This question already has answers here:
Why does std::getline() skip input after a formatted extraction?
(5 answers)
Closed 1 year ago.
To allow a multiple words user input I use getline and I flush the stream according to this answer with this code :
while(!everyoneAnswered()){
string name;
std::cout << "enter your name : ";
if (getline(std::cin, name)) {
std::cin.clear();
std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n'); // flush stream
std::cout << "enter your age : ";
int age;
std::cin >> age;
std::cin.clear();
std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');
// stuff
}
}
The problem is that now the user has to hit enter two times for the input to get accepted. I guess it's because getline consumes the first \n.
How can I avoid this, so that the user only hit enter once for the input to validate?
Here is an example of the result without the flush:
enter your name : foo
enter your age : 42
enter your name : enter your age : 69
--crashing because second name is empty--
Here is an example of the result with the flush :
enter your name : foo
enter your age : 42
enter your name : bar
enter your age : 69
Finally the flush without \n delimiter hangs on the first try waiting for MAX_INT characters entered.
I use gcc version 7.5.0 (Ubuntu 7.5.0-3ubuntu1~18.04).
The problem are these two lines:
if (getline(std::cin, name)) {
std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');
The getline call includes the newline, so your ignore call will read the next line of input.
You need the ignore call after reading the age (with >>), but not after getline.
The solution was to also use getline() to read the age and parse it to an int. I don't know why mixing getline() and operator>> produces this behavior, though.
Related
I have the following piece of code that prompts the user for their cat's age and name:
#include <iostream>
#include <string>
int main()
{
int age;
std::string name;
std::cin >> age;
std::getline(std::cin, name);
if (std::cin)
{
std::cout << "My cat is " << age << " years old and their name is " << name << std::endl;
}
}
What I find is that the age has been successfully read, but not the name. Here is the input and output:
Input:
"10"
"Mr. Whiskers"
Output:
"My cat is 10 years old and their name is "
Why has the name been omitted from the output? I've given the proper input, but the code somehow ignores it. Why does this happen?
Why does this happen?
This has little to do with the input you provided yourself but rather with the default behavior std::getline() has. When you provided your input for the age (std::cin >> age), you not only submitted the following characters, but also an implicit newline was appended to the stream when you typed Enter:
"10\n"
A newline is always appended to your input when you select Enter or Return when submitting from a terminal. It is also used in files for moving toward the next line. The newline is left in the buffer after the extraction into age until the next I/O operation where it is either discarded or read. When the flow of control reaches std::getline(), it will see "\nMr. Whiskers" and the newline at the beginning will be discarded, but the input operation will stop immediately. The reason this happens is because the job of std::getline() is to attempt to read characters and stop when it finds a newline. So the rest of your input is left in the buffer unread.
Solution
cin.ignore()
To fix this, one option is to skip over the newline before doing std::getline(). You can do this by calling std::cin.ignore() after the first input operation. It will discard the next character (the newline character) so that it is no longer in the way.
std::cin >> age;
std::cin.ignore();
std::getline(std::cin, name);
assert(std::cin);
// Success!
std::ws
Another way to discard the whitespace is to use the std::ws function which is a manipulator designed to extract and discard leading whitespace from the beginning of an input stream:
std::cin >> age;
std::getline(std::cin >> std::ws, name);
assert(std::cin);
// Success!
The std::cin >> std::ws expression is executed before the std::getline() call (and after the std::cin >> age call) so that the newline character is removed.
The difference is that ignore() discards only 1 character (or N characters when given a parameter), and std::ws continues to ignore whitespace until it finds a non-whitespace character. So if you don't know how much whitespace will precede the next token you should consider using this.
Match the operations
When you run into an issue like this it's usually because you're combining formatted input operations with unformatted input operations. A formatted input operation is when you take input and format it for a certain type. That's what operator>>() is for. Unformatted input operations are anything other than that, like std::getline(), std::cin.read(), std::cin.get(), etc. Those functions don't care about the format of the input and only process raw text.
If you stick to using a single type of formatting then you can avoid this annoying issue:
// Unformatted I/O
std::string age, name;
std::getline(std::cin, age);
std::getline(std::cin, name);
or
// Formatted I/O
int age;
std::string firstName, lastName;
std::cin >> age >> firstName >> lastName;
If you choose to read everything as strings using the unformatted operations you can convert them into the appropriate types afterwards.
