Transcript Variable Length Arguments

Variable Argument Lists
CS 112
(slides from Marc Lihan)
Definition
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Also called Variadic function
Methods containing parameter(s) that can
accept different number of arguments
Support for variadic functions differs among
different programming languages
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C
C#
LISP
PHP
- C++
- Java
- Visual Basic
C Programming Language
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Most common variable argument functions
are
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void printf(const char* fmt, …);
void scanf(const char* fmt, …);
Parameters include
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One named argument usually a format string
Ellipsis indicating the function may take any
number of arguments and their types
Creating a Variadic Function
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Use standard header stdarg.h
4 macros (not prototypes) needed from
this header file
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va_list
va_start
va_args
va_end
Variable arguments in C
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va_list
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Stores the list of arguments (argument pointer)
Declared like any other variable
Ex. va_list ap
va_start
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initializes the list, point to the first argument
Accepts two arguments
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va_list
Name of the variable that directly precedes the ellipsis
Ex. va_start(ap, fmt)
Variable arguments in C
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va_args
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returns the next argument of whatever type it is told
moves down to the next argument in the list
Accepts two arguments
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va_list
Variable argument type
Ex. va_args(ap, int) //returns next int value in the argument
va_end
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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#
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Variadic functions are also possible
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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
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public void sum(params int[] nums)
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];
OUTPUT:
return total;
Number of arguments: 0
}
Sum of the first problem is 0
}
Number of arguments: 5
Sum of the second problem is 25
Java
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Newly implemented feature Java 5.0
Basic syntax
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type … variableName
Argument passed to a method is
converted into an array of the sametyped values
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sum (10,20) sum(new int[] {10,20})
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];
OUTPUT:
return total;
The sum is 60
}
Formatting in Java 1.5
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Employs variable arguments
printf() and format() method of PrintStream
and String
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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)
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“%.2f” round of to the nearest hundredths: 1234.57
“%04d” integer should be formatted with a minimum of
4 digits: 01234
Visual Basic
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Parameter array allows a procedure
to accept an array of values for an
argument.
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Must be passed by value.
Syntax
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Sub MethodName
(ByVal ParamArray array() As String)
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 )
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OUTPUT: The sum is 83.
Caveats
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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
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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
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C
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Java
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… method( type … varName )
C#
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… function_name( type first_arg, … )
… Method( params type num[] )
Visual Basic
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Sub MethodName(ByVal ParamArray array() As String)
PHP
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Unlike other programming language, method
signatures of PHP are purely for syntax checking
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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
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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