Variadic function

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"Varargs" redirects here. For the varargs.h library in C, see varargs.h.

In computer programming, a variadic function is a function of variable arity; that is, one which can take different numbers of arguments. Support for variadic functions differs widely among programming languages.

There are many mathematical and logical operations which come across naturally as variadic functions. For instance, the summing of numbers or the concatenation of strings or other sequences are operations that can logically apply to any number of operands.

Another operation which has been implemented as a variadic function in many languages is output formatting. The C function printf and the Common Lisp function format are two such examples. Both take one argument which specifies the formatting of the output, and any number of arguments which provide the values to be formatted.

Variadic functions can expose type-safety problems in some languages. For instance, C's printf, if used uncautiously, can give rise to a class of security holes known as format string attacks. The attack is possible because the language support for variadic functions is not type-safe; it permits the function to attempt to pop more arguments off the stack than were placed there -- corrupting the stack and leading to unexpected behavior.

Contents

[edit] Specific implementations

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The following provides an overview of specific implementations in different programming languages and environments.

[edit] Variadic functions in C and C++

To portably implement variadic functions in the C programming language, the standard stdarg.h header file should be used. The older varargs.h header has been deprecated in favor of stdarg.h. In C++, the header file cstdarg should be used.

The variadic function feature is going to be present in the upcoming C++ language standard, C++0x; this feature is called "variadic templates". This allows the creation of variadic template classes and variadic template functions.

template<typename T>
void print( const T& t )
{
        std::cout << t;
}
 
template< typename T, typename... Args>
void print( const T& t, const Args&... args )
{
       print( t );
       print( args... );
}
 
template<typename... T>
class tuple;
template<typename T, typename... Ts>
class tuple<T,Ts...>;
template<>
class tuple<>;

[edit] Variadic functions in C# and Java

Other languages, such as C# and Java use a different approach—they just allow that a variable number of arguments of the same (super)type may be passed to a variadic function. Inside the method they are simply collected in an array.

This feature, called "varargs", has been present in Java since J2SE 5.0. Using autoboxing, this can also be used to pass primitive arguments to for example a print function, as can be seen in the Java example below:

public static void printSpaced(Object... objects) {
   for (Object o : objects)
     System.out.print(o + " ");
 }
 
 // Can be used to print:
 // printSpaced(1, 2, "three");

C# Example:

public static void PrintSpaced(params Object[] objects)
{
    foreach (Object o in objects)
        Console.Write(o + " ");
}
 
 // Can be used to print:
 // PrintSpaced(1, 2, "three");

[edit] Variadic functions in JavaScript

In JavaScript, the arguments to a function may be accessed as local variables within the function. They also exist as members of a local array within the function, called arguments [1].

function printSpaced() {
   for (var i = 0; i < arguments.length; i++) {
     document.write(arguments[i] + " ");
   }
 }

[edit] Variadic functions in Perl

In Perl 5, functions don't have argument lists (except optional prototypes that are used for limited type checking). All arguments are stored in an array called @_, which can be accessed in the function. Therefore no special effort is needed to accept variable number of arguments.

sub print_spaced {
  my @objects = @_;
  print "@objects\n";
}
 
sub print_names {
  my ($title, @names) = @_;
  print "This is the list of $title names:\n"
  foreach (@names) {
    print "Name: $_\n";
  }
}
 
print_spaced(1, 2, "three");

[edit] Variadic functions in PHP

In PHP, variable-length argument lists are natively supported (without security risks) since version 4; dedicated functions (func_num_args, func_get_arg, func_get_args) allow the programmer to determine the number and values of unspecified arguments.

[edit] Variadic functions in Python

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Further information might be found on the talk page or at requests for expansion.

Python supports very flexible variadic functions. By marking variables with one asterisk (e.g. *var) the given variable is defined to be a tuple of all the extra arguments. By marking variables with two asterisks (e.g. **var) the given variable is a dictionary of all extra keyword arguments; the keys are strings, which are the names that were. Conventionally these are called "args" and "kwargs" respectively, but they may be something else, and packages often make good use of this ability to improve readability (e.g. BeautifulSoup). If they exist, these arguments must be the last one in the list.

def f(*args, **kwargs):
    print args
    print kwargs
 
>>> f(1, 2, "cow", "kitty")
(1, 2, "cow", "kitty")
{}
 
>>> f(arg1=1, sample=2, name="cow", hero="kitty")
()
{"arg1": 1, "sample": 2, "name": "cow", "hero": "kitty"}
 
