From e6819292b0d0a45a5d3f74af9ef320e544f42c82 Mon Sep 17 00:00:00 2001
From: zlg <zlg@zlg.space>
Date: Sun, 5 May 2013 00:54:37 -0500
Subject: Solve Exercise 4-1: strrindex()

---
 ch4/4-01_strrindex.c | 87 ++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 87 insertions(+)
 create mode 100644 ch4/4-01_strrindex.c

(limited to 'ch4')

diff --git a/ch4/4-01_strrindex.c b/ch4/4-01_strrindex.c
new file mode 100644
index 0000000..b6400c4
--- /dev/null
+++ b/ch4/4-01_strrindex.c
@@ -0,0 +1,87 @@
+#include <stdio.h>
+#include <string.h>
+
+/* The C Programming Language: 2nd Edition
+ *
+ * Exercise 4-1: Write the function strrindex(s, t), which returns the position
+ * of the /rightmost/ occurrence of 't' in 's', or -1 if there is none.
+ *
+ * Answer: The directions are unclear. Is it asking for the rightmost index of
+ * the first result from the beginning of the string, or the first index of the
+ * first occurrence from the end of the string? I will assume the leftmost
+ * index of the first occurrence from the end of the string, since it seems
+ * more difficult and relevant.
+ *
+ * This exercise was made simpler by remembering the work we did in Chapter 3.
+ * Specifically, itoa2. We made a reverse(s) function which reverses strings.
+ * If we reverse our two strings, we can pass them to strindex(), do a little
+ * math, and get the rightmost equivalent, all without copying and modifying
+ * another function! This strengthens the concept of writing functions that
+ * don't need to be fully understood in order to use or expand on their
+ * functionality.
+ *
+ * In this instance, I reversed strindex's behavior without needing to know
+ * anything beyond the concept of what strings are.
+ *
+ * (For the record I understand how strindex works :P)
+ */
+
+#define MAXLINE 1000
+
+void reverse(char s[]) {
+	int c, i, j;
+
+	for (i = 0, j = strlen(s)-1; i < j; i++, j--) {
+		c = s[i];
+		s[i] = s[j];
+		s[j] = c;
+	}
+}
+
+int strindex(char s[], char t[]) {
+	int i, j, k;
+
+	for (i = 0; s[i] != '\0'; i++) {
+		for (j = i, k = 0; t[k] != '\0' && s[j] == t[k]; j++, k++) {
+		}
+		if (k > 0 && t[k] == '\0') {
+			return i;
+		}
+	}
+	return -1;
+}
+
+int strrindex(char s[], char t[]) {
+	/* Instead of starting at the end and going back, let's be super lazy
+	 * and just reverse it, call our buddy strindex, and reverse the
+	 * strings again.
+	 */
+	reverse(s);
+	reverse(t);
+	/* We add strlen(t) to point to the first index of t when it's
+	 * reversed again */
+	int o = strindex(s, t) + strlen(t);
+	reverse(s);
+	reverse(t);
+
+	// Be sure to return the correct offset with some math :3
+	return (strlen(s) - o);
+}
+
+int main() {
+	char haystack[MAXLINE] = "charred and burned, ouch! My stomach churned... I hope I'll be okay!";
+	char needle[MAXLINE] = "ned";
+	int pos = strrindex(haystack, needle);
+	int i = 0;
+
+	/* This is mostly just to provide a handy visual aid */
+	printf("The phrase %s's rightmost occurrence is %d:\n", needle, pos);
+	printf("%s\n", haystack);
+	for (i = 0; i < pos; i++) {
+		putchar(' ');
+	}
+	for (i = 0; i < strlen(needle); i++) {
+		putchar('^');
+	}
+	printf("\n");
+}
-- 
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