Everything will be OK if you change your initial code in the following way:
if ((cin >> name).get() && std::getline(cin, state))
This happens because an implicit line feed also known as newline character \n is appended to all user input from a terminal as it's telling the stream to start a new line. You can safely account for this by using std::getline when checking for multiple lines of user input. The default behavior of std::getline will read everything up to and including the newline character \n from the input stream object which is std::cin in this case.
#include <iostream>
#include <string>
int main()
{
std::string name;
std::string state;
if (std::getline(std::cin, name) && std::getline(std::cin, state))
{
std::cout << "Your name is " << name << " and you live in " << state;
}
return 0;
}
Input:
"John"
"New Hampshire"
Output:
"Your name is John and you live in New Hampshire"
I am really wondering. C++ has a dedicated function for eating up any remaining or whatever white spaces. It is called std::ws. And then, you can simply use
std::getline(std::cin >> std::ws, name);
That should be the idomatic approach. For each transistion between formatted to unformatted input that should be used.
If we are not talking about white spaces, but entering for example letters where a number is expected, then we should follow the CPP reference and use
.ignore(std::numeric_limits<std::streamsize>::max(), '\n'); to eliminate the wrong stuff.
Please read here
Since everyone above has answered the problem for input 10\nMr Whisker\n, I would like to answer a different approach. All the solution above published the code for if the buffer is like 10\nMr Whisker\n. But what if we don't know how user will behave in giving input. The user might type 10\n\nMr. Whisker\n or 10 \n\n Mr. whisker\n by mistake. In that case, codes above may not work. So, I use the function below to take string input to address the problem.
string StringInput() //returns null-terminated string
{
string input;
getline(cin, input);
while(input.length()==0)//keep taking input as long as valid string is taken
{
getline(cin, input);
}
return input.c_str();
}
So, the answer would be:
#include <iostream>
#include <string>
int main()
{
int age;
std::string name;
std::cin >> age;
name = StringInput();
std::cout << "My cat is " << age << " years old and it's name is " << name << std::endl;
}
Extra:
If user inputs a \n10\n \nmr. whiskey;
To check whether int input is valid or not, this function can be used to check int input (program will have undefined behavior if char is given as input instead of int):
//instead of "std::cin>>age;" use "get_untill_int(&age);" in main function.
void get_Untill_Int(int* pInput)//keep taking input until input is `int or float`
{
cin>> *pInput;
/*-----------check input validity----------------*/
while (!cin)
{
cin.clear();
cin.ignore(100, '\n');
cout<<"Invalid Input Type.\nEnter again: ";
cin >>*pInput;
}
/*-----------checked input validity-------------*/
}
This question already has answers here:
Why does std::getline() skip input after a formatted extraction?
(5 answers)
Closed 8 months ago.
can you help me
Why this code can't be swap
cout << "Enter a string: ";
getline(cin, str1);
cout << "Enter another string: ";
cin.get(str, 100, '\n');
Into
cout << "Enter string: ";
cin.get(str, 100, '\n');
cout << "Enter a string: ";
getline(cin, str1);
when i ran
First code
Output :
Enter a string: hai
Enter another string: hello
Second code
Output :
Enter another string: hello
Enter a string:
I can't input anymore, it just directly returned 0
Is it because of delimiters?
std::istream::get leaves the newline character in the stream so if you use std::getline afterwards, it'll directly read that newline character.
You can get rid of it like so:
std::cin.get(str, 100, '\n');
std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');
std::getline(std::cin, str1);
Demo
But, it's easier if you don't mix std::getline and std::istream::get.
As you can see in the documentation for std::istream::get:
The delimiting character is not extracted from the input sequence if
found, and remains there as the next character to be extracted from
the stream (see getline for an alternative that does discard the
delimiting character).
I.e. the difference is that std::getline disacrds the newline delimiter character. If you use std::istream::get it stays in the stream buffer, and when you try to extract the 2nd string you will get it into your variable.
Regarding your specific example, it's a better to use either std::getline or std::istream::get consistently, rather than mixing them up.
