strstr
- C course
- Beginner
- Lesson 89/116
- 5 min
Learn to search for a substring with strstr.
strstr: searching for a substring
The strstr function searches for one string inside another. Its signature is char *strstr(const char *s, const char *sub). It comes with the <string.h> header.
char *p = strstr("merhaba dunya", "dunya");
// p points at the "dunya" partTo get the starting index of the match, you again use p - s. It returns NULL if not found. strchr searches for a single character; strstr searches for a multi-character substring.
strstr example: computing the index
By subtracting the start of the string from the pointer that strstr returns, you get the substring's starting index.
char text[] = "merhaba dunya";
char *p = strstr(text, "dunya");
long index = p - text;
// index = 8Here the "dunya" substring starts at index 8. Always check that p is not NULL before subtracting. This pattern is very useful to find where a word appears in a string.
Practice Print the index of the "dunya" substring
Use strstr to find the starting index of the "dunya" substring in "merhaba dunya" and print it with printf. You must include the <string.h> header.
Starter code
#include <stdio.h>
int main(void) {
char text[] = "merhaba dunya";
/* Find the starting index of "dunya" with strstr and print it */
return 0;
}Sample solution
#include <string.h>
char *p = strstr(text, "dunya");
if (p != NULL) {
printf("%ld\n", (long)(p - text));
}Expected output
8
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Match function to task
| Term | Meaning |
|---|---|
strstr(s, sub) |
Searches substring |
strchr(s, c) |
Searches character |
strcmp(a, b) |
Compares |
strlen(s) |
Returns length |
2 What does strstr("merhaba", "ha") return?
- The value 0
- Pointer starting at "ha" Correct answer
- The index of the first 'h'
- NULL
Explanation: strstr returns a pointer to the start of the match.
3 Fill in the blank: strstr call
char *p = ____(text, "dunya");char *p = strstr(text, "dunya");Explanation: strstr searches a substring.