The formal parameter list

Note the syntax of the formal parameter list for the function in Listing 3.

An empty subscript operator is used in the declaration of the first parameter to specify that the first parameter is an array.

(As mentioned earlier, this is really a reference to the array rather than a copy of the array although that isn't shown explicitly by the declaration syntax.)

In this case, the second parameter is the size of the array.  This value will be used in the conditional clause of a while loop that gets integer input values from the user and stores those values in the elements of the array.