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-rw-r--r--ch4/4-13_recursive-reverse.c48
1 files changed, 48 insertions, 0 deletions
diff --git a/ch4/4-13_recursive-reverse.c b/ch4/4-13_recursive-reverse.c
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+++ b/ch4/4-13_recursive-reverse.c
@@ -0,0 +1,48 @@
+#include <stdio.h>
+#include <string.h>
+
+/* The C Programming Language: 2nd Edition
+ *
+ * Exercise 4-13: Write a recursive version of the function reverse(s), which
+ * reverses the string `s` in place.
+ *
+ * Answer: Much like the last exercise, the trick is in terminating your flow
+ * early and resetting state for the next string.
+ */
+
+void reverse(char[]);
+
+int main() {
+ char foo[40] = "Learning C is fun and challenging.";
+ printf("Before: %s\n", foo);
+ reverse(foo);
+ printf(" After: %s\n", foo);
+ /* I don't know how to set a char array to another string... */
+ char bar[30] = "Hello out there!";
+ printf("Before: %s\n", bar);
+ reverse(bar);
+ printf(" After: %s\n", bar);
+ return 0;
+}
+
+void reverse(char s[]) {
+ static int i = 0;
+ static int j = 0;
+ int c;
+
+ if (j == 0) {
+ j = strlen(s) - 1;
+ }
+
+ if (i < j) {
+ c = s[i];
+ s[i] = s[j];
+ s[j] = c;
+ i++;
+ j--;
+ reverse(s);
+ return;
+ }
+ i = 0;
+ j = 0;
+}