K and R C, Solution to Exercise 6-6:
K and R C Programs Exercises provides the solution to all the exercises in the C Programming Language (2nd Edition). You can learn and solve K&R C Programs Exercise.
C Program to implement a simple version of the #define processor(i.e , no arguments) suitable for use with C programs, based on the routines of this section, You may also find getch and ungetch helpful.Read more about C Programming Language .

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#include<stdio.h>
#include<ctype.h>
#include<string.h>

#define MAXWORD 100

struct nlist{
 struct nlist *next;
 char *name;
 char *defn;
};
void error(int, char*);
int getch(void);
void getdef(void);
int getword(char *,int);
struct nlist *install(char *,char *);
struct nlist *lookup(char *);
void skipblanks(void);
void undef(char *);
void ungetch(int);
void ungets(char *);


//simple version of #define processor
main()
{
 char w[MAXWORD];
 struct nlist *p;
 while(getword(w, MAXWORD) != EOF)
  if(strcmp(w, "#") == 0)
   getdef();
  else if(!isalpha(w[0]))
   printf("%s",w);
  else if ((p == lookup(w)) == NULL)
   printf("%s",w);
  else
   ungets(p->defn);
}

//getdef:get defination and install it
void getdef()
{
 int c,i;
 char def[MAXWORD], dir[MAXWORD], name[MAXWORD];
 skipblanks();
 if(!isalpha(getword(dir,MAXWORD)))
  error(dir[0],"getdef:expecting a directve after #");
 else if(strcmp(dir,"define") == 0) {
  skipblanks();
  if(!isalpha(getword(name, MAXWORD)))
   error(name[0],"getdef:non alpha name expected"):
   else{
    skipblanks();
    for(i = 0;i <  MAXWORD-1; i++)
     if((def[i] = getch()) == EOF || def[i] == 'n')
      break;
    def[i] = '';
    if(i <= 0)
     error('n',"getdef:non alpha in define");
    else
     install(name, def);
   }
 }else if(strcmp(dir,"undef") == 0) {
  skipblanks();
  if(!isalpha(getword(name, MAXWORD)))
   error(name[0],"getdef:non alphain undef"):
   else
    undef(name);
 }else
  error(dir[0],"getdef:expecting a directive after #");
}

//undef:remove a name and defination from the table
int undef(char * name) {
 struct nlist * np1, * np2;

 if ((np1 = lookup(name)) == NULL)  /*  name not found  */
  return 1;

 for ( np1 = np2 = hashtab[hash(name)]; np1 != NULL;
   np2 = np1, np1 = np1->next ) {
  if ( strcmp(name, np1->name) == 0 ) {  /*  name found  */

   /*  Remove node from list  */

   if ( np1 == np2 )
    hashtab[hash(name)] = np1->next;
   else
    np2->next = np1->next;

   /*  Free memory  */

   free(np1->name);
   free(np1->defn);
   free(np1);

   return 0;
  }
 }

 return 1;  /*  name not found  */
}

//error:print error message and skip the rest of the line
void error(int c, char *s)
{
 printf("error:%sn",s);
 while(c != EOF && c != 'n')
  c = getch();
}

//skipblanks:skip blan and tab characters
void skipblanks()
{
 int c;
 while((c = getch()) == ' ' ||c == 't')
  ;
 ungetch(c);
}

/*ungets: push string back onto the input*/
void ungets(char s[])
{
 int len=strlen(s);
 void ungetch(int);
 while (len >0)
  ungetch(s[--len]);
}
/*note: modify the getword function to return 
spaces so that the output resembles the input data.*/

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