functions with variable length argument lists

[kalar]

I found many pages about my problem(topic title) and i understand how it works
and all the examples
but after i found an example which i want to explain me
look especially in function maxof and the comments
[PHP]#include <stdio.h>
#include <stdarg.h>
#include <stdlib.h>

main(){
f();

exit(EXIT SUCCESS)
}

void f(void){
int i=5;
int j[256];
j[42]=24;
printf("%d\n", maxof(3, i, j[42], 0);

int maxof(int n_args, ... ){ /*what is n-args */
/* i understand that n_args is one of the numbers(the first) that we pass in the function to find the max*/
register int i;
int max, a;
va_list ap;

max=va_arg(ap,int)/*why set max with i(the second argument) and not with the first which is n_args or 3*/
for(i=2; i <=n_args; i++){
if((a = va_arg(ap, int)) > max
max = a;
}
va_end(ap);
return max;
}
[/PHP]
I think that we don't check if the first argument is max, because we set max in the beginning with the second argument and we check the rest numbers from i to 0.
Where is my mistake?Maybe don't understand the meaning of n_args

[JosAH]

The parameter n_args is simply the number of arguments that should be considered
in determining the maximum number. As in:

int max= f(3, -7, 0, 8);
 

Three numbers are to be considered: -7, 0 and 8. This way you don't need a
special 'sentinel' value such as -9999 (ugly) to indicate the last argument.
The disadvantage is that you can also do this by mistake:

int max= f(2, -7, 0, 8);
 

The number 8 is not considered one of the list for which a maximum must be found.

kind regards,

Jos

[kalar]

The disadvantage is that you can also do this by mistake:

int max= f(2, -7, 0, 8);
 

The number 8 is not considered one of the list for which a maximum must be found.

kind regards,

Jos

Why number 8 ?As you say the first argument(2 in the example) n_args is simply the number of arguments that should be considered.So number 2 is not considered one of the list for which a maximum must be found and not 8.Am i wrong??

[JosAH]

Why number 8 ?As you say the first argument(2 in the example) n_args is simply the number of arguments that should be considered.So number 2 is not considered one of the list for which a maximum must be found and not 8.Am i wrong??

I'm afraid I didn't express myself clear enough: the first parameter of that function
indicates the number of following parameters for which the maximum must be found. So:

int max= f(2, -7, 0, 8)
 

Tells the function to take the first two arguments *following the very first argument*,
i.e. -7 and 0 and find the maximum of them. Number 8 is not considered.

kind regards,

Jos

[kalar]

Thank you very very much ,now i understand it

[JosAH]

Thank you very very much ,now i understand it

Good, you're welcome of course. In general: using C there is no way for a varargs
function to tell how many parameters were actually passed to it, i.e. it has to trust
the caller. I can't remember if C++ could do any better than that ...

kind regards,

Jos