Add: Exercise 6-5

Author: satyajitghana

Chapter-6/Exercise 6-5/buffer.c

@@ -0,0 +1,22 @@
+#include <stdio.h>
+#include <string.h>
+#define BUFSIZE 100
+
+int buf[BUFSIZE]; /* buffer for ungetch */
+int bufp = 0; /* buffer pointer (not a real pointer) */
+
+int getch(void) /* get a (possibly pushed back) character */ {
+    return (bufp > 0) ? buf[--bufp] : getchar();
+}
+
+void ungetch(int c) /* push character back on input */ {
+    if (bufp >= BUFSIZE)
+        printf("ungetch : too many characters\n");
+    else
+        buf[bufp++] = c;
+}
+
+void ungets(char s[]) /* push string s back to input */ {
+    int i = strlen(s);
+    while (i > 0) ungetch(s[--i]);
+}

Chapter-6/Exercise 6-5/buffer.h

@@ -0,0 +1,10 @@
+#ifndef BUFFER_H
+#define BUFFER_H
+
+int getch(void);
+
+void ungetch(int);
+
+void ungets(char []);
+
+#endif

Chapter-6/Exercise 6-5/io.c

@@ -0,0 +1,51 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <ctype.h>
+#include "buffer.h"
+#include "io.h"
+
+char *_strdup(char *s) { /* make a duplicate of s */
+    char *p;
+
+    p = (char *) malloc(strlen(s) + 1); /* +1 for '\0' */
+    if (p != NULL)
+        strcpy(p, s);
+    return p;
+}
+
+/* _strndup: make a duplicate of at most 6 characters of s*/
+char *_strndup(char *s, int n) {
+    char *p;
+
+    p = (char *) malloc(n +1);
+    if (p != NULL)
+        strncpy(p, s, n);
+    return p;
+}
+
+/* getword: get next word or character from input */
+int getword(char *word, int lim) {
+    int c;
+    char *w = word;
+
+    /* skip the whitespaces */
+    while (isspace(c = getch()))
+        ;
+    if (c != EOF)
+        *w++ = c;
+    if (!isalpha(c)) {
+        *w = '\0';
+        return c;
+    }
+
+    while (--lim) {
+        if (!isalnum(*w = getch())) {
+            ungetch(*w);
+            break;
+        }
+        w++;
+    }
+    *w = '\0';
+    return word[0];
+}

Chapter-6/Exercise 6-5/io.h

@@ -0,0 +1,9 @@
+#ifndef IO_H
+#define IO_H
+
+char *_strdup(char *s);
+char *_strndup(char *s, int n);
+
+int getword(char *word, int lim);
+
+#endif

Chapter-6/Exercise 6-5/main.c

@@ -0,0 +1,17 @@
+#include <stdio.h>
+#include "nlist.h"
+
+int main() {
+    install("satyajit", "ghana");
+
+    struct nlist *search;
+    if ((search = lookup("satyajit")) != NULL)
+        printf("name : %s, defn : %s\n", search -> name, search -> defn);
+    /* delete the name */
+    if (undef("satyajit") == 0 ) {
+        printf("deleted !");
+        printf(" lookup : %d\n", lookup("satyajit"));
+    }
+
+    return 0;
+}

Chapter-6/Exercise 6-5/nlist.c

@@ -0,0 +1,77 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+#include "nlist.h"
+#include "io.h"
+
+#define HASHSIZE 101
+
+static struct nlist *hashtab[HASHSIZE]; /* pointer table */
+
+/* hash: form hash value for string s */
+unsigned hash(char *s) {
+    unsigned hashval;
+
+    for (hashval = 0; *s != '\0' ; s++)
+        hashval = *s + 31 * hashval;
+
+    return hashval % HASHSIZE;
+}
+
+/* lookup: look for s in hashtab */
+struct nlist *lookup(char *s) {
+    struct nlist *np;
+
+    for (np = hashtab[hash(s)] ; np != NULL ; np = np -> next)
+        if (strcmp(s, np -> name) == 0)
+            return np;  /* found */
+    return NULL;        /* not found */
+}
+
+/* install: put(name, defn) in hashtab */
+struct nlist *install(char* name, char* defn) {
+    struct nlist *np;
+    unsigned hashval;
+
+    if ((np = lookup(name)) == NULL) { /* not found */
+        np = (struct nlist *) malloc(sizeof(*np));
+        if (np == NULL || (np -> name = _strdup(name)) == NULL)
+            return NULL;
+        hashval = hash(name);
+        np -> next = hashtab[hashval];
+        hashtab[hashval]  = np;
+    } else /* that means its already there */
+        free((void*) np -> defn); /* free previous defn */
+    if ((np -> defn = _strdup(defn)) == NULL)
+        return NULL;
+    return np;
+}
+
+/* undef: removes a name from the hashtab */
+int undef(char* name) {
+    struct nlist *np;
+
+    if ((np = lookup(name)) == NULL)
+        return 1; /* the name was not found */
+
+    struct nlist *prev_np = NULL;
+    unsigned hashval = hash(name);
+    for (np = hashtab[hashval] ; np != NULL; prev_np = np, np = np -> next) {
+        if (strcmp(np -> name, name) == 0) { /* found the node with that name */
+            if (prev_np == NULL) /* at the beginning ? */
+                hashtab[hashval] = np -> next;
+            else /* somewhere in the middle */
+               prev_np -> next = np -> next;
+            /* free the memory */
+            free(np -> name);
+            free(np -> defn);
+            free(np);
+
+            return 0;
+        }
+    }
+
+    return 1; /* name not found */
+}
+

Chapter-6/Exercise 6-5/nlist.h

@@ -0,0 +1,18 @@
+#ifndef NLIST_H
+#define NLIST_H
+
+struct nlist {          /* table entry */
+    struct nlist *next; /* next entry in the chain */
+    char *name;         /* defined name */
+    char *defn;         /* replacement text */
+};
+
+unsigned hash(char*);
+
+struct nlist *lookup(char*);
+
+struct nlist *install(char*, char*);
+
+int undef(char*);
+
+#endif