# include <stdio.h>

void version1 ( );
void version2 ( );

int main ( )
{
  version1 ( );
  version2 ( );

  return 0;
}
void version1 ( )
{
  int i;
  int j;
  float x_old[5] = { 1.0, 1.0, 2.0, 2.0, 10.0 };
  float x_new[5];

  for ( i = 1; i <= 10; i++ )
  {
//
//  New data.
//
    x_new[0] = x_old[0];
    for ( j = 1; j < 4; j++ )
    {
      x_new[j] = x_old[j] + 0.1 * ( x_old[j-1] - 2 * x_old[j] + x_old[j+1] );
    }
    x_new[4] = x_old[4];
//
//  Update.
//
    for ( j = 0; j < 5; j++ )
    {
      x_old[j] = x_new[j];
    }
  }
//
//  Print final result.
//
  printf ( "\n" );
  printf ( "Version 1 final result:\n" );
  printf ( "\n" );
  for ( j = 0; j < 5; j++ )
  {
    printf ( "  %g", x_old[j] );
  }
  printf ( "\n" );

  return;
}
void version2 ( )
{
  int i;
  int j;
  float *new;
  float *old;
  float *temp;
  float x1[5] = { 1.0, 1.0, 2.0, 2.0, 10.0 };
  float x2[5];
//
//  Where is the "old" information initially?
//
  old = x1;
  new = x2;

  for ( i = 1; i <= 10; i++ )
  {
//
//  New data.
//
    new[0] = old[0];
    for ( j = 1; j < 4; j++ )
    {
      new[j] = old[j] + 0.1 * ( old[j-1] - 2 * old[j] + old[j+1] );
    }
    new[4] = old[4];
//
//  Swap the pointers.
//
    temp = old;
    old = new;
    new = temp;
  }
//
//  Print final result.
//
  printf ( "\n" );
  printf ( "Version 2 final result:\n" );
  printf ( "\n" );
  for ( j = 0; j < 5; j++ )
  {
    printf ( "  %g", old[j] );
  }
  printf ( "\n" );

  return;
}