If you have a good reason to mix them, see how to in #TedLyngmo's answer.
I have the following piece of code that prompts the user for their cat's age and name:
#include <iostream>
#include <string>
int main()
{
int age;
std::string name;
std::cin >> age;
std::getline(std::cin, name);
if (std::cin)
{
std::cout << "My cat is " << age << " years old and their name is " << name << std::endl;
}
}
What I find is that the age has been successfully read, but not the name. Here is the input and output:
Input:
"10"
"Mr. Whiskers"
Output:
"My cat is 10 years old and their name is "
Why has the name been omitted from the output? I've given the proper input, but the code somehow ignores it. Why does this happen?
Why does this happen?
This has little to do with the input you provided yourself but rather with the default behavior std::getline() has. When you provided your input for the age (std::cin >> age), you not only submitted the following characters, but also an implicit newline was appended to the stream when you typed Enter:
"10\n"
A newline is always appended to your input when you select Enter or Return when submitting from a terminal. It is also used in files for moving toward the next line. The newline is left in the buffer after the extraction into age until the next I/O operation where it is either discarded or read. When the flow of control reaches std::getline(), it will see "\nMr. Whiskers" and the newline at the beginning will be discarded, but the input operation will stop immediately. The reason this happens is because the job of std::getline() is to attempt to read characters and stop when it finds a newline. So the rest of your input is left in the buffer unread.
Solution
cin.ignore()
To fix this, one option is to skip over the newline before doing std::getline(). You can do this by calling std::cin.ignore() after the first input operation. It will discard the next character (the newline character) so that it is no longer in the way.
std::cin >> age;
std::cin.ignore();
std::getline(std::cin, name);
assert(std::cin);
// Success!
std::ws
Another way to discard the whitespace is to use the std::ws function which is a manipulator designed to extract and discard leading whitespace from the beginning of an input stream:
std::cin >> age;
std::getline(std::cin >> std::ws, name);
assert(std::cin);
// Success!
The std::cin >> std::ws expression is executed before the std::getline() call (and after the std::cin >> age call) so that the newline character is removed.
The difference is that ignore() discards only 1 character (or N characters when given a parameter), and std::ws continues to ignore whitespace until it finds a non-whitespace character. So if you don't know how much whitespace will precede the next token you should consider using this.
Match the operations
When you run into an issue like this it's usually because you're combining formatted input operations with unformatted input operations. A formatted input operation is when you take input and format it for a certain type. That's what operator>>() is for. Unformatted input operations are anything other than that, like std::getline(), std::cin.read(), std::cin.get(), etc. Those functions don't care about the format of the input and only process raw text.
If you stick to using a single type of formatting then you can avoid this annoying issue:
// Unformatted I/O
std::string age, name;
std::getline(std::cin, age);
std::getline(std::cin, name);
or
// Formatted I/O
int age;
std::string firstName, lastName;
std::cin >> age >> firstName >> lastName;
If you choose to read everything as strings using the unformatted operations you can convert them into the appropriate types afterwards.
Everything will be OK if you change your initial code in the following way:
if ((cin >> name).get() && std::getline(cin, state))
This happens because an implicit line feed also known as newline character \n is appended to all user input from a terminal as it's telling the stream to start a new line. You can safely account for this by using std::getline when checking for multiple lines of user input. The default behavior of std::getline will read everything up to and including the newline character \n from the input stream object which is std::cin in this case.
#include <iostream>
#include <string>
int main()
{
std::string name;
std::string state;
if (std::getline(std::cin, name) && std::getline(std::cin, state))
{
std::cout << "Your name is " << name << " and you live in " << state;
}
return 0;
}
Input:
"John"
"New Hampshire"
Output:
"Your name is John and you live in New Hampshire"
I am really wondering. C++ has a dedicated function for eating up any remaining or whatever white spaces. It is called std::ws. And then, you can simply use
std::getline(std::cin >> std::ws, name);
That should be the idomatic approach. For each transistion between formatted to unformatted input that should be used.
If we are not talking about white spaces, but entering for example letters where a number is expected, then we should follow the CPP reference and use
.ignore(std::numeric_limits<std::streamsize>::max(), '\n'); to eliminate the wrong stuff.
