I have the following function:
void playersetup(string &name){
char verify;
cout<<"Enter player name: ";
getline(cin,name);
cout<<endl<<"Your name is: " + name + ", is that correct? (Y for yes, any other key for no): ";
cin>>verify;
if ((verify=='y')||(verify=='Y')) {
verify=='n';
} else {
playersetup (name);
}
}
It asks for the player's name, asks the user to verify thats the name they intended, if not it calls itself to start over. However this is what the output looks like:
Enter Player Name:
user enters: John Smith
Your name is: John Smith, is that correct? (Y for yes, any other key for no):
user enters: n
Enter Player Name:
(user is not given a chance to input anything)
Your name is: , is that correct? (Y for yes, any other key for no):
Is there logic error here or an aspect of getline(cin) that Im missing?
The newline character remains in standard input as the >> char consumes the users single character response only. When std::getline() is next invoked it reads the newline character; a blank line. Consume, or skip, the newline character before asking the user for the name again:
#include <limits> // for 'std::numeric_limits'
std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n')
If you're using std::getline after std::cin >> verify you need to flush the newline '\n' out of the buffer in between. Otherwise getline reads '\n' and returns. You can add a call to std::basic_istream::ignore to ignore '\n':
#include <limits>
cin>>verify;
cin.ignore ( std::numeric_limits<std::streamsize>::max(), '\n');
basic_istream/ignore
Related
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.
I wrote a very basic program in C++ which asked the user to input a number and then a string. To my surprise, when running the program it never stopped to ask for the string. It just skipped over it. After doing some reading on StackOverflow, I found out that I needed to add a line that said:
cin.ignore(256, '\n');
before the line that gets the string input. Adding that fixed the problem and made the program work. My question is why does C++ need this cin.ignore() line and how can I predict when I will need to use cin.ignore()?
Here is the program I wrote:
#include <iostream>
#include <string>
using namespace std;
int main()
{
double num;
string mystr;
cout << "Please enter a number: " << "\n";
cin >> num;
cout << "Your number is: " << num << "\n";
cin.ignore(256, '\n'); // Why do I need this line?
cout << "Please enter your name: \n";
getline (cin, mystr);
cout << "So your name is " << mystr << "?\n";
cout << "Have a nice day. \n";
}
ignore does exactly what the name implies.
It doesn't "throw away" something you don't need. Instead, it ignores the number of characters you specify when you call it, up to the char you specify as a delimiter.
It works with both input and output buffers.
Essentially, for std::cin statements you use ignore before you do a getline call, because when a user inputs something with std::cin, they hit enter and a '\n' char gets into the cin buffer. Then if you use getline, it gets the newline char instead of the string you want. So you do a std::cin.ignore(1000,'\n') and that should clear the buffer up to the string that you want. (The 1000 is put there to skip over a specific number of chars before the specified delimiter, in this case, the '\n' newline character.)
You're thinking about this the wrong way. You're thinking in logical steps each time cin or getline is used. Ex. First ask for a number, then ask for a name. That is the wrong way to think about cin. So you run into a race condition because you assume the stream is clear each time you ask for a input.
If you write your program purely for input you'll find the problem:
int main()
{
double num;
string mystr;
cin >> num;
getline(cin, mystr);
cout << "num=" << num << ",mystr=\'" << mystr << "\'" << endl;
}
In the above, you are thinking, "first get a number." So you type in 123 press enter, and your output will be num=123,mystr=''. Why is that? It's because in the stream you have 123\n and the 123 is parsed into the num variable while \n is still in the stream. Reading the doc for getline function by default it will look in the istream until a \n is encountered. In this example, since \n is in the stream, it looks like it "skipped" it but it worked properly.
For the above to work, you'll have to enter 123Hello World which will properly output num=123,mystr='Hello World'. That, or you put a cin.ignore between the cin and getline so that it'll break into logical steps that you expect.
This is why you need the ignore command. Because you are thinking of it in logical steps rather than in a stream form so you run into a race condition.
Take another code example that is commonly found in schools:
int main()
{
int age;
string firstName;
string lastName;
cout << "First name: ";
cin >> firstName;
cout << "Last name: ";
cin >> lastName;
cout << "Age: ";
cin >> age;
cout << "Hello " << firstName << " " << lastName << "! You are " << age << " years old!" << endl;
}
The above seems to be in logical steps. First ask for first name, last name, then age. So if you did John enter, then Doe enter, then 19 enter, the application works each logic step. If you think of it in "streams" you can simply enter John Doe 19 on the "First name:" question and it would work as well and appear to skip the remaining questions. For the above to work in logical steps, you would need to ignore the remaining stream for each logical break in questions.
