Thread: String manipulation

What about using strncmp() and a for loop ?

What about writing say myStrncmp() and myStrstr() ?
Just because you're not allowed to use the functions doesn't mean you can write your own similarly named functions, with the same API.

Well writing two functions (of whatever name you would like to call them), each only 5 lines long, is going to result in something a lot easier to think about, and a lot easier to debug.

When you've got it all working (or even using strncmp() just to prove the point, then writing your own), then you can inline it all in main() if you still want to.

A problem: if i change str2 to "s " (s followed by a space character) I get a NOT FOUND outcome...

Code:

/* Get String Length function */
int getlength(char str[]) {
	int len = 0;
	while(str[len] != '\0') len++; return len;
}

That indentation is just confusing.

Consider using strncmp() in your strstr(), as Salem has already said.

If you want a really short solution, consider recursion. It's really inefficient and everything, but you can basically code strstr like this:

Code:

const char *recursive_strstr(const char *in, const char *what) {
    if(!in || !*in) return 0;
    if(strncmp(in, what, strlen(what)) == 0) return in;
    return recursive_strstr(in + 1, what);
}

Well, that's absolutely stupid.

Be that as it may, as Salem has already mentioned, there's no harm in using the built-in strlen() or strncmp() or whatever for your strstr() as you're writing it. And (I seem to be repeating Salem a lot!) it would be best if you modularized your strstr() so that it called strncmp() for example, rather than doing everything itself in an unreadable mess.

Or you could develop it in a bottom-up approach, writing your own strncmp() before tackling strstr().

Start like this

Code:

int myStrncmp ( const char *a, const char *b, size_t len ) {
    return strncmp( a, b, len );
}

size_t myStrlen ( const char *a ) {
    return strlen( a );
}

char *myStrstr ( const char *haystack, const char *needle ) {
    return strstr( haystack, needle );
}

Then refine one of the functions to say

Code:

char *myStrstr ( const char *haystack, const char *needle ) {
    size_t h_len = myStrlen( haystack );
    size_t n_len = myStrlen( needle );
    size_t i;
    for ( i = 0 ; i < (h_len-n_len) ; i++ ) {
        if ( myStrncmp( &haystack[i], needle, n_len ) == 0 ) {
            return &haystack[i];
        }
    }
    return NULL;
}

The point is, you start with something which works, which means when you write your own version and it stops working, you have some idea of where the problem is.

> Well, we've been told that for the exams we wont be allowed to use any library fuctions
That's only in the answer you hand in. Nothing wrong with using them to get you moving forwards.

Originally Posted by Karpaty

Code:

	if(result == 1) printf("Not Found");
    else printf("Found");

This one's misaligned! Try indenting it to match the if.
And don't mix spaces and tabs - use one! It will mess up the indentation outside your editor. I recommend only tabs.

Without the use of any function of string.h which i agree of being a stupid thing to do here is my code for strstr.

Code:

//---------------------------------------------------------------------------

#include <stdio.h>

int my_strlen(const char *str)
{
	int i;
	for(i = 0; str[i] != '\0'; i++);
	return i;
}

char *my_strstr(char *source, char *str)
{

	int i,u, j = 0;
	/* Return Value. */
	char *ptrToStr = NULL;
	/* Get the lengths. */
	int nLen = my_strlen(source);
	int sLen = my_strlen(str);
	if(!source)
		return NULL;
	if(sLen > nLen)
		return NULL;
	for(i = 0; i < nLen; i++)
	{
		if(source[i] == str[j])
		{
			u = i;
			for(j = 0; j < sLen && str[j] == source[u]; j++, u++);
			if(j == sLen)
			{
				/* Save a pointer to the start position. */
				ptrToStr = &source[i];
				return ptrToStr;
			}
		}
	}
	/* Return NULL. */
	return NULL;
}

//---------------------------------------------------------------------------

int main(int argc, char* argv[])
{
	char *mainstr = "This is a nice day.";
	char *substr = "This";
	char *strptr = my_strstr(mainstr, substr);
	printf("&#37;s\n", strptr);
	printf("Hit enter to quit.....\n");
	getchar();
	return 0;
}
//---------------------------------------------------------------------------

Intuition tells me you could probably just return i from my_strlen since both strLen and i are incremented on each loop (though you might have to do strLen - 1).