9.2 Fundamentals of Subprograms 395Lua uses a simple mechanism for supporting a variable number of param-
eters—such parameters are represented by an ellipsis (.. .). This ellipsis can be
treated as an array or as a list of values that can be assigned to a list of variables.
For example, consider the following two function examples:function multiply (.. .)
local product = 1
for i, next in ipairs{.. .} do
product = product * next
end
return sum
endipairs is an iterator for arrays (it returns the index and value of the elements
of an array, one element at a time). {.. .} is an array of the actual parameter
values.function DoIt (.. .)
local a, b, c =......
end
Suppose DoIt is called with the following call:doit(4, 7, 3)In this example, a, b, and c will be initialized in the function to the values 4 ,
7 , and 3 , respectively.
The three-period parameter need not be the only parameter—it can appear
at the end of a list of named formal parameters.9.2.4 Procedures and Functions
There are two distinct categories of subprograms—procedures and functions—
both of which can be viewed as approaches to extending the language. All sub-
programs are collections of statements that define parameterized computations.
Functions return values and procedures do not. In most languages that do not
include procedures as a separate form of subprogram, functions can be defined not
to return values and they can be used as procedures. The computations of a proce-
dure are enacted by single call statements. In effect, procedures define new state-
ments. For example, if a particular language does not have a sort statement, a user
can build a procedure to sort arrays of data and use a call to that procedure in place
of the unavailable sort statement. In Ada, procedures are called just that; in Fortran,
they are called subroutines. Most other languages do not support procedures.
Procedures can produce results in the calling program unit by two meth-
ods: (1) If there are variables that are not formal parameters but are still visible