Just remember to think of your program input as it is reading from a "stream" and not in logical steps. Each time you call cin it is being read from a stream. This creates a rather buggy application if the user enters the wrong input. For example, if you entered a character where a cin >> double is expected, the application will produce a seemingly bizarre output.
Short answer
Why? Because there is still whitespace (carriage returns, tabs, spaces, newline) left in the input stream.
When? When you are using some function which does not on their own ignores the leading whitespaces. Cin by default ignores and removes the leading whitespace but getline does not ignore the leading whitespace on its own.
Now a detailed answer.
Everything you input in the console is read from the standard stream stdin. When you enter something, let's say 256 in your case and press enter, the contents of the stream become 256\n. Now cin picks up 256 and removes it from the stream and \n still remaining in the stream.
Now next when you enter your name, let's say Raddicus, the new contents of the stream is \nRaddicus.
Now here comes the catch.
When you try to read a line using getline, if not provided any delimiter as the third argument, getline by default reads till the newline character and removes the newline character from the stream.
So on calling new line, getline reads and discards \n from the stream and resulting in an empty string read in mystr which appears like getline is skipped (but it's not) because there was already an newline in the stream, getline will not prompt for input as it has already read what it was supposed to read.
Now, how does cin.ignore help here?
According to the ignore documentation extract from cplusplus.com-
istream& ignore (streamsize n = 1, int delim = EOF);
Extracts characters from the input sequence and discards them, until
either n characters have been extracted, or one compares equal to
delim.
The function also stops extracting characters if the end-of-file is
reached. If this is reached prematurely (before either extracting n
characters or finding delim), the function sets the eofbit flag.
So, cin.ignore(256, '\n');, ignores first 256 characters or all the character untill it encounters delimeter (here \n in your case), whichever comes first (here \n is the first character, so it ignores until \n is encountered).
Just for your reference, If you don't exactly know how many characters to skip and your sole purpose is to clear the stream to prepare for reading a string using getline or cin you should use cin.ignore(numeric_limits<streamsize>::max(),'\n').
Quick explanation: It ignores the characters equal to maximum size of stream or until a '\n' is encountered, whichever case happens first.
When you want to throw away a specific number of characters from the input stream manually.
A very common use case is using this to safely ignore newline characters since cin will sometimes leave newline characters that you will have to go over to get to the next line of input.
Long story short it gives you flexibility when handling stream input.
Ignore function is used to skip(discard/throw away) characters in the input stream. Ignore file is associated with the file istream.
Consider the function below
ex: cin.ignore(120,'/n');
the particular function skips the next 120 input character or to skip the characters until a newline character is read.
As pointed right by many other users. It's because there may be whitespace or a newline character.
Consider the following code, it removes all the duplicate characters from a given string.
#include <bits/stdc++.h>
using namespace std;
int main() {
int t;
cin>>t;
cin.ignore(); //Notice that this cin.ignore() is really crucial for any extra whitespace or newline character
while(t--){
vector<int> v(256,0);
string s;
getline(cin,s);
string s2;
for(int i=0;i<s.size();i++){
if (v[s[i]]) continue;
else{
s2.push_back(s[i]);
v[s[i]]++;
}
}
cout<<s2<<endl;
}
return 0;
}
So, You get the point that it will ignore those unwanted inputs and will get the job done.
It is better to use scanf(" %[^\n]",str) in c++ than cin.ignore() after cin>> statement.To do that first you have to include < cstdio > header.
I don't think this question is not a duplicate of this. Even though it is caused by the same thing, it is a different manifestation of the problem.
The terminal output when I run this program looks like this (my input is 1 and Rock Lee followed by enter):
Enter a number: 1
Enter your name: Rock Lee
Name is blank.
However, the name shouldn't be blank. Why is the name variable ""? How do I fix this bug? Here is the code:
#include <iostream>
#include <iomanip>
#include <string>
using namespace std;
int main () {
int num;
string name;
cout << "Enter a number: ";
cin >> num;
cout << "Enter your name: ";
getline(cin, name, '\n');
cin.ignore (1000, '\n');
if (name == "")
{
cout << "Name is blank.";
return -1;
}
return 0;
}
Also, please note that I want to use Getline() in the solution, and I want to be able to read an entire line (so the name can be anything, not just first and last name).
