K & R C Programs Exercise 6-6.

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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