Variable Argument Lists

1. Variable Argument Lists CS 112 (slides from Marc Lihan)

2. Definition • Also called Variadic function • Methods containing parameter(s) that can accept different number of arguments • Support for variadic functions differs among different programming languages • C - C++ • C# - Java • LISP - Visual Basic • PHP

3. C Programming Language • Most common variable argument functions are • void printf(const char* fmt, …); • void scanf(const char* fmt, …); • Parameters include • One named argument usually a format string • Ellipsis indicating the function may take any number of arguments and their types

4. Creating a Variadic Function • Use standard header stdarg.h • 4 macros (not prototypes) needed from this header file • va_list • va_start • va_args • va_end

5. Variable arguments in C • va_list • Stores the list of arguments (argument pointer) • Declared like any other variable • Ex. va_list ap • va_start • initializes the list, point to the first argument • Accepts two arguments • va_list • Name of the variable that directly precedes the ellipsis • Ex. va_start(ap, fmt)

6. Variable arguments in C • va_args • returns the next argument of whatever type it is told • moves down to the next argument in the list • Accepts two arguments • va_list • Variable argument type • Ex. va_args(ap, int) //returns next int value in the argument • va_end • Cleans up the variable argument list • Ex. va_end(ap)

7. C Sample Code #include <stdio.h> #include <stdarg.h> double average ( int num, ... ) { va_list ap; double sum = 0; va_start ( ap, num ); //initializes, store all values int x; for ( x = 0; x < num; x++ ) sum += va_arg ( ap, double ); va_end ( ap ); return sum/num; } int main() { printf( "%f\n", average ( 3, 12.2, 22.3, 4.5 )); printf("%f\n", average ( 5, 3.3, 2.2, 1.1, 5.5, 3.3 )); return 0; }

8. C# • Variadic functions are also possible • Console.WriteLine(“Hi {0} and {1} nice to meet you.”, “John”, “Marc” ); • Output: Hi John and Marc nice to meet you. • C# supports this behavior with the params keyword • public void sum(params int[] nums)

9. Code Sample using System; class Total { static void Main(string[] args) { //call sum with no arguments, optional parameter Console.WriteLine( "Sum of the first problem is {0}", Sum()); //call with 5 arguments Console.WriteLine( "Sum of the second problem is {0}", Sum(3,2,6,5,9)); Console.ReadLine(); } public static int Sum(params int[] args) { int total = 0; Console.WriteLine("Number of arguments: " + args.GetLength(0).ToString()); for (int i = 0; i<args.GetLength(0); i++) total += args[i]; return total; } } OUTPUT: Number of arguments: 0 Sum of the first problem is 0 Number of arguments: 5 Sum of the second problem is 25

10. Java • Newly implemented feature Java 5.0 • Basic syntax • type … variableName • Argument passed to a method is converted into an array of the same-typed values • sum (10,20) sum(new int[] {10,20})

11. Sample Code public static void main(String[] args) { System.out.println(“The sum is ” + sum(10,20,30)); } public int sum (double … numbers) { int total = 0; for (int i = 0; i < numbers.length; i++) total += numbers[i]; return total; } OUTPUT:The sum is 60

12. Formatting in Java 1.5 • Employs variable arguments • printf() and format() method of PrintStream and String • System.out.printf( “%5d %6.2f”, 23, 45.6 ); • Formats data values of arbitrary data types and outputs the formatted results • String s = String.format(“%.2f”, 1234.567) • “%.2f” round of to the nearest hundredths: 1234.57 • “%04d” integer should be formatted with a minimum of 4 digits: 01234

13. Visual Basic • Parameter array allows a procedure to accept an array of values for an argument. • Must be passed by value. • Syntax • Sub MethodName(ByVal ParamArray array() As String)

14. Sample Code Sub Sum(ByVal ParamArray Nums() As Integer) Dim I As Integer Dim Total As Integer For I = 0 To UBound(Nums) Total = Total + Nums(I) Next I Debug.WriteLine("The sum is " & Total & ".") End Sub • Sum( 10, 26, 32, 15 ) • OUTPUT: The sum is 83.

15. Caveats • No enforceable limit to the number of arguments without writing the code at runtime inside the function itself • No way to make the array typesafe • If multiple types are needed use the LCD approach, usually array as type Object • Only one parameter may be marked using params or ellipsis • It must be the last parameter • The arguments are automatically optional, it can be one, many or none at all

16. Syntax Summary • C • … function_name( type first_arg, … ) • Java • … method( type … varName ) • C# • … Method( params type num[] ) • Visual Basic • Sub MethodName(ByVal ParamArray array() As String)

17. PHP • Unlike other programming language, method signatures of PHP are purely for syntax checking • Will never complain if you call a function with more parameters than what's defined • It will only complain if you call it with less • Related methods in PHP • func_get_args(void) – returns an array comprising a function’s argument list • func_num_args(void) – returns the total length of the array • func_get_arg(int arg_num) – returns an item in the argument list

18. PHP Sample Code <?php function average() {   $array = func_get_args(); $sum = array_sum($array); $num = func_num_args(); if ($num == 0) return 0; else return $sum/$num;}echo average(10, 15, 20); ?> • array_sum() – calculates the sum of the values in an array • OUTPUT: 15