While loop re-initiates itself!

[abrownin]

Hi All,

I've created a while loop which, once the condition is satisfied, stops. But then a few lines later if the condition is "unsatisfied", the while loop decides to reinitiate itself.. I'll show you what I mean:

bool is_line_valid()
	 {
        //Return true if there is a new line character after the last #
		list<char>::iterator list_iter = row_chars.end();
		while(*list_iter != '#')
		{
		list_iter--;
		}
		cout << "Finished while loop; print what is at current container: " << *list_iter << "\n";
		cout << "Now move container forward (to last position)... \n";
		list_iter++;
		if(*list_iter == '\n')
		return true;
		else
		return false;
	 }

And here's the output (perpetual of course):

Finished while loop; print what is at current container: #
Now move container forward (to last position)...
Finished while loop; print what is at current container: #
Now move container forward (to last position)...
Finished while loop; print what is at current container: #
Now move container forward (to last position)...

..etc.

Can someone tell me why this happens please?

[tabstop]

It doesn't. You're probably calling this function in a loop, I would guess.

[abrownin]

If I remove the line:

list_iter++;

It actually works perfectly. Yes, I am calling the function in a loop. But the loop does not finish it's first iteration, as the WHILE loop in the function shown will not finish...

[abrownin]

What I mean is, it never returns true or false, unless I remove the aforementioned line.

[tabstop]

What I mean is, it never returns true or false, unless I remove the aforementioned line.

Give us complete code.

EDIT: Or better yet, do something like this:

std::cout << std::boolalpha << is_line_valid() << std::endl;

and see if it doesn't print true/false every time.

[abrownin]

Thanks for the help! I ended up not needing to use this piece of code, so I won't need to work on it anymore. But I appreciate the suggestions.

[smasherprog]

Yeah, the above code you posted is not an infinite loop. You are doing something else wrong. I bet you are calling the function from some infinite loop that you are now going to look at and go... uhh ohh. I bet we will never see you post a reply to this too. Don't feel bad, I have done it too

[abrownin]

Haha don't worry if you miss me I've already posted a new thread with a new completely unrelated problem.

You know, just to spice this place up a bit.

[anon]

The above code contains several errors, though, which makes the results undefined:

bool is_line_valid()
	 {
        //Return true if there is a new line character after the last #
		list<char>::iterator list_iter = row_chars.end();
		while(*list_iter != '#') //dereferences row_chars.end() right away; what happens if there is no '#'? 
		{
		list_iter--;
		}
		cout << "Finished while loop; print what is at current container: " << *list_iter << "\n";
		cout << "Now move container forward (to last position)... \n";
		list_iter++;
		if(*list_iter == '\n') //what if '#' was the last character?
		return true;
		else
		return false;
	 }

You could rather make use of the standard algorithms, for example:

bool is_line_valid()
{
    std::list<char>::const_reverse_iterator it = std::find(row_chars.rbegin(), row_chars.rend(), '#');
    if (it != row_chars.rend()) //found
    {
        std::list<char>::const_iterator next = it.base();
        return next != row_chars.end() && *next == 'n'; //if next character exists and is line-break
    }
    return false;
}

[abrownin]

Wow anon, that's some really great code -- thanks so much for the advice! It's definitely something I wouldn't have thought of in my beginner stage, but it makes a lot of sense.

Sadly, after re-reading my assignment specification, it turns out I'm not meant to test for if the line is valid (for that test in particular) - as it's assumed that this is so. So I can't use it

Again, thanks for the response! Helps me notice more things