exercise08.c

// C Primer Plus
// Chapter 11 Exercise 8:

// Write a function called string_in() that takes two string pointers as
// arguments. If the second string is contained in the first string, have the
// function return the address at which the contained string begins. For
// instance, string_in("hats", "at") would return the address of the a in hats.
// Otherwise, have the function return the null pointer. Test the function in a
// complete program that uses a loop to provide input values for feeding to the
// function.

#include <stdio.h>

#define LIMIT 50

char * string_in(char *string, char *substring);
char * get(char *string, int n);

int main(void)
{
	// test string_in()

	char string[LIMIT];
	char substring[LIMIT];

	char *substr_loc;

	printf("Enter a string: ");
	get(string, LIMIT);
	while (string[0] != '\0')
	{
		printf("Enter a substring to look for: ");
		get(substring, LIMIT);

		substr_loc = string_in(string, substring);

		if (substr_loc == NULL)
			printf("%s not in %s\n", substring, string);
		else
			printf("%s found in %s at index %lu\n",
				   substring, string, substr_loc - string);

		printf("Enter a string (empty line to quit): ");
		get(string, LIMIT);
	}

	puts("Bye");

	return 0;
}

char * string_in(char *string, char *substring)
{
	// checks if substring is in string
	// returns pointer to first location of substring
	// in string or NULL if substring not in string

	int i;

	while (*string != '\0')
	{
		i = 0;

		// check for substring at current location
		while (*(string + i) == *(substring + i))
		{
			i++;

			// if next char in substring is null, then match
			// is found. return current location
			if (*(substring + i) == '\0')
				return string;
		}

		string++;
	}

	// no match
	return NULL;
}


char * get(char *string, int n)
{
	// wrapper for fgets that replaces first newline with null

	char *return_value = fgets(string, n, stdin);

	while (*string != '\0')
	{
		if (*string == '\n')
		{
			*string = '\0';
			break;
		}

		string++;
	}

	return return_value;
}