>>> f(1, 2, name="cow", hero="kitty")
(1, 2)
{"name": "cow", "hero": "kitty"}
 
>>> f(arg1=1, sample=2, name="cow", "kitty")
SyntaxError "Non-keyword arg after keyword arg"

Conversely you may also pass in a tuple or dictionary using the same asterisk-notation and have it automatically expand to fill.

def g(a, b, c):
    print a, b, c
 
>>> mytuple = 1,2,3
>>> mydict = {"a": "first", "b": "second", "c": "third"}
>>> g(*mytuple)
1 2 3
>>> g(**mydict)
first second third
>>> g(**{"a": "first"})
TypeError "g() takes exactly 3 non-keyword arguments (got 1)"
>>> g(**{"a": "first", "b": "second", "c": "third", "d": "fourth"})
TypeError "g() got an unexpected keyword argument 'd'"

[edit] Variadic functions in S-Lang

S-Lang supports two mechanisms for passing optional arguments to a function. The two mechanisms are illustrated by the readascii function that is distributed as part of the S-Lang library. The function is used to read one or more data columns from an ascii (as opposed to binary) file:

readascii ("xy.dat", &x, &y);
readascii ("xydy.dat", &x, &y, &dy);
readascii ("score.dat", &name, &score, &date, &flags;
            format="%s %lf %s %d", skip=5);

The optional keyword/value pairs that follow the semi-colon in the argument list are called qualifiers in S-Lang parlance.

When a function is called, the "_NARGS" variable contains the number of arguments passed to the function. This value does not include the number of qualifiers. Here is a simple example that performs the sum of the arguments (assumed here to be numeric) passed to a function:

define sum_values ()
{
   if (_NARGS == 0)
     usage ("result = sum_values (val1, val2, ...);");
   variable args = __pop_args (_NARGS);
   variable total = 0, arg;
   foreach arg (args) total += arg.value;
   return total;
}
vmessage ("Sum = %g", sum_values (1,2,3,4,5));

Here is a variant of the above that uses a qualifier to alternate between the sum and difference of the successive arguments. This example makes use of the built-in sum function to add the elements of an array.

define sum_values ()
{
   if (_NARGS == 0)
     usage ("result = sum_values (val1, val2, ... ; alternate);");
   variable args = __pop_args (_NARGS);
 
   % Create an array from the arguments
   variable array = [__push_args(args)];
 
   % If the sum is to alternate, multiply the odd elements by -1
   if (qualifier_exists ("alternate"))
     array[[1::2]] *= -1;
 
   return sum(array);
}
vmessage ("Sum = %g", sum_values (1,2,3,4,5 ; alternate));

[edit] Variadic functions in Ruby

Ruby supports variadic functions natively. An asterisk before the last parameter in the function declaration declares that parameter to be an array into which all remaining input arguments are placed.

def printNames(listTitle, *nameList)
  puts "This is the list of #{listTitle} names:"
  nameList.each do |name|
    puts "Name: #{name}"
  end
end
 
printNames("employee", "John", "Bob", "Chris")
 
#This code produces the following output:
#
#This is the list of employee names:
#Name: John
#Name: Bob
#Name: Chris

[edit] Variadic functions in Scheme

In Scheme, there is a dotted notation for representing improper lists (chains of pairs whose last cdr is not null). You can use this notation in argument lists to specify an argument to "dump" the rest of the arguments into.

If a function is defined with the "define" syntax, then you put a dot before the varargs argument

(define (print-spaced . args)
  (for-each
    (lambda (x) (display x) (display " "))
    args))
 
(define (print-names title . names)
  (display "This is the list of ")
  (display title)
  (display " names:")
  (newline)
  (for-each
    (lambda (x) (display "Name: ") (display x) (newline))
    names))
 
(print-spaced 1 2 "three")

If a function is defined with the "lambda" syntax, then it is the same thing, except that if there are no arguments before the varargs argument (i.e. if you want to put all the arguments into one thing), then you remove the parentheses altogether; as if to say, let this be the entire argument list.

(define print-spaced
  (lambda args
    (for-each
      (lambda (x) (display x) (display " "))
      args)))
 
(define print-names
  (lambda (title . names)
    (display "This is the list of ")
    (display title)
    (display " names:")
    (newline)
    (for-each
      (lambda (x) (display "Name: ") (display x) (newline))
      names)))

[edit] See also