C - Pass multiple function addresses as parameters in a variadic function - c

Question

I'm trying to write a function that will take the first n integers and a variable number of functions and build a table that has the number as "i" in the first column and "function(i)" in the others.
But I cannot seem to be able to pass the addresses of my functions to the table generator because I get an access violation error. What did I do wrong?
#include <stdio.h>
#include <math.h>
#include <stdarg.h>
typedef float(*f)(float);
// Some examples of f-type functions.
float square(float x) { return x*x; };
float root(float x) { return sqrt(x); };
float timesPi(float x) { return x * 3.14; };
// Display a table with first colon being the numbers from 1 to n,
// then the other columns to be f(i)
void table(unsigned int n, unsigned int nr_functions, ...)
{
va_list func;
va_start(func, nr_functions);
for (float i = 1; i <= n; i += 1)
{
printf("\n%6.0f |", i);
for (unsigned int j = 0; j < nr_functions; j++)
{
f foo = va_arg(func, f);
printf("%6.3f |", foo(i));
}
va_end(func);
}
}
// Main function
int main()
{
table(5, 3, &square, &root, &timesPi);
system("pause");
return 0;
}
For the example above
table(5, 3, &square, &root, &timesPi);
I want to get back
1 | 1.000 | 3.140 |
2 | 1.141 | 6.280 |
3 | 1.732 | 9.420 |
4 | 2.000 | 12.560 |
5 | 2.236 | 15.700 |

Answer 1

You need to reuse the variable section of the argument list, which means you need the va_start() and va_end() in the right places — inside the outer loop:
void table(unsigned int n, unsigned int nr_functions, ...)
{
for (unsigned int i = 1; i <= n; i++)
{
va_list func;
printf("\n%6.0f |", (double)i);
va_start(func, nr_functions);
for (unsigned int j = 0; j < nr_functions; j++)
{
f foo = va_arg(func, f);
printf("%6.3f |", foo(i));
}
va_end(func);
}
}
Otherwise, you're marching off the end of the list, except that you called va_end() inside the loop, leading to goodness only knows what damage.
Note that the loop should use integer arithmetic — with a consequential change to the printf() — here I cast the value, but changing the format to %6d would also be sane (possibly better, in fact).
With this function, I got the output:
1 | 1.000 | 1.000 | 3.140 |
2 | 4.000 | 1.414 | 6.280 |
3 | 9.000 | 1.732 | 9.420 |
4 |16.000 | 2.000 |12.560 |
5 |25.000 | 2.236 |15.700 |