/******************************************************************************
* FILE: opt02.c
* DESCRIPTION:
*   Example program that illustrates how certain optimizations can effect 
*   program correctness.  In the nested loop below, if the j loop is the 
*   innermost loop, then the negative exponents on the data will accumulate 
*   until x is rounded off to zero.  If the i loop is the innermost loop, 
*   then the exponent of x never becomes so small as to be rounded off to 
*   zero.  
******************************************************************************/
#include <stdio.h>

double a[4][4] = {3.234e-80, 3.234e-70, 3.234e-60, 3.234e-50,
                  3.234e-70, 3.234e-60, 3.234e-50, 3.234e-40,
                  3.234e70,  3.234e60,  3.234e50,  3.234e40,
                  3.234e80,  3.234e70,  3.234e60,  3.234e50};
main()
{
   double x;
   int i, j;

   x = 1.0;
   for(i=0;i<4;i++)
      for(j=0;j<4;j++)
         x = x * a[i][j];
   printf("Result of the original loop is %f\n", x);
/*****************************************************************************/
/*	Interchange the next two loops to witness a vast change in x	     */
/*****************************************************************************/
   x = 1.0;
   for(j=0;j<4;j++)
      for(i=0;i<4;i++)
         x = x * a[i][j];
   printf("Result of the interchanged loop is %f\n", x);
}