strpos in C- how does it work - c

Question

I am really new to C.
I want to use the strpos function but it is telling me it doesnt exist?

Answer 1

Here a complete snippet code to solve you problem.
PS: Isn't too late to help. ;)
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define NOT_FOUND -1
int main (){
int pos = NOT_FOUND;
if ( (pos = strpos( "subsstring", "string")) != NOT_FOUND )
printf("found at %d\n", pos);
else
printf("not found!\n");
return 0;
}
int strpos(const char *haystack, const char *needle)
{
const char *p = strstr(haystack, needle);
if (p)
return p - haystack;
return NOT_FOUND;
}
Edit: Answering Can Vural question:
No. I really think that it would be as it is. At structured programming paradigm, it's a common practice to use the scope structure as first parameter on every function that belongs to the structure's scope itself. The strstr function defined at string.h follow the same approach.
On OOP you have haystack.indexOf( needle ). At structured programming, you have indexOf( haystack, needle ).

Answer 2

The function you are looking for might be either strstr or strchr. You then need to include string.h. There is no strpos in the POSIX interface.

Answer 3

Yes. It's called strstr, related to strpos like (pseudo-code):
strpos(str, target) {
res = strstr(str, target);
if (res == NULL) return false;
else return res - str;
}

Answer 4

I have written strpos() function from scratch with position feature(Like PHP's strpos() function). Return value will be starting position of searched string. Enjoy! :)
In this example code output will be 12
#include <stdio.h>
#include <string.h>
int strpos(char *haystack, char *needle, int pos);
int main(){
printf("%d",strpos("abcdefabcdefabcdef asdfgavcabcddd","abc",10));
return 0;
}
int strpos(char *haystack, char *needle, int pos){
int i,j,check,result = -1;
int len_needle=strlen(needle);
int len_haystack=strlen(haystack);
i = pos;
if (len_needle>len_haystack || *needle==NULL || i>(len_haystack-1)) return result;
for(;i<len_haystack;i++){
check = 0;
for(j=0;j<len_needle;j++){
if(haystack[i+j]==needle[j]){
check++;
}
}
if(check==len_needle){
result = i;
break;
}
}
return result;
}

Answer 5

This is in response to Miere and Can Vural. I can't add comments yet so will add this as an answer.
Shouldn't it be strpos("string", "substring") – Can Vural
At structured programming, you have indexOf( haystack, needle ). Miere
In your code, you have:
int strpos(char *haystack, char *needle)
but you also have:
(pos = strpos( "subsstring", "string"))
I fully agree with the "int strpos(char *haystack, char *needle)" where the string to be searched comes first and the string to search FOR comes second. But to me, "subsstring" (in the context of "one is a substring and one is a string"), "subsstring" implies that IT is the shorter of the two and that you're trying to find "substring" in "string."
So the one part:
(pos = strpos( "subsstring", "string"))
should be:
(pos = strpos( "string" /*that which is being searched within*/, "substring" /*that which is being searched for in the previous parameter*/))
which would be the same as:
(pos = strpos( "haystack", "needle"))
Edit: One of the C comments above wasn't closed properly due to a typo.

Answer 6

There is no function strpos defined in the Standard C library nor in the POSIX Standard. PHP has a function strpos with this definition:
strpos(string $haystack, string $needle, int $offset = 0): int|false
This function locates a substring needle inside a string haystack and returns the offset from the beginning of the string haystack.
C has function strstr that can be used for this purpose and returns a pointer to the substring or NULL if no match is found:
char *strstr(const char *haystack, const char *needle);
Here is an implementation for a C equivalent of strpos relying on strstr for the dirty work. Note however that the name strpos is reserved for future functions in <string.h>:
#include <string.h>
int strpos(const char *haystack, const char *needle, int offset) {
char *p;
size_t len, pos;
len = strlen(haystack);
pos = 0;
if (offset < 0) {
if (len > INT_MAX || offset < -(int)len)
pos = len + offset;
} else {
if (len <= INT_MAX && offset > len)
return NULL;
pos = offset;
}
p = strstr(haystack + pos, needle);
if (p != NULL && p - haystack <= INT_MAX)
return (int)(p - haystack);
else
return -1;
}