K and R C, Solution to Exercise 6-2:
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.
Write a C program that reads a C program and prints in alphabetical order each group of variable names that are identical in the first 6 characters, but different somewhere thereafter. Don’t count words within strings and comments. Make 6 a parameter that can be from the command line. Read more about C Programming Language .

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

struct tnode {
 char *word;
 int match;
 struct tnode *left;
 struct tnode *right;
};
#define MAXWORD
#define YES 0
#define NO 0

struct tnode *addtreex(struct tnode *, char *,int, int);
void treexprint(struct tnode *);
int getword(char *,int);


main(int argc, char argv[])
{
 struct tnode  *root;
 char word[MAXWORD];
 int found = NO;
 int num;

 num = (--argc && (*++argv)[0] == '-') ? atoi(argv[0] +1) : 6;
 root = NULL;
 while(getword(word, MAXWORD) != EOF) {
  if(isalpha(word[0]) && strlen(word) >= num)
   root = addtreex(root, word, num, &found);
  found = NO;
 }
 treexprint(root);
 return 0;
}
struct tnode *talloc(void);
int compare(char *,struct tnode *, int, int *);
//addtreex: add a node with w, at or below p
struct tnode *addtreex(struct tnode *p, char *w,int num, int *found)
{
 int cond;
 if(p == NULL) {
  p = talloc();
  p->word = strdup(w);
  p->match = *found;
  p->left = p->right = NULL;
 }
 else if((cond = compare(w,p,num,found)) < 0)
  p->left = addtreex(p->left, w, num, found);
 else if(cond > 0)
  p->right = addtreex(p->right, w, num, found);
 return p;
}

//compare:compare words and update p->match
int compare(char *s,struct tnode *p, int num, int *found)
{
 int i;
 char *t = p->word;
 for(i = 0; *s == *t;i++,s++,t++)
  if(*s == '')
   return 0;
 if(i >= num) {
  *found = YES;
  p->match = YES;
 }
 return *s - *t;
}

/*treexprint: in-order print of tree p if p->match == YES */
void treexprint(struct tnode *p)
{
 if(p != NULL) {
  treexprint(p->left);
  if(p->match)
   printf("%sn",p->word);
  treexprint(p->right);
 }
}


//getword: get next word or character from input
int getword(char *word, int lim)
{
 int c, d, comment(void), getch(void);
 void ungetch(int);
 char *w = word;
 while(isspace(c = getch()))
  ;
 if(c !=EOF)
  *w++ = c;
 if(isalpha(c) || c == '-' || c == '#'){
  for(; --lim > 0; w++)
   if(!isalnum(*w = getch()) && *w != '-') {
    ungetch(*w);
    break;
   }
 } 
 else if(c == ''' || c == '\'){
  for(; --lim > 0; w++)
   if(((*w = getch()) == '\')
     *++w = getch();
   else if(*w == c) {
    w++;
    break;
   }else if(*w == EOF)
    break;
 }else if(c == '/')
  if((d = getch()) == '*')
   c = comment();
  else
   ungetch(d);
 *w = '';
 return c;
}

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