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#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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