Please read here
Since everyone above has answered the problem for input 10\nMr Whisker\n, I would like to answer a different approach. All the solution above published the code for if the buffer is like 10\nMr Whisker\n. But what if we don't know how user will behave in giving input. The user might type 10\n\nMr. Whisker\n or 10 \n\n Mr. whisker\n by mistake. In that case, codes above may not work. So, I use the function below to take string input to address the problem.
string StringInput() //returns null-terminated string
{
string input;
getline(cin, input);
while(input.length()==0)//keep taking input as long as valid string is taken
{
getline(cin, input);
}
return input.c_str();
}
So, the answer would be:
#include <iostream>
#include <string>
int main()
{
int age;
std::string name;
std::cin >> age;
name = StringInput();
std::cout << "My cat is " << age << " years old and it's name is " << name << std::endl;
}
Extra:
If user inputs a \n10\n \nmr. whiskey;
To check whether int input is valid or not, this function can be used to check int input (program will have undefined behavior if char is given as input instead of int):
//instead of "std::cin>>age;" use "get_untill_int(&age);" in main function.
void get_Untill_Int(int* pInput)//keep taking input until input is `int or float`
{
cin>> *pInput;
/*-----------check input validity----------------*/
while (!cin)
{
cin.clear();
cin.ignore(100, '\n');
cout<<"Invalid Input Type.\nEnter again: ";
cin >>*pInput;
}
/*-----------checked input validity-------------*/
}
I have the following piece of code that prompts the user for their cat's age and name:
#include <iostream>
#include <string>
int main()
{
int age;
std::string name;
std::cin >> age;
std::getline(std::cin, name);
if (std::cin)
{
std::cout << "My cat is " << age << " years old and their name is " << name << std::endl;
}
}
What I find is that the age has been successfully read, but not the name. Here is the input and output:
Input:
"10"
"Mr. Whiskers"
Output:
"My cat is 10 years old and their name is "
Why has the name been omitted from the output? I've given the proper input, but the code somehow ignores it. Why does this happen?
Why does this happen?
This has little to do with the input you provided yourself but rather with the default behavior std::getline() has. When you provided your input for the age (std::cin >> age), you not only submitted the following characters, but also an implicit newline was appended to the stream when you typed Enter:
"10\n"
A newline is always appended to your input when you select Enter or Return when submitting from a terminal. It is also used in files for moving toward the next line. The newline is left in the buffer after the extraction into age until the next I/O operation where it is either discarded or read. When the flow of control reaches std::getline(), it will see "\nMr. Whiskers" and the newline at the beginning will be discarded, but the input operation will stop immediately. The reason this happens is because the job of std::getline() is to attempt to read characters and stop when it finds a newline. So the rest of your input is left in the buffer unread.
Solution
cin.ignore()
To fix this, one option is to skip over the newline before doing std::getline(). You can do this by calling std::cin.ignore() after the first input operation. It will discard the next character (the newline character) so that it is no longer in the way.
std::cin >> age;
std::cin.ignore();
std::getline(std::cin, name);
assert(std::cin);
// Success!
std::ws
Another way to discard the whitespace is to use the std::ws function which is a manipulator designed to extract and discard leading whitespace from the beginning of an input stream:
std::cin >> age;
std::getline(std::cin >> std::ws, name);
assert(std::cin);
// Success!
The std::cin >> std::ws expression is executed before the std::getline() call (and after the std::cin >> age call) so that the newline character is removed.
The difference is that ignore() discards only 1 character (or N characters when given a parameter), and std::ws continues to ignore whitespace until it finds a non-whitespace character. So if you don't know how much whitespace will precede the next token you should consider using this.
Match the operations
When you run into an issue like this it's usually because you're combining formatted input operations with unformatted input operations. A formatted input operation is when you take input and format it for a certain type. That's what operator>>() is for. Unformatted input operations are anything other than that, like std::getline(), std::cin.read(), std::cin.get(), etc. Those functions don't care about the format of the input and only process raw text.
If you stick to using a single type of formatting then you can avoid this annoying issue:
// Unformatted I/O
std::string age, name;
std::getline(std::cin, age);
std::getline(std::cin, name);
or
// Formatted I/O
int age;
std::string firstName, lastName;
std::cin >> age >> firstName >> lastName;
If you choose to read everything as strings using the unformatted operations you can convert them into the appropriate types afterwards.
