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.
Related
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 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 the following code:
int main()
{
char str1[20];
int a;
cout << "Enter Integer:"
cin >> a;
cout << "Integer:"<<a<<endl;
cout << "Enter string:"<<endl;
cin.getline(str1,20);
cout << "Input String is:"<<str1;
return 0;
}
and OUTPUT is:
Enter Integer:20
Integer:20
Enter string:
Input String is:
I am able to enter the string when not accepting integer using cin, but when I try to use cin.getline() after cin, its not working.
Can anybody help?
The problem is that operator>> ignores whitespace (i.e. ' ', '\t', '\n') before a field, i.e. it reads until before the next whitespace.
getline on the other hand reads until and including the next line break, and returns the text before the linebreak.
Consequently, if you do first operator>> before a line-break and then getline, the operator>> will read until before the line-break, and getline will read only until after the line-break, returning an empty string.
Note: what you have in the input buffer after entering "20, 20, mystring" is effectively
20\n20\nmystring
Hence
the first operator>> reads and returns 20
the second operator>> reads until after the second 20, swallows the first \n and returns the second 20
getline reads until the second \n and returns the text before that, i.e. nothing.
Try out the function gets(), i prefer it for accepting strings and the only parameter that you need to pass is the string name.
I'm creating a simple console application in C++ that gets string and char inputs from the user. To make things simple, I would like to use the string and char data types to pass input from cin to.
To get string inputs, I'm using the getline method:
string var;
cin.ignore(); //I used ignore() because it prevents skipping a line after using cin >> var
getline(cin, var);
To get char inputs, I'm using the cin >> var method:
char var;
cin >> var;
This works fine for the most part. However, when I enter a string using getline, it ignores the first character of my string.
Is it possible to use getline and cin >> without having to use ignore, or a method I can call to ensure that my first character isn't skipped?
This is a full sample of code where I use both getline and cin >>:
string firstName;
string lastName;
char gender = 'A';
cout << "First Name: ";
cin.ignore();
getline(cin, firstName);
cout << "Last Name: ";
cin.ignore();
getline(cin, lastName);
while(genderChar != 'M' && genderChar != 'F')
{
cout << "Gender (M/F): ";
cin >> genderChar;
genderChar = toupper(genderChar);
}
cin>>var;
only grabs the var from the buffer, it leaves the \n in the buffer,
which is then immediately grabbed up by the getline
So, following is just fine, (if I understood correctly your problem)
cin>>var;
cin.ignore(); //Skip trailing '\n'
getline(cin, var);
As per your edited post
You don't have to use cin.ignore(); for geline
This extracts characters from buffer and stores them into firstName or (lastName) until the delimitation character here -newline ('\n').
ignore() does not skip a line, it skips a character. Could you send example code and elaborate on the need for cin.ignore()?
std::cin.ignore() will ignore the first character of your input.
For your case, use std::cin.ignore() after std::cin and then getline() to ignore newline character as:
cin>>ch;
cin.ignore(); //to skip the newline character in the buffer
getline(cin,var);
You are using std::isstream::ignore() before std::getline(). std::cin.ignore() will extract the first character from the input sequence and discard that.
http://www.cplusplus.com/reference/istream/istream/ignore/
So basically, cin>>var leaves the '\n' character out of its buffer. So now when you call
getline it reads the '\n' character and stops. Therefore we use cin.ignore() to ignore the first character getline reads i.e '\n' when we use it after cin.
But getline doesn't leave '\n' character instead it stores everything in its buffer till it find '\n' character, then stores '\n' character as well and then stops.
So in your code when you are using cin.ignore() after a getline and again uses getline to take input, it ignores the first character of the string instead of '\n'.
That is why the first character is missing.
Hope this answers your question.
This question already has answers here:
Why does std::getline() skip input after a formatted extraction?
(5 answers)
Closed 9 years ago.
This is my main program,
int main () {
string command;
cin>>command;
if(command == "keyword")
{
string str, str2, str3, str4;
cout << "Enter first name: ";
getline (cin,str);
cout << "Enter last name: ";
getline (cin,str2);
cout << "Enter age: ";
getline (cin,str3);
cout<<"Enter country: ";
getline (cin,str4);
cout << "Thank you, " << str <<" "<<str2 <<" "<<str3<<" "<<str4<< ".\n";
}
}
When keyword is entered, the program immediately outputs :
Enter first name: Enter last name:
completely bypassing the ability to enter the first name.
string command;
cin>>command;
after this just eat the end of the line
string restOfLine;
getline(cin, restOfLine);
Otherwise the '\n' in the line where you input command is not consumed and the next readline reads just it. HTH
cin >> command does not extract the newline character ('\n') from the input stream; it's still there when you call getline(). Therefore, you need an extra dummy call to getline() (or ignore()) to deal with this.
As mentioned by others, the problem is that while reading the command you are leaving the end of line character in the buffer. Besides the alternative proposed by #Armen Tsirunyan, you can use two other approaches:
Use std::istream::ignore for that: cin.ignore( 1024, '\n' ); (assuming that lines will not be greater than 1024 characters in width.
Just replace cin >> command with getline( cin, command ).
Neither alternative requires creating an extra string, the first is weaker (in the event of very long lines), the second alternative modifies the semantics, as now the whole first line (not just the first word) is processed as the command, but this might be fine as it allows you to perform tighter input checking (the command is spelled as required in the first word, and there are no extra options in the command line.
If you have different set of commands and some might need an argument, you can read the command line in one pass, and then read the command and arguments from there:
std::string commandline;
std::vector<std::string> parsed_command;
getline( cin, commandline );
std::istringstream cmdin( commandline );
std::copy( std::istream_iterator<std::string>(cmdin), std::istream_iterator(),
std::back_inserter( parsed_command ) );
// Here parsed_command is a vector of word tokens from the first line:
// parsed_command[0] is the command, parsed_command[1] ... are the arguments