// testing one function at two points,
//  using function pointer arguments to a function
//
#include <stdio.h>
#include <math.h>

double my_sqrt( double x) { return exp(log(x)/2); }

typedef double (*Func)(double);

void test( const char *s, Func f, Func g, double x) {
  double y1 = f(x); // or: (*f)(x)
  double y2 = g(x);
  printf( "%s %g %g %g\n", s, x, y1, y2);
}

int main( void)
{
  test( "sqrt", my_sqrt, sqrt, 2);
  test( "sqrt", my_sqrt, sqrt, 10);
}

/* output:

sqrt 2 1.41421 1.41421
sqrt 10 3.16228 3.16228

*/