Diff.c

#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#define MAXN 10
#define PI   3.14159

int main (void) {
  int i;
  double a=0, b=2.0, h;
  double x[MAXN + 1], f[MAXN + 1], df[MAXN + 1];
  
  h = (b - a)/MAXN;
  x[0] = a;
  for (i=1; i< MAXN; i++)
    x[i] = x[i-1] + h;
  x[i] = b;

  for (i=0; i<= MAXN; i++)
    f[i] = x[i] * x[i] + 1;

  df[0] = ( f[1] - f[0] )/( x[1] - x[0] );  
  for (i=1; i< MAXN; i++)
    df[i] = ( f[i+1] - f[i-1] )/( x[i+1] - x[i-1] );  
  df[i] = ( f[i] - f[i-1] )/( x[i] - x[i-1] );  

  printf("- - - - - - - - - - - - - - - - - - - -\n");
  printf("  i    x[i]       f[i]       df[i]\n");
  printf("- - - - - - - - - - - - - - - - - - - -\n");
  for (i=0; i<= MAXN; i++)
    printf("%3d %10lf %10lf %10lf\n",i,x[i],f[i],df[i] );
  printf("- - - - - - - - - - - - - - - - - - - -\n");


  return EXIT_SUCCESS;
}