Solve Exercise 5-6: pointer-based functions

This exercise was not very fun or interesting. While I somewhat enjoyed
learning to use pointers, this was a bunch of busywork. It felt like
homework, and that's terrible. It's the first exercise I've outright
disliked. If you're writing a programming book, *don't* make exercises
like this one!

1 files changed, 154 insertions, 0 deletions

diff --git a/ch5/5-06_pointer-funcs.c b/ch5/5-06_pointer-funcs.c
new file mode 100644
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--- /dev/null
+++ b/ch5/5-06_pointer-funcs.c
@@ -0,0 +1,154 @@
+#include <stdio.h>
+#include <ctype.h>
+
+/* The C Programming Language: 2nd Edition
+ *
+ * Exercise 5-6: Rewrite appropriate programs from earlier chapters and
+ * exercises with pointers instead of array indexing. Good possibilites
+ * include getline (Chapters 1 and 4), atoi, itoa, and their variants
+ * (Chapters 2, 3, and 4), reverse (Chapter 3), and strindex and getop
+ * (Chapter 4).
+ *
+ * Answer: This is more of the same. Practice or homework, basically. Thing is,
+ * I hate homework, so let's do the bare minimum and only do the functions
+ * that were explicitly listed. :)
+ *
+ * A cool side effect of using pointers is pointer-based functions seem to be
+ * shorter!
+ */
+
+int get_line(char *, int);
+int atoi(char *);
+void itoa(int, char *);
+void reverse(char *);
+int strindex(char *, char *);
+int getop(char *);
+
+int main() {
+	// simplistic testing, nothing fancy, screw busywork
+	char foo[20] = "foobar";
+	char bar[50];
+	int baz;
+	reverse(foo);
+	printf("Let's test reverse(\"foobar\"): %s\n", foo);
+	printf("Enter some text to test get_line(): ");
+	if ((baz = get_line(bar, 50)) > 0) {
+		printf("get_line() returns %d and updates to: %s", baz, bar);
+	}
+	printf("atoi(): %d\n", atoi("7529"));
+	itoa(-4321, bar);
+	printf("itoa(): %s\n", bar);
+	printf("strindex(\"foobarness\", \"bar\"): %d\n", strindex("foobarness", "bar"));
+	printf("Enter a RPN equation to test getop(): ");
+	printf("getop(): %d, %s\n", getop(bar), bar);
+	return 0;
+}
+
+void reverse(char *s) {
+	char *end = s;
+	char tween;
+	while (*end != '\0') {
+		end++;
+	}
+	end--; // get behind the \0
+	while (end > s) {
+		tween = *end;
+		*end-- = *s;
+		*s++ = tween;
+	}
+}
+
+int get_line(char *s, int lim) {
+	int c, i;
+	for (i = 0; i < lim - 1 && (c=getchar()) != EOF && c != '\n'; ++i) {
+		*s++ = c;
+	}
+	if (c == '\n') {
+		*s++ = c;
+		i++;
+	}
+	*s = '\0';
+	return i;
+}
+
+int atoi(char *s) {
+	int n, sign;
+	while (isspace(*s)) {
+		s++;
+	}
+	sign = (*s == '-') ? -1 : 1;
+	if (*s == '+' || *s == '-') {
+		s++;
+	}
+	for (n = 0; isdigit(*s); n = 10 * n + (*s - '0'), s++);
+	return n * sign;
+}
+
+void itoa(int n, char *s) {
+	char *start = s;
+	int sign = 0;
+	if (n < 0) {
+		sign = 1;
+		n = -n;
+	}
+	do {
+		*s = '0' + (n % 10);
+		s++;
+	} while ((n /= 10) > 0);
+	if (sign) {
+		*s = '-';
+		s++;
+	}
+	*s = '\0';
+	reverse(start);
+}
+
+int strindex(char *s, char *t) {
+	int loc;
+	int len;
+	for (loc = 0; *s != '\0'; loc++, s++) {
+		for (len = 0; *t != '\0' && *s == *t; len++, s++, t++);
+		if (len > 0 && *t == '\0') {
+			return loc;
+		}
+	}
+	return -1;
+}
+
+/* This function is useless on its own. It should be used in a calculator
+ * application that has an input buffer. Regardless, it should still store the
+ * operand in a string and return an integer; that's all testable.
+ */
+int getop(char *s) {
+	int c, next;
+	while ((c = *s = getchar()) == ' ' || c == '\t');
+	s++;
+	*s = '\0';
+	if (!isdigit(c) && c != '.' && c != '-') {
+		return c;
+	}
+	if (c == '-') {
+		next = getchar();
+		if (!isdigit(next) && next != '.') {
+			return c;
+		} else {
+			c = next;
+		}
+	} else {
+		c = getchar();
+	}
+	while (isdigit(*s = c)) {
+		s++;
+		c = getchar();
+	}
+	if (c == '.') {
+		*s = c;
+		s++;
+		while (isdigit(*s = c = getchar())) {
+			s++;
+		}
+	}
+	s++;
+	*s = '\0';
+	return 1; // number found
+}