Everything will be OK if you change your initial code in the following way:
if ((cin >> name).get() && std::getline(cin, state))
This happens because an implicit line feed also known as newline character \n is appended to all user input from a terminal as it's telling the stream to start a new line. You can safely account for this by using std::getline when checking for multiple lines of user input. The default behavior of std::getline will read everything up to and including the newline character \n from the input stream object which is std::cin in this case.
#include <iostream>
#include <string>
int main()
{
std::string name;
std::string state;
if (std::getline(std::cin, name) && std::getline(std::cin, state))
{
std::cout << "Your name is " << name << " and you live in " << state;
}
return 0;
}
Input:
"John"
"New Hampshire"
Output:
"Your name is John and you live in New Hampshire"
I am really wondering. C++ has a dedicated function for eating up any remaining or whatever white spaces. It is called std::ws. And then, you can simply use
std::getline(std::cin >> std::ws, name);
That should be the idomatic approach. For each transistion between formatted to unformatted input that should be used.
If we are not talking about white spaces, but entering for example letters where a number is expected, then we should follow the CPP reference and use
.ignore(std::numeric_limits<std::streamsize>::max(), '\n'); to eliminate the wrong stuff.
Please read here
Since everyone above has answered the problem for input 10\nMr Whisker\n, I would like to answer a different approach. All the solution above published the code for if the buffer is like 10\nMr Whisker\n. But what if we don't know how user will behave in giving input. The user might type 10\n\nMr. Whisker\n or 10 \n\n Mr. whisker\n by mistake. In that case, codes above may not work. So, I use the function below to take string input to address the problem.
string StringInput() //returns null-terminated string
{
string input;
getline(cin, input);
while(input.length()==0)//keep taking input as long as valid string is taken
{
getline(cin, input);
}
return input.c_str();
}
So, the answer would be:
#include <iostream>
#include <string>
int main()
{
int age;
std::string name;
std::cin >> age;
name = StringInput();
std::cout << "My cat is " << age << " years old and it's name is " << name << std::endl;
}
Extra:
If user inputs a \n10\n \nmr. whiskey;
To check whether int input is valid or not, this function can be used to check int input (program will have undefined behavior if char is given as input instead of int):
//instead of "std::cin>>age;" use "get_untill_int(&age);" in main function.
void get_Untill_Int(int* pInput)//keep taking input until input is `int or float`
{
cin>> *pInput;
/*-----------check input validity----------------*/
while (!cin)
{
cin.clear();
cin.ignore(100, '\n');
cout<<"Invalid Input Type.\nEnter again: ";
cin >>*pInput;
}
/*-----------checked input validity-------------*/
}
I have the following piece of code that prompts the user for their cat's age and name:
#include <iostream>
#include <string>
int main()
{
int age;
std::string name;
std::cin >> age;
std::getline(std::cin, name);
if (std::cin)
{
std::cout << "My cat is " << age << " years old and their name is " << name << std::endl;
}
}
What I find is that the age has been successfully read, but not the name. Here is the input and output:
Input:
"10"
"Mr. Whiskers"
Output:
"My cat is 10 years old and their name is "
Why has the name been omitted from the output? I've given the proper input, but the code somehow ignores it. Why does this happen?
Why does this happen?
This has little to do with the input you provided yourself but rather with the default behavior std::getline() has. When you provided your input for the age (std::cin >> age), you not only submitted the following characters, but also an implicit newline was appended to the stream when you typed Enter:
"10\n"
A newline is always appended to your input when you select Enter or Return when submitting from a terminal. It is also used in files for moving toward the next line. The newline is left in the buffer after the extraction into age until the next I/O operation where it is either discarded or read. When the flow of control reaches std::getline(), it will see "\nMr. Whiskers" and the newline at the beginning will be discarded, but the input operation will stop immediately. The reason this happens is because the job of std::getline() is to attempt to read characters and stop when it finds a newline. So the rest of your input is left in the buffer unread.
