Finding the last occurrence of a string in a sentence in C - c

Question

I am practicing and I came across an exercise. The exercise says I am to manually write a function that finds the index of the last occurrence in a string. Now, I get that this has maybe been asked before but I cannot find what is the problem with my code. It works for almost all instances, but not so when the last occurrence of the word is at the beginning of the string.
What I have tried: I used pointers to store the addresses of the ends of both the sentence and the word we are looking for. I then used a while loop to iterate through the string. If the current character matches the last character of the word we are searching for, we enter another while loop which compares the two. If the pointer that points to the beginning of the word and the one we used to iterate through the word are equal, the word is found.
Here is some code:
#include <stdio.h>
int find_last( char *str, char *word)
{
char *p, *q;
char *s, *t;
p=str; /* Pointer p now points to the last character of the sentence*/
while(*p!='\0') p++;
p--;
q = word;
while(*q!='\0') q++; /* Pointer q now points to the last character of the word*/
q--;
while(p != str) {
if(*p == *q) {
s=p; /* if a matching character is found, "s" and "t" are used to iterate through */
/* the string and the word, respectively*/
t=q;
while(*s == *t) {
s--;
t--;
}
if(t == word-1) return s-str+1; /* if pointer "t" is equal by address to pointer word-1, we have found our match. return s-str+1. */
}
p--;
}
return -1;
}
int main()
{
char arr[] = "Today is a great day!";
printf("%d", find_last(arr, "Today"));
return 0;
}
So, this code should return 0 but it returns -1.
It works in every other instance I tested! When ran in CodeBlocks, the output is as expected (0), but using any other online IDE I could find the output is still -1.

Answer 1

For starters the parameters of the function shall have the qualifier const and its return type should be either size_t or ptrdiff_t.
For example
ptrdiff_t find_last( const char *str, const char *word );
In any case the function shall be declared at least like
int find_last( const char *str, const char *word );
The function should emulate the behavior of the standard C function strstr. That is when the second argument is an empty string the function should return 0.
If either of the arguments is an empty string then your function has undefined behavior due to these statements
p=str; /* Pointer p now points to the last character of the sentence*/
while(*p!='\0') p++;
p--;
^^^^
q = word;
while(*q!='\0') q++; /* Pointer q now points to the last character of the word*/
q--;
^^^^
If the string pointed to by str contains only one symbol then your function returns -1 because the condition of the loop
while(p != str) {
evaluates to false independent on whether the both strings are equal each other or not.
This loop
while(*s == *t) {
s--;
t--;
}
again can invoke undefined behavior because there can be an access to memory that precedes the string word.
And this statement
if(t == word-1) return s-str+1;
also can invoke the undefined behavior by the same reason.
The function can be defined as it is shown in the demonstrative program below.
#include <stdio.h>
int find_last( const char *str, const char *word )
{
const char *p = str;
int found = !*word;
if ( !found )
{
while ( *p ) ++p;
const char *q = word;
while ( *q ) ++q;
while ( !found && !( p - str < q - word ) )
{
const char *s = p;
const char *t = q;
while ( t != word && *( s - 1 ) == *( t - 1) )
{
--s;
--t;
}
found = t == word;
if ( found ) p = s;
else --p;
}
}
return found ? p - str : -1;
}
int main(void)
{
const char *str = "";
const char *word = "";
printf( "find_last( str, word ) == %d\n", find_last( str, word ) );
word = "A";
printf( "find_last( str, word ) == %d\n", find_last( str, word ) );
str = "A";
printf( "find_last( str, word ) == %d\n", find_last( str, word ) );
str = "ABA";
printf( "find_last( str, word ) == %d\n", find_last( str, word ) );
str = "ABAB";
printf( "find_last( str, word ) == %d\n", find_last( str, word ) );
str = "ABCDEF";
printf( "find_last( str, word ) == %d\n", find_last( str, word ) );
str = "ABCDEF";
word = "BC";
printf( "find_last( str, word ) == %d\n", find_last( str, word ) );
return 0;
}
The program output is
find_last( str, word ) == 0
find_last( str, word ) == -1
find_last( str, word ) == 0
find_last( str, word ) == 2
find_last( str, word ) == 2
find_last( str, word ) == 0
find_last( str, word ) == 1