#include <stdio.h>
#include <math.h>

double derivative1( double f( double ), double x )
{
  double delta = x / 1000000;
  return ( f( x + delta ) - f( x ) ) / delta;
}

double derivative2( double (*f)( double ), double x )
{
  double delta = x / 1000000;
  return ( (*f)( x + delta ) - (*f)( x ) ) / delta;
}

int main( void )
{
  printf( "Deriv is %f\n", derivative1( sqrt, 1.0 ) );
  printf( "Deriv is %f\n", derivative1( (*sqrt), 1.0 ) );
  printf( "Deriv is %f\n", derivative2( sqrt, 1.0 ) );
  printf( "Deriv is %f\n", derivative2( (*sqrt), 1.0 ) );
}