Thread: How do I write this function?

Here is the problem that I have read it several times but still don't know whether I must return the memory address (is a number) or the character of the string (a character).

Write a function find with two parameters: str and substr. The parameters str and substr are pointers to char. {*the following sentence is what I dont understand*} The function find returns the address of the first occurrence of the string pointed by substr in the string pointed to by str. If the string pointed to by substr does not occur in the string pointed to by str, find returns NULL.

This is like a strchr() function already written for the compiler. But how can we write our own?

Well have you got as far as

Code:

char *foo ( char *str ) {
  char *p;
  for ( p = str ; *p != '\0' ; p++ ) {
    printf( "%c\n", *p );
  }
  return str;
}

That's your basic string scanning loop - now add an if() or two as appropriate

Hello,

Here are replica's of the functions strstr() and strchr(), rename them to whatever you want, etc...

Code:

#include <stdlib.h> // for printf()

char* strchr(const char *, int);
char* strstr(const char *, const char *);

int main() {
	int spaces=0;
	char *pch;
	char data[255] = {"This is a string"};

	/*
	** Search for first ' '
	** and all there after
	*/
	pch = strchr(data, ' ');
	while (pch != NULL) {			// while found, and not the end...
		spaces++;
		pch = strchr(pch+1, ' ');	// search again
	}
	printf("%i spaces found in sentence\n", spaces);

	/*
	** Search for "is"
	*/
	pch = strstr(data, "is");
	while (pch != NULL) {			// while found, and not the end...
		printf("is was found at position %i\n", pch-data);
		pch = strstr(pch+1, "is");	// search again
	}

	return 0;
}

char* strchr(const char *string, int c) {
	while( *string ) {
		if( *string == c )
			return (char *)string;
		string++;
	}

	return (char *)0;
}

char* strstr(const char *string1, const char *string2) {
	int i;

	while( *string1 ) {
		for( i = 0; string2[i]; i++ ) {
			if( string1[i] != string2[i] )
				break;
		}

		if( !string2[i] )
			return (char *)string1;
		string1++;
	}

	return (char *)0;
}

I also wrote an example in main() of how they are used. They respond exactly like the ones in the string library, so there shouldn't be to much of a difference. If you have any further questions, feel free to ask.


I hope this helps,
- Stack Overflow

Well,

Because for some compilers it compiles the code from the top to the bottom, so if I dont have the static definitions, my compiler gives errors saying it doesn't know what that function is. I could just move the strstr() and strchr() examples above main() removing the static definitions.

I just wanted to give a complete example in case anyone likes their function(s) underneath the main() call and come back to me saying 'strstr() is undeclared', etc...

Edit: Besides, I've been reading the book "The C Programming Language" by K & R, and most of there examples always have the codes underneath main() and static defintions above main(). That's how C works, C++ may be different.


- Stack Overflow

I'm not sure what its actually called. The best name I can think of is the function prototype declaration.

I guess its supposed to help some by saying char* strstr(const char *, const char *); is a function that expects two char * arguments and returns a char *. That declaration, psuedo function prototype, has to complywith the defintion and uses of strstr().


- Stack Overflow

You mis-used static for function prototype

What linuxdude was asking (as am I) is why

Code:

char* strstr(const char *string1, const char *string2) {
	static int i;   // <<<---  this is static

Ah,

Now I understand. Well i'm not sure why that was there, I think I made a mis-type a couple of weeks back when I wrote the function. Thanks again for pointing that out, and I modified my original post.


- Stack Overflow

Code:

while(*pconfig != '*' || *(pconfig-1) != '/')
{
	if(*pconfig == '/' && *(pconfig-1) == '*')

What on earth are you doing? You're just trying to make a dupe of strstr, right? If so, why are you checking for specific characters in the middle of your search function?

Code:

while you're not at the end of the string to search
    while the current letter of the string to search is not the first letter of what to find
        increment pointer using for string to search
    while the current letter of the string to search is the same as the string to find
        increment both pointers
    if you've run out of the string to find
        then it's been found so return something saying as much
return null

I've left off error checking, but it should be trivial to make sure you're staying within the bounds of your strings.

Quzah.