When I tried commenting out cin.ignore(1000, '\n');, it gave this output which is also incorrect:
Enter a number: 1
Enter your name: Name is blank.
It doesn't even give me a chance to type in my name. How do I fix this?
cin >> num reads until it sees whitespace, but it doesn't discard the whitespace, so it leaves '\n' in the input. Your call to getline sees it and immediately returns, filling name with an empty string. You need to call cin.ignore before getline to ignore the '\n' that cin::operator>> left laying around.
I wrote a very basic program in C++ which asked the user to input a number and then a string. To my surprise, when running the program it never stopped to ask for the string. It just skipped over it. After doing some reading on StackOverflow, I found out that I needed to add a line that said:
cin.ignore(256, '\n');
before the line that gets the string input. Adding that fixed the problem and made the program work. My question is why does C++ need this cin.ignore() line and how can I predict when I will need to use cin.ignore()?
Here is the program I wrote:
#include <iostream>
#include <string>
using namespace std;
int main()
{
double num;
string mystr;
cout << "Please enter a number: " << "\n";
cin >> num;
cout << "Your number is: " << num << "\n";
cin.ignore(256, '\n'); // Why do I need this line?
cout << "Please enter your name: \n";
getline (cin, mystr);
cout << "So your name is " << mystr << "?\n";
cout << "Have a nice day. \n";
}
ignore does exactly what the name implies.
It doesn't "throw away" something you don't need. Instead, it ignores the number of characters you specify when you call it, up to the char you specify as a delimiter.
It works with both input and output buffers.
Essentially, for std::cin statements you use ignore before you do a getline call, because when a user inputs something with std::cin, they hit enter and a '\n' char gets into the cin buffer. Then if you use getline, it gets the newline char instead of the string you want. So you do a std::cin.ignore(1000,'\n') and that should clear the buffer up to the string that you want. (The 1000 is put there to skip over a specific number of chars before the specified delimiter, in this case, the '\n' newline character.)
You're thinking about this the wrong way. You're thinking in logical steps each time cin or getline is used. Ex. First ask for a number, then ask for a name. That is the wrong way to think about cin. So you run into a race condition because you assume the stream is clear each time you ask for a input.
If you write your program purely for input you'll find the problem:
int main()
{
double num;
string mystr;
cin >> num;
getline(cin, mystr);
cout << "num=" << num << ",mystr=\'" << mystr << "\'" << endl;
}
In the above, you are thinking, "first get a number." So you type in 123 press enter, and your output will be num=123,mystr=''. Why is that? It's because in the stream you have 123\n and the 123 is parsed into the num variable while \n is still in the stream. Reading the doc for getline function by default it will look in the istream until a \n is encountered. In this example, since \n is in the stream, it looks like it "skipped" it but it worked properly.
For the above to work, you'll have to enter 123Hello World which will properly output num=123,mystr='Hello World'. That, or you put a cin.ignore between the cin and getline so that it'll break into logical steps that you expect.
This is why you need the ignore command. Because you are thinking of it in logical steps rather than in a stream form so you run into a race condition.
Take another code example that is commonly found in schools:
int main()
{
int age;
string firstName;
string lastName;
cout << "First name: ";
cin >> firstName;
cout << "Last name: ";
cin >> lastName;
cout << "Age: ";
cin >> age;
cout << "Hello " << firstName << " " << lastName << "! You are " << age << " years old!" << endl;
}
The above seems to be in logical steps. First ask for first name, last name, then age. So if you did John enter, then Doe enter, then 19 enter, the application works each logic step. If you think of it in "streams" you can simply enter John Doe 19 on the "First name:" question and it would work as well and appear to skip the remaining questions. For the above to work in logical steps, you would need to ignore the remaining stream for each logical break in questions.
Just remember to think of your program input as it is reading from a "stream" and not in logical steps. Each time you call cin it is being read from a stream. This creates a rather buggy application if the user enters the wrong input. For example, if you entered a character where a cin >> double is expected, the application will produce a seemingly bizarre output.
Short answer
Why? Because there is still whitespace (carriage returns, tabs, spaces, newline) left in the input stream.
