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	TABLE OF CONTENTS Introduction Rules in C Control Another Way to Control Execution

Rules in C

In most rule-based systems the rules are written in a language resembling English, like the rule in the previous example, and the rule interpreter converts them into language the machine can deal with. To dispense with the rule interpreter, we are going to write our rules directly in C. To someone familiar with the language these rules will be easy to read, and judicious use of the preprocessor can make them even clearer. (Similar techniques could be used in many programming languages.) Here is the first rule from Listing One.

if (av_speed > 60)
 fly =1:

When the function rules() is called, the global variable av_speed has a value calculated from the numbers entered by the user. The variable fly has its initial value, which is 0. If av_speed is less than or equal to 60, the rule does not fire and fly keeps the value of 0.

Nothing here is different from a normal C program. We are just using different names and structuring things a bit differently.

#include "stdio.h"

int av_speed;

char like_scenery = 0,
   is_pilot = 0,
   fly = 0,
   drive = 0,
   fly_com = 0,
   fly_tcart = 0,
   fly_bon = 0,
   m_cycle = 0,
   car = 0;

main()

{
   int done,
      distance,  /* in miles */
      time:      /* in hours */
   char c;
   char str[10];
   printf("This is a program to help with travel planning. \n");
   printf("\n\n\nHow far are you going? (miles)\n");
   gets(str);
   distances = atio(str);
   printf("\n\nHow much time do you have for the trip? (hours)\n:);
   gets(str);
   time = atio(str);
   av_speed = distance/time;
   printf("\n%d\n",av_speed);
   printf("Do you prefer scenery over speed? (Y/N)\n");
   gets(str);
   if(str[0] = 'Y')   
      like_scenery = 1;
   printf("Are you a pilot? (Y/N)\n");
   gets(str);
   if(str[0] = 'Y')
      is_pilot = 1;
   rules();
   if(fly_com)
      printf("\nFly commercial.");
   if(fly_tcart)
      printf("\nRent a Taylorcraft and fly low.");
   if(fly_bon)
      printf("\nRent a Bonanza and fly high.");
   if(m_cycle)
      printf("\nTake your motorcycle and ride the back roads.");
   if(car)
      printf("\nThere's nothing for it but to drive a car.");
}

rule(s)
{
   if(av_speed > 60)
     fly = 1;
   if (av_speed) <= 60)
     drive = 1;
   if (fly
      && !is_pilot)
         fly_com = 1;
   if(fly
     && is_pilot
     && like_scenery
     && av_speed < 100
        fly_tcart = 1;
   if(fly
   && is_pilot
   && (100 < av_speed)
   && (av_speed < 200))
       fly_bon = 1;
   if(drive
   && like_scenery)
      m_cycle = 1;
   if(drive
   && !m_cycl)
      car = 1;
}

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