Variable Argument Lists

CS 112

(slides from Marc Lihan)

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

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

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

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)

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)

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;

}

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)

Code Sampleusing System;class Total{

static void Main(string[] args){

//call sum with no arguments, optional parameterConsole.WriteLine( "Sum of the first problem is {0}", Sum());//call with 5 argumentsConsole.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: 0Sum of the first problem is 0Number of arguments: 5Sum of the second problem is 25

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})

Sample Codepublic 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

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

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)

Sample CodeSub Sum(ByVal ParamArray Nums() As Integer)

Dim I As IntegerDim Total As IntegerFor I = 0 To UBound(Nums)

Total = Total + Nums(I) Next IDebug.WriteLine("The sum is " & Total & ".")

End Sub Sum( 10, 26, 32, 15 )

OUTPUT: The sum is 83.

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

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)

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

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