Solve Exercise 5-5: strn{cpy|cat|cmp}

This exercise seemed like it was designed to "pull it all together"
with things learned in the previous few pages. Not much new in terms
of techniques, but more a test to see how you could blend everything
together.

1 files changed, 76 insertions, 0 deletions

diff --git a/ch5/5-05_strn-funcs.c b/ch5/5-05_strn-funcs.c
new file mode 100644
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--- /dev/null
+++ b/ch5/5-05_strn-funcs.c
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+#include <stdio.h>
+
+/* The C Programming Language: 2nd Edition
+ *
+ * Exercise 5-5: Write versions of the library functions strncpy, strncat, and
+ * strncmp, which operate on at most the first n charecetr of their argument
+ * strings. For example, strncpy(s,t,n) copies at most n character of t to s.
+ * Full descriptions are in Appendix B.
+ */
+
+char *strn_cpy(char *, const char *, size_t);
+char *strn_cat(char *, const char *, size_t);
+int strn_cmp(const char *, const char *, size_t);
+
+int main() {
+	/* This is to fill the array up beyond where I'll be filling it so I can
+	 * test. */
+	char foo[50] = "Meepineeeeeessssss";
+	char *bar = "Merpy";
+	printf("strn_cpy: %s\n", strn_cpy(foo, bar, 9));
+	/* Test to see that the last element of the range is indeed a \0, and
+	 * some other part further is something else. */
+	if (foo[8] == '\0' && foo[14] != '\0') {
+		printf("strn_cpy works as expected.\n");
+	}
+	bar = " hai";
+	printf("strn_cat: %s\n", strn_cat(foo, bar, 9));
+	// bar is 4 characters long. Let's test comparisons.
+	printf("strn_cmp: %d\n", strn_cmp(bar, "hai lits", 10)); // -4
+	printf("strn_cmp: %d\n", strn_cmp(bar, "no", 10)); // 2
+	printf("strn_cmp: %d\n", strn_cmp(bar, "derp", 10)); // 0
+	return 0;
+}
+
+char *strn_cpy(char *s, const char *t, size_t n) {
+	char *start = s;
+	while (n > 0 && *t != '\0') {
+		*s++ = *t++;
+		n--;
+	}
+	while (n > 0) {
+		*s++ = '\0';
+		n--;
+	}
+	return start;
+}
+
+char *strn_cat(char *s, const char *t, size_t n) {
+	char *start = s;
+	while (*s != '\0') {
+		s++;
+	}
+	while (*t != '\0' && n > 0) {
+		*s++ = *t++;
+		n--;
+	}
+	*s = '\0';
+	return start;
+}
+
+int strn_cmp(const char *s, const char *t, size_t n) {
+	int slen, tlen;
+	slen = tlen = 0;
+	while (n > 0) {
+		if (*s != '\0') {
+			s++;
+			slen++;
+		}
+		if (*t != '\0') {
+			t++;
+			tlen++;
+		}
+		n--;
+	}
+	return slen - tlen;
+}