K & R C Programs Exercise 6-2.

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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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;
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) {

//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 != '-') {
else if(c == ''' || c == '\'){
for(; --lim > 0; w++)
if(((*w = getch()) == '\')
*++w = getch();
else if(*w == c) {
}else if(*w == EOF)
}else if(c == '/')
if((d = getch()) == '*')
c = comment();
*w = '';
return c;

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