When? When you are using some function which does not on their own ignores the leading whitespaces. Cin by default ignores and removes the leading whitespace but getline does not ignore the leading whitespace on its own.
Now a detailed answer.
Everything you input in the console is read from the standard stream stdin. When you enter something, let's say 256 in your case and press enter, the contents of the stream become 256\n. Now cin picks up 256 and removes it from the stream and \n still remaining in the stream.
Now next when you enter your name, let's say Raddicus, the new contents of the stream is \nRaddicus.
Now here comes the catch.
When you try to read a line using getline, if not provided any delimiter as the third argument, getline by default reads till the newline character and removes the newline character from the stream.
So on calling new line, getline reads and discards \n from the stream and resulting in an empty string read in mystr which appears like getline is skipped (but it's not) because there was already an newline in the stream, getline will not prompt for input as it has already read what it was supposed to read.
Now, how does cin.ignore help here?
According to the ignore documentation extract from cplusplus.com-
istream& ignore (streamsize n = 1, int delim = EOF);
Extracts characters from the input sequence and discards them, until
either n characters have been extracted, or one compares equal to
delim.
The function also stops extracting characters if the end-of-file is
reached. If this is reached prematurely (before either extracting n
characters or finding delim), the function sets the eofbit flag.
So, cin.ignore(256, '\n');, ignores first 256 characters or all the character untill it encounters delimeter (here \n in your case), whichever comes first (here \n is the first character, so it ignores until \n is encountered).
Just for your reference, If you don't exactly know how many characters to skip and your sole purpose is to clear the stream to prepare for reading a string using getline or cin you should use cin.ignore(numeric_limits<streamsize>::max(),'\n').
Quick explanation: It ignores the characters equal to maximum size of stream or until a '\n' is encountered, whichever case happens first.
When you want to throw away a specific number of characters from the input stream manually.
A very common use case is using this to safely ignore newline characters since cin will sometimes leave newline characters that you will have to go over to get to the next line of input.
Long story short it gives you flexibility when handling stream input.
Ignore function is used to skip(discard/throw away) characters in the input stream. Ignore file is associated with the file istream.
Consider the function below
ex: cin.ignore(120,'/n');
the particular function skips the next 120 input character or to skip the characters until a newline character is read.
As pointed right by many other users. It's because there may be whitespace or a newline character.
Consider the following code, it removes all the duplicate characters from a given string.
#include <bits/stdc++.h>
using namespace std;
int main() {
int t;
cin>>t;
cin.ignore(); //Notice that this cin.ignore() is really crucial for any extra whitespace or newline character
while(t--){
vector<int> v(256,0);
string s;
getline(cin,s);
string s2;
for(int i=0;i<s.size();i++){
if (v[s[i]]) continue;
else{
s2.push_back(s[i]);
v[s[i]]++;
}
}
cout<<s2<<endl;
}
return 0;
}
So, You get the point that it will ignore those unwanted inputs and will get the job done.
It is better to use scanf(" %[^\n]",str) in c++ than cin.ignore() after cin>> statement.To do that first you have to include < cstdio > header.
I am trying to input a string. It's working fine when I input something like John.
But if I input something like John Smite I end up in an endless loop and a terminal crash.
string fullname;
do{
cout << "Please input the Full Name of the user: ";
cin >> fullname;
}while(fullname=="");
The space is throwing cin off. You should use getline.
You can try this one:
do{
cout << "Please input the Full Name of the user: ";
cin >> fullname;
}
while(fullname.length() == 0);
As to why you get an infinite loop - operator>> overload for std::string will first discard any leading whitespace and then read up to either next whitespace or the end of avaliable input.
When you enter "John Smite", first iteration reads "John", second iteration reads "Smite" and then there's no more input for subsequent iterations. The problem is that your implementation seems to clear fullname before it attempts a read. But because the stream is not in good state anymore, no more reads are possible and there's your infinite loop.
You could do something like this instead:
string temp;
string fullname;
do {
cin.clear(); // clear any error flags
do {
if (cin >> temp) fullname += temp + ' ';
} while (cin.good());
} while (fullname.empty());
This has a (good) side effect that it collapses multiple adjacent whitespace characters, but it's still pretty clunky.
Much better way would be to just say
do std::getline(std::cin, fullname); while (fullname.empty());