This first edition was written for Lua 5.0. While still largely relevant for later versions, there are some differences.
The third edition targets Lua 5.2 and is available at Amazon and other bookstores.
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5.2 – Variable Number of Arguments

Some functions in Lua receive a variable number of arguments. For instance, we have already called print with one, two, and more arguments.

Suppose now that we want to redefine print in Lua: Perhaps our system does not have a stdout and so, instead of printing its arguments, print stores them in a global variable, for later use. We can write this new function in Lua as follows:

    printResult = ""
    function print (...)
      for i,v in ipairs(arg) do
        printResult = printResult .. tostring(v) .. "\t"
      printResult = printResult .. "\n"
The three dots (...) in the parameter list indicate that the function has a variable number of arguments. When this function is called, all its arguments are collected in a single table, which the function accesses as a hidden parameter named arg. Besides those arguments, the arg table has an extra field, n, with the actual number of arguments collected.

Sometimes, a function has some fixed parameters plus a variable number of parameters. Let us see an example. When we write a function that returns multiple values into an expression, only its first result is used. However, sometimes we want another result. A typical solution is to use dummy variables; for instance, if we want only the second result from string.find, we may write the following code:

    local _, x = string.find(s, p)
    -- now use `x'
An alternative solution is to define a select function, which selects a specific return from a function:
    print(string.find("hello hello", " hel"))         --> 6  9
    print(select(1, string.find("hello hello", " hel"))) --> 6
    print(select(2, string.find("hello hello", " hel"))) --> 9
Notice that a call to select has always one fixed argument, the selector, plus a variable number of extra arguments (the returns of a function). To accommodate this fixed argument, a function may have regular parameters before the dots. Then, Lua assigns the first arguments to those parameters and only the extra arguments (if any) go to arg. To better illustrate this point, assume a definition like
    function g (a, b, ...) end
Then, we have the following mapping from arguments to parameters:
    CALL            PARAMETERS
    g(3)             a=3, b=nil, arg={n=0}
    g(3, 4)          a=3, b=4, arg={n=0}
    g(3, 4, 5, 8)    a=3, b=4, arg={5, 8; n=2}

Using those regular parameters, the definition of select is straightforward:

    function select (n, ...)
      return arg[n]

Sometimes, a function with a variable number of arguments needs to pass them all to another function. All it has to do is to call the other function using unpack(arg) as argument: unpack will return all values in arg, which will be passed to the other function. A good example of this use is a function to write formatted text. Lua provides separate functions to format text (string.format, similar to the sprintf function from the C library) and to write text (io.write). Of course, it is easy to combine both functions into a single one, except that this new function has to pass a variable number of values to format. This is a job for unpack:

    function fwrite (fmt, ...)
      return io.write(string.format(fmt, unpack(arg)))