Solution
cin.ignore()
To fix this, one option is to skip over the newline before doing std::getline(). You can do this by calling std::cin.ignore() after the first input operation. It will discard the next character (the newline character) so that it is no longer in the way.
std::cin >> age;
std::cin.ignore();
std::getline(std::cin, name);
assert(std::cin);
// Success!
std::ws
Another way to discard the whitespace is to use the std::ws function which is a manipulator designed to extract and discard leading whitespace from the beginning of an input stream:
std::cin >> age;
std::getline(std::cin >> std::ws, name);
assert(std::cin);
// Success!
The std::cin >> std::ws expression is executed before the std::getline() call (and after the std::cin >> age call) so that the newline character is removed.
The difference is that ignore() discards only 1 character (or N characters when given a parameter), and std::ws continues to ignore whitespace until it finds a non-whitespace character. So if you don't know how much whitespace will precede the next token you should consider using this.
Match the operations
When you run into an issue like this it's usually because you're combining formatted input operations with unformatted input operations. A formatted input operation is when you take input and format it for a certain type. That's what operator>>() is for. Unformatted input operations are anything other than that, like std::getline(), std::cin.read(), std::cin.get(), etc. Those functions don't care about the format of the input and only process raw text.
If you stick to using a single type of formatting then you can avoid this annoying issue:
// Unformatted I/O
std::string age, name;
std::getline(std::cin, age);
std::getline(std::cin, name);
or
// Formatted I/O
int age;
std::string firstName, lastName;
std::cin >> age >> firstName >> lastName;
If you choose to read everything as strings using the unformatted operations you can convert them into the appropriate types afterwards.
Everything will be OK if you change your initial code in the following way:
if ((cin >> name).get() && std::getline(cin, state))
This happens because an implicit line feed also known as newline character \n is appended to all user input from a terminal as it's telling the stream to start a new line. You can safely account for this by using std::getline when checking for multiple lines of user input. The default behavior of std::getline will read everything up to and including the newline character \n from the input stream object which is std::cin in this case.
#include <iostream>
#include <string>
int main()
{
std::string name;
std::string state;
if (std::getline(std::cin, name) && std::getline(std::cin, state))
{
std::cout << "Your name is " << name << " and you live in " << state;
}
return 0;
}
Input:
"John"
"New Hampshire"
Output:
"Your name is John and you live in New Hampshire"
I am really wondering. C++ has a dedicated function for eating up any remaining or whatever white spaces. It is called std::ws. And then, you can simply use
std::getline(std::cin >> std::ws, name);
That should be the idomatic approach. For each transistion between formatted to unformatted input that should be used.
If we are not talking about white spaces, but entering for example letters where a number is expected, then we should follow the CPP reference and use
.ignore(std::numeric_limits<std::streamsize>::max(), '\n'); to eliminate the wrong stuff.
Please read here
Since everyone above has answered the problem for input 10\nMr Whisker\n, I would like to answer a different approach. All the solution above published the code for if the buffer is like 10\nMr Whisker\n. But what if we don't know how user will behave in giving input. The user might type 10\n\nMr. Whisker\n or 10 \n\n Mr. whisker\n by mistake. In that case, codes above may not work. So, I use the function below to take string input to address the problem.
string StringInput() //returns null-terminated string
{
string input;
getline(cin, input);
while(input.length()==0)//keep taking input as long as valid string is taken
{
getline(cin, input);
}
return input.c_str();
}
So, the answer would be:
#include <iostream>
#include <string>
int main()
{
int age;
std::string name;
std::cin >> age;
name = StringInput();
std::cout << "My cat is " << age << " years old and it's name is " << name << std::endl;
}
Extra:
If user inputs a \n10\n \nmr. whiskey;
To check whether int input is valid or not, this function can be used to check int input (program will have undefined behavior if char is given as input instead of int):
//instead of "std::cin>>age;" use "get_untill_int(&age);" in main function.
void get_Untill_Int(int* pInput)//keep taking input until input is `int or float`
{
cin>> *pInput;
/*-----------check input validity----------------*/
while (!cin)
{
cin.clear();
cin.ignore(100, '\n');
cout<<"Invalid Input Type.\nEnter again: ";
cin >>*pInput;
}
/*-----------checked input validity-------------*/
}