or 'within the outer closure'.

Here's an example:

function postman(thistable)
  function psord(a,b)
    if thistable[a] < thistable[b] then
      return true
      end
    return false
    end
  local keys = {}
  for k,_ in pairs(thistable) do
    keys[#keys+1] = k
    end
  table.sort(keys,psord)
  rs = ""
  for k,v in ipairs(keys) do
    rs = rs .. k .. " " .. v .. " at no. ".. thistable[v].."\n"
    end
  return rs
end
 
deliveries = {John = 4, Jim = 10, Jean = 7, Joanna = 3, Jo = 9}
hegoes = postman(deliveries)
io.write (hegoes)

You'll notice that "thistable" is a local (parameter) to postman, but never the less can be accessed from within psord ... which is a pleasant surprise if you're converting from other languages.

Running the code, I get

1 Joanna at no. 3
2 John at no. 4
3 Jean at no. 7
4 Jo at no. 9
5 Jim at no. 10

Source of the example here



(written 2009-07-31)

 U106 - Lua - Functions.
  [1685] Short circuit evaluation (lazy operator) in Lua - (2008-06-22)
  [1697] Sorting in lua - specifying your own sort routine - (2008-07-05)
  [1699] If you are learning Lua, here are some more examples - (2008-07-06)
  [1743] First class functions in Lua lead to powerful OO facilities - (2008-08-07)
  [1744] Lua examples, Lua Courses - (2008-08-08)
  [1847] Lua - IAQ (Infrequently Answered Questions) - (2008-10-18)
  [2347] Dot, dot, dot in Lua - variable length parameter lists - (2009-08-11)
  [2356] Passing multiple results back from a function - Lua, Python, etc - (2009-08-13)
  [2455] Lua examples - coroutines, error handling, objects, etc - (2009-10-15)
  [2723] Returning multiple values from a function - Lua - (2010-04-15)
  [2949] Lua - variable function arguments and select - (2010-09-08)
  [3695] Functions are first class variables in Lua and Python - (2012-04-13)

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New Lua Examples - for last weeks delegates
Passing parameters to a coroutine in Lua
Seeing Guadalajara - a first glimpse downtown
What are closures in Lua?
Floor to ceiling
Learning to write high quality code in Lua
Lua training class in Spanish
A new place in pictures
Breakfast in Mexico