HOW TO DETECT NO STRING INPUT WITHOUT USING A FUNCTION

[phztfte1]

Write your question here.
How do I get the user to be re-prompted if the user initially presses the enter key?

 /*	strlen
*	function returns the length of s.
*	take care to return 0 if s is NULL
*	or if s is of length 0
*	you may not call an functions in your function
*	int strlen (string s);
*	Get string s
*	Loop through s array
*	if s[i] == \0, return 0
*	else
*	increment counter
*/


#include <cstdio>
#include <string>
#include <iostream>

using namespace std;

// function prototype
int strlen(string s);

int main(void)
{
	// Get string s
	string str;
	printf_s("Please give me a string of characters: ");
	do
	{
		cin >> str;
	} 
	while (str[0] == '\0');
		
	// pass s to function to determine length
	int x = strlen(str);
	printf_s("The length of s = %i\n", x);
}

int strlen(string s)
{
	// initialize counter i to zero
	int i = 0;
	do
	{
		if (s[i] == '\0')
		{
			return 0;
		}
		else
		{
			i++;
		}
	} 
	while (s[i] != '\0');
	return i;
}

[AbstractionAnon]

What version of C++ are you using?
C++98:
Accessing the value at data()+size() produces undefined behavior: There are no guarantees that a null character terminates the character sequence pointed by the value returned by this function. See string::c_str for a function that provides such guarantee.
http://www.cplusplus.com/reference/string/string/data/

[Smac89]

To accomplish this, use the std::istream::get method.

#include <cctype>
do
{
     printf_s("Please give me a string of characters: ");
} while (!isalnum(std::cin.get()));

std::cin.unget() >> str;
...

[phztfte1]

AbstractionAnon, Smac89,

I read my problem wrong. Instead of re-prompting the user, I need to return a zero if s is NULL. I would like to let the user know that the string length is zero.

[phztfte1]

I solved the problem. Thank you for jogging my mind. Here is my code:

/*	strlen
*	function returns the length of s.
*	take care to return 0 if s is NULL
*	or if s is of length 0
*	you may not call any functions in your function
*	int strlen (string s);
*	Get string s
*	Loop through s array
*	if s[i] == \0, return 0
*	else
*	increment counter
*/


#include <cstdio>
#include <string>
#include <iostream>

using namespace std;

// function prototype
int strlen(string s);

int main(void)
{
	// Get string s
	string str;
	printf_s("Please give me a string of characters: ");
	getline(cin, str);


	if (str[0] != NULL)
	{
		// pass s to function to determine length
		int x = strlen(str);
		printf_s("The length of s = %i\n", x);
	}
	else
	{	
		printf_s("The string length is 0!\n");
	}
}


int strlen(string s)
{
	// initialize counter i to zero
	int count = 0;
	while(s[count] != '\0')
	{
		count = count + 1;
		printf_s("The count is: %i\n", count);
	}
	return count;
}