#include <stdio.h>
#include <string.h>
/*
Find all starting indices of a word inside a larger text.
This function uses strstr, which returns a pointer to the next
occurrence of the substring. We loop until no more matches exist.
*/
void find_all_occurrences(const char *text, const char *word)
{
// If the word is empty, searching is meaningless
if (word[0] == '\0') {
return;
}
const char *pos = text; // Start scanning from the beginning
const char *match = NULL; // Pointer to each found occurrence
while ((match = strstr(pos, word)) != NULL) {
/*
strstr returns a pointer to the first character of the match.
To convert this pointer into an index, subtract the base pointer.
*/
size_t index = (size_t)(match - text);
printf("%zu\n", index);
/*
Move forward by one character to continue searching.
This ensures overlapping matches are also found.
*/
pos = match + 1;
}
}
int main(void)
{
const char *text =
"the quick brown fox jumps over the lazy dog. the fox is clever.";
const char *word = "the";
printf("Text: %s\n", text);
printf("Word: \"%s\"\n\n", word);
printf("Occurrences at indices:\n");
find_all_occurrences(text, word);
return 0;
}
/*
run:
Text: the quick brown fox jumps over the lazy dog. the fox is clever.
Word: "the"
Occurrences at indices:
0
31
45
*/
