Re: sum(2,4,3,5,-1,...)
- From: "Michael Tiomkin" <tmk@xxxxxxxxxxxxxxxx>
- Date: 10 Aug 2006 09:01:03 GMT
Allan Adler wrote:
I don't understand how to write functions that have variable numbers
of arguments. The only example I have to imitate is the one on p.289
of C: A Reference Manual, 2d ed (1987), by Harbison and Steele, for
a function named printargs, which is also my only source of information
about how to write such functions. I tried it on my PC running RH 7.1
and it compiles and runs fine. It depends, however, on an array at[]
for which &at[0] is the first argument. There is no natural choice
of an array related to the first argument of the function sum I'm
trying to write, but GCC complains that the first argument to sum
is not a pointer, so I think I have to give it one. Basically, I
wrote sum by imitating the code for printargs mentioned above.
#include <stdio.h>
#include <stdarg.h>
int sum(int *argtypep, ...)
{
va_list ap;
int argtype;
int total=0;
va_start(ap,argtypep);
while((argtype = *argtypep++) != 0) total += va_arg(ap,int);
va_end(ap);
return total;
}
The function sum is supposed to take a variable number of arguments
of type int and add them up. Thus, sum(0) returns 0 and sum(3,5,2,-7)
returns 3.
You cannot do this in C. In most implementations of C, e.g. using the
stack to push the arguments from right to left, the function with
ellipsis cannot determine what is the number of its arguments unless
there are some constraints on them.
One example: the number of arguments is the first arg:
int sum (int num_args, ...);
....
... sum(4, 3,5,2,-7) ... /* will return 3 */
Other example: you set a special number to end the sequence:
int sum(int first, ...); /* put -9999 at the end */
...
.... sum(3,5,2,-7,-999) ... /* will return -3 */
The problem with these solutions is that you cannot call sum(), with
no args, and get 0, like you can do in Scheme or C++.
In C++ you can use the default arguments for the 1st style, with the
number of args to sum:
int sum(int num_args=0, ...);
In this case your 'sum()' will return 0.
Michael
--
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.
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