Question

I’m completely confused by Lua‘s variable scoping and function argument passing (value or reference).

See the code below:

local a = 9        -- since it's define local, should not have func scope
local t = {4,6}    -- since it's define local, should not have func scope

function moda(a)
  a = 10           -- creates a global var?
end
function modt(t)
  t[1] = 7         -- create a global var?
  t[2] = 8
end

moda(a)
modt(t)
print(a)  -- print 9 (function does not modify the parent variable)
print(t[1]..t[2])  -- print 78 (some how modt is modifying the parent t var) 

As such, this behavior completely confuses me.

• Does this mean that table variables
  are passed to the function by
  reference and not value?

• How is the global variable creation
  conflicting with the already define
  local variable?

  • Why is modt able to
    modify the table yet moda is not able
    to modify the a variable?

Answer 1

You guessed right, table variables are passed by reference. Citing Lua 5.1 Reference Manual:

There are eight basic types in Lua: nil, boolean, number, string, function, userdata, thread, and table.
….

Tables, functions, threads, and (full) userdata values are objects: variables do not actually contain these values, only references to them. Assignment, parameter passing, and function returns always manipulate references to such values; these operations do not imply any kind of copy.

So nil, booleans, numbers and strings are passed by value. This exactly explains the behavior you observe.