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variable argument list

Posted on 2004-09-14
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Last Modified: 2010-04-01
Hello

I want my function to take in a varialbe number of arguments,
so for example if i had just one argument  I would call it by
function(1, arg1)

or if I had 2 arguments then I'd call
function(2, arg1, arg2)

or 3 then
function(3, arg1, arg2, arg3)
etc...

so I would have to have something like a switch statement to handle these variable cases

but what if I could possibly have 200 arguments
then how would I handle this?

i.e., how would I have a variable arugment list that doesnt require knowing the number of arguments in case the number of arguments could be from 1 to a million!

thanks..
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Question by:lost_bits1110
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7 Comments
 

Author Comment

by:lost_bits1110
ID: 12057383
or if someone could let me know if its possible at all...
Thanks
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Expert Comment

by:jkr
ID: 12057413
There are a few ways using 'va_arg', e.g.

/* VA.C: The program below illustrates passing a variable
 * number of arguments using the following macros:
 *      va_start            va_arg              va_end
 *      va_list             va_dcl (UNIX only)
 */

#include <stdio.h>
#define ANSI            /* Comment out for UNIX version     */
#ifdef ANSI             /* ANSI compatible version          */
#include <stdarg.h>
int average( int first, ... );
#else                   /* UNIX compatible version          */
#include <varargs.h>
int average( va_list );
#endif

void main( void )
{
   /* Call with 3 integers (-1 is used as terminator). */
   printf( "Average is: %d\n", average( 2, 3, 4, -1 ) );

   /* Call with 4 integers. */
   printf( "Average is: %d\n", average( 5, 7, 9, 11, -1 ) );

   /* Call with just -1 terminator. */
   printf( "Average is: %d\n", average( -1 ) );
}

/* Returns the average of a variable list of integers. */
#ifdef ANSI             /* ANSI compatible version    */
int average( int first, ... )
{
   int count = 0, sum = 0, i = first;
   va_list marker;

   va_start( marker, first );     /* Initialize variable arguments. */
   while( i != -1 )
   {
      sum += i;
      count++;
      i = va_arg( marker, int);
   }
   va_end( marker );              /* Reset variable arguments.      */
   return( sum ? (sum / count) : 0 );
}
#else       /* UNIX compatible version must use old-style definition.  */
int average( va_alist )
va_dcl
{
   int i, count, sum;
   va_list marker;

   va_start( marker );            /* Initialize variable arguments. */
   for( sum = count = 0; (i = va_arg( marker, int)) != -1; count++ )
      sum += i;
   va_end( marker );              /* Reset variable arguments.      */
   return( sum ? (sum / count) : 0 );
}
#endif
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Author Comment

by:lost_bits1110
ID: 12057565
Okay, but how in my code could I make your following statement

printf( "Average is: %d\n", average( 2, 3, 4, -1 ) )

more general?

because what if I have say 200 arguments then I would have to do

if (numArgs == 1)
printf( "Average is: %d\n", average( 1,1) );
if (numArgs == 2)
printf( "Average is: %d\n", average( 2, 1, 2 ) );
....etc....
if (numArgs == 200)
printf( "Average is: %d\n", average( 200, 3, 4, -1, etc.............. ) );

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LVL 86

Expert Comment

by:jkr
ID: 12057587
You mean you want to arrange/create the arguments at runtime?
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Author Comment

by:lost_bits1110
ID: 12057632
yes precisely......
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LVL 86

Accepted Solution

by:
jkr earned 50 total points
ID: 12057689
Well, then, a variable list of arguments is not what you want - you'd pass an array of arguments instead, e.g.

int average ( int a[], int count) {

    int sum = 0;

    for ( int i = 0; i < count; ++i) sum += a[i];

    return sum / count;
}

int an [] = { 1,2,3,4,5,6,7,8,9};

int avg;

avg = average ( an, 4);


avg = average ( an, 9);

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Author Comment

by:lost_bits1110
ID: 12058450
okay, even if the arguments i'm passing are arrays?
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