std::cin and whitespace - not consistent?

[zapshe]

Alright, I've been having a weird experience with std::cin - hope someone can clarify things for me. std::cin doesn't accept white space as part of the input. However, this can sometimes cause problems since if you type 2 words, then the second word will be used as the input of the next std::cin input. Up to here, this is fine. However, under what circumstances will std::cin also use '\n' as input for the next std::cin? For example :

double x;
double y;
char z;

int main()
{
	std::cout << "Type In Equation: ";
	std::cin >> x; 
	std::cin >> z;
	if (z == '!') // To Skip Getting A "y" Value For Factorials
	{
		goto Operation;
	}
	std::cin >> y;

(This is code taken from a calculator I made) - No matter how much white space I put between x/z/y - the program has no trouble differentiating between these different values.

However, for this code :

int main()
{
	std::cout << "Pick 1 or 2: ";
	int choice;
	std::cin >> choice;

	std::cin.ignore(32767, '\n'); // Code Won't Work Without This Line

	std::cout << "Now enter your name: ";
	std::string name;
	std::getline(std::cin, name);

	std::cout << "Hello, " << name << ", you picked " << choice << '\n';

	return 0;
}

For this code, without std::cin.ignore(32767, '\n');, it wont work. Apparently, in this instance, when putting a number in for std::cin >> choice; - it'll also take '\n' as an input (which I assume is the space created when pressing enter). Since it's taken as an input, it's put into the next std::cin that's asking for a name.

So now, how come it's perfectly fine to have as much white space as you want (or pressing enter as many times as you want) in the first example without it being taken as an input - yet in the second one it is taken as an input and doesn't work as it was expected to without that extra line of code? Thanks.

[JLBorges]

See: http://www.cplusplus.com/forum/beginner/233165/#msg1048524

[zapshe]

I didn't even see you replied to that post since I had stopped looking at it once I realized what was happening.

But to be sure I understood - std::cin >> - discards white space at the beginning, but keeps white space at the end in the buffer. There for, it another std::cin >> was used, it would discard that beginning white space. However, since in this case we were using std::getline - which doesn't discard white space anywhere - it took the '\n' or white space in the buffer as a value and moved on - correct?

Thanks JLBorges, don't know what I'd do without you !

[Peter87]

If you want to discard leading whitespace before using std::getline you can use std::ws.

std::getline(std::cin >> std::ws, name);

Note that this approach is no good if you want to allow empty lines as input, or if you care about whitespace characters at the beginning of lines, but otherwise this is probably the easiest way to do it.

[JLBorges]

> it took the '\n' or white space in the buffer as a value and moved on - correct?

Yes.

[zapshe]

Thanks Peter, that looks pretty handy, I'll make sure to remember it for future uses!