From fa70b1c7303cce993ed6a1785b72bf11b21cc6b6 Mon Sep 17 00:00:00 2001
From: zlg <zlg@zlg.space>
Date: Fri, 20 Nov 2015 23:28:24 -0800
Subject: Solve Exercise 6-5: undef()

We're getting close to more useful data structures! Wee!
---
 ch6/6-05_undef.c | 107 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 107 insertions(+)
 create mode 100644 ch6/6-05_undef.c

(limited to 'ch6')

diff --git a/ch6/6-05_undef.c b/ch6/6-05_undef.c
new file mode 100644
index 0000000..9ffd248
--- /dev/null
+++ b/ch6/6-05_undef.c
@@ -0,0 +1,107 @@
+#include <stdio.h>
+#include <string.h> // strdup, strcmp
+#include <stdlib.h> // malloc, free
+
+/* The C Programming Language: 2nd Edition
+ *
+ * Exercise 6-5: Write a function `undef` that will remove a name and
+ * definition from the table maintai\ed by lookup() and install().
+ *
+ * Notes: since undef() operates on the same data structure as lookup()
+ * and install(), I chose to depend on lookup() to find the correct node
+ * to remove. The tricky part is juggling two pointers and cleaning up
+ * the pointers to other nodes on the list.
+ */
+
+#define HASHSIZE 101
+
+struct nlist {
+	struct nlist *next;
+	char *name;
+	char *defn;
+};
+
+static struct nlist *hashtab[HASHSIZE];
+
+unsigned hash(char *);
+struct nlist *install(char *, char *);
+struct nlist *lookup(char *);
+int undef(char *);
+void print_table(struct nlist *);
+
+int main(int argc, char **argv) {
+	/* Install a few words */
+	install("foo", "1");
+	install("bar", "2");
+	install("baz", "4");
+	/* Make up scenarios for changing and removing values */
+	printf("We don't like baz, because his value is %s.\n", hashtab[hash("baz")]->defn);
+	printf("Let's ressign baz to 3.\n");
+	install("baz", "3");
+	printf("baz is now %s.\n", hashtab[hash("baz")]->defn);
+	printf("baz is a weird name. Let's try removing it.\n");
+	undef("baz") ? : printf("baz was removed from the table! Let's check to make sure it's gone...\n");
+	if (lookup("baz") == NULL) {
+		printf("Success! baz is gone!\n");
+	}
+	return 0;
+}
+
+/* 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;
+}
+
+/* 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;
+		}
+	}
+	return NULL;
+}
+
+/* put name and defn in hashtab */
+struct nlist *install(char *name, char *defn) {
+	struct nlist *np;
+	unsigned hashval;
+	if ((np = lookup(name)) == NULL) {
+		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 {
+		free ((void *) np->defn);
+	}
+	if ((np->defn = strdup(defn)) == NULL) {
+		return NULL;
+	}
+	return np;
+}
+
+int undef(char *name) {
+	struct nlist *np, *np2;
+	if ((np = lookup(name)) == NULL) {
+		return 1;
+	}
+	for (np = np2 = hashtab[hash(name)]; np != NULL; np2 = np, np = np->next) {
+		if (np == np2) {
+			hashtab[hash(name)] = np->next;
+		} else {
+			np2->next = np->next;
+		}
+		free(np->name);
+		free(np->defn);
+		free(np);
+		return 0;
+	}
+}
-- 
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