Strange behaviour on a std::getline

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#1

Hi all! I would like to show you my script because there something strange about the "bool Ripetere() " function.

The program seems to ignore the “std::getline” inside of that function, and if you try to type something, it will just close itself.

I’ve manage to do it the first time during the BullCowGame, but I can’t replicate it, and I don’t know why. Can someone help me? thank you very much!

#include <iostream>
#include <string>

int number;
int multiplier;

int InserisciNumero();
int MoltiplicaPer();
int Risultato();
bool Ripetere();

void main()
{
	bool bRisposta = false;	
	do 
	{
		InserisciNumero();
		MoltiplicaPer();
		Risultato();
		bRisposta = Ripetere();
	} 	
	while (bRisposta);

	std::cout << std::endl;

	return;
}

int InserisciNumero()
{
	std::cout << "Inserisci un numero intero: ";
	std::cin >> number;

	return 0;
}

int MoltiplicaPer() 
{
	std::cout << "Per quale valore intero vuoi moltiplicarlo? Inserisci un numero: ";
	std::cin >> multiplier;

	return 0;
}

int Risultato() 
{
	std::cout << number << " * " << multiplier << " = " << number * multiplier << std::endl;
	std::cout << std::endl;

	return 0;
}

bool Ripetere() 
{
	std::cout << "Vuoi fare un altro calcolo? (y/n)" << std::endl;
	std::string Response = "";
	std::getline(std::cin, Response);

	return (Response[0] == 'y') || (Response[0] == 'Y');
}
1 Like
Bonus challenge done, but a couple of issues
#2

@sampattuzzi can you shed any light on this.

#3

Firstly you should be using int main not void main. See here for reasons.

Your problem stems from you using std::cin >> number. When you enter a number, say 1 and then press enter, the input buffer will be “1\n”, the 1 will go into number and then you will be left with “\n” in the buffer which would be used the next time you use std::cin (in std::getline), to solve this you will have to throw away the buffer, or you could use std::getlline and use std::stoi to get that as an int. Solution for the first would be

std::cin >> number;
//clear error flags
std::cin.clear();
//reads max streamsize upto and including a newline and discards it
std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');
2 Likes
Your guess was: blank error
#4

Thanks @DanM

1 Like
#5

Thank you all for you help!

So, if I get the logic behind it, std::cin >> number collect ALL the inputs I give it, including the “Enter” button, that is trasnlated in a “\n”. So if I digit for example 5 and then Enter, the system will read it like “5\n”. When I ask to insert again another number to multiply the first, the “\n” from the “5\n” is returned and given to the second std::cin, but is ignored because std::cin won’t accept “\n” as an input alone, so everything work fine. I then input, for example, 6 and then press Enter, so, again, is a “6\n”. The third time I won’t have a std::cin, but ai std::getline, that will accept the last “\n” as an input e automaticly end the program because \n != (y or Y). We can then solve the problem with your script put inside “int MoltiplicaPer()” so that we can ignore the last “\n” OR I can use another std::cin and then return the input inside “std::string Response” because std::cin automaticly ignore the last “\n”.

Maybe my reasoning was a little bit intricated, I’m sorry, I’m not very experienced, but I hope I got the meaning of your script and how std::cin work. Can you explain to me what every line of yout script exactly do, if you don’t mind? Thank you again!

#6

Close, you are correct at the first part because std::cin >> will ignore the first whitespace character when reading input so using std::cin >> the next time is fine since the newline is ignored.

The actual problem isn’t that “\n != (y or Y)”. It’s that std::getline reads up to and including a newline and then discards the newline, so in actuality your string is empty.

I’m not sure how to describe it in more detail than what the comments provide.

//clear error flags
std::cin.clear();

This clears the error state flags (eofbit, failbit and badbit)

//reads max streamsize upto and including a newline and discards it
std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');

std::numeric_limits<std::streamsize>::max() would just be whatever the maximum possible size a stream (like iostream) can be. So it would just be like std::cin.ignore(9223372036854775807, '\n');, since apparently that’s what that number is.

And whilst I was making sure everything I’m saying is correct, I learned that you can use std::ws to discard leading whitespace. So you could just do

std::cin >> number >> std::ws;

and whereever else you use std::cin >> and you shouldn’t have a problem reading with std::getline

1 Like
#7

Thank you very much, you where amazing! That was a nasty sneaky issue for a newbie like me. Now it’s far more clear for me and we managed to solve the problem! Thank you again!

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