For reference I have already looked at Why does std::getline() skip input after a formatted extraction?
I want to understand cin and getline behavior. I am imagining cin and getline to be implemented with a loop over the input buffer, each iteration incrementing a cursor. Once the current element of the input buffer equals some "stopping" value (" " or "\n" for cin, "\n" for getline), the loop breaks.
The question I have is the difference between the reading behavior of cin and getline. With cin, it seems to stop at "\n", but it will increment the cursor before breaking from the loop. For example,
string a, b;
cin >> a;
cin >> b;
cout << a << "-" << b << endl;
// Input: "cat\nhat"
// Output: "cat-hat"
So in the above code, the first cin read up until the "\n". once it hit that "\n", it increments the cursor to the next position "h" before breaking the loop. Then, the next cin operation starts reading from "h". This allows the next cin to actually process characters instead of just breaking.
When getline is mixed with cin, this is not the behavior.
string a, b;
cin >> a;
getline(cin, b);
cout << a << "-" << b << endl;
// Input: "cat\nhat"
// Output: "cat-"
In this example, the cin reads up to the "\n". But when getline starts reading, it seems to be reading from the "\n" instead of the "h". This means that the cursor did not advance to "h". So the getline processed the "\n" and advances the cursor to the "h" but does not actually save the getline to "b".
So in one example, cin seems to advance the cursor at "\n", whereas in another example, it does not. getline also exhibits different behaviors. For example
string a, b;
getline(cin, a);
getline(cin, b);
cout << a << "-" << b << endl;
// Input: "cat\nhat"
// Output: "cat-hat"
Now getline actually advances the cursor on the "\n". Why is there different behavior and what is the actual implementation of cin vs getline when it comes to delimeter characters?
reading behavior of cin and getline.
cin does not "read" anything. cin is an input stream. cin is getting read from. getline reads from an input stream. The formatted extraction operator, >>, reads from an input stream. What's doing the reading is >> and std::getline. std::cin does no reading of its own. It's what's being read from.
first cin read up until the "\n". once it hit that "\n", it increments the
cursor to the next position
No it doesn't. The first >> operator reads up until the \n, but does not read it. \n remains unread.
The second >> operator starts reading with the newline character. The >> operator skips all whitespace in the input stream before it extracts the expected value.
The detail that you're missing is that >> skips whitespace (if there is any) before it extracts the value from the input stream, and not after.
Now, it is certainly possible that >> finds no whitespace in the input stream before extracting the formatted value. If >> is tasked with extracting an int, and the input stream has just been opened and it's at the beginning of the file, and the first character in the file is a 1, well, the >> just doesn't skip any whitespace at all.
Finally, std::getline does not skip any whitespace, it just reads from the input stream until it reads a \n (or reaching the end of the input stream).
tl;dr: it's because how std::cin is intra-line-oriented while getline is line-oriented.
Historically, in C's standard library, we had the functions scanf() and getline():
When you tell scanf() to expect a string, it
... stops at white space or at the maximum field width, whichever occurs first.
and more generally,
Most conversions [e.g. readings of strings] discard initial white space characters
(from the scanf() man page)
When you call getline(), it:
reads an entire line ... the buffer containing the text ... includes the newline character, if one was found.
(from the getline() man page)
Now, C++'s std::cin mechanism replaced scanf() for formatted input matching, but with type safety. (Actually std::cin and std::cout are quite problematic as replacements, but never mind that now.) As a substitute for scanf(), it inherits many of its features, including being averse to picking up white space.
Thus, just like scanf(), running std::cin >> a for a string a will stop before a \n character, and keep that line break in the input stream for future use. Also, just like scanf(), std::cin's >> operator skips leading whitespace, so if you use it a second time, the \n will be skipped, and the next string picked up starting from the next line's first non-whitespace character.
With std::getline(), you get the exact same getline() behavior of decades past.
PS - you can control the whitespace-skipping behavior using the skipws format-flag of std::cin
Related
I'm having an issue where I want to input elements of a string array but the first letter is ignored for all the elements except for the first one. For example, i write pepperoni, olvies, cheese, it will print out pepperoni, lives, heese. If you look at my code, I think it is because of the cin.ignore() that comes before inputting name.
cin.ignore();
getline(cin, name);
petes.setCustomerName(name);
for (int i = 0; i < numToppings; i++)
{
cin.ignore();
getline(cin, toppingNames[i]);
}
You must have a cin before your first cin.ignore.
To remember a simple trick, always use cin.ignore right after any cin >> any_variable you use in your code.
cin stores an "Enter" (i.e. '\n' character) value in the input stream. Because of this character in the input stream, if you use a command like getline anywhere after the cin in your code, your getline will be ignored.
For example, in your case, the first getline(cin, name) was being ignored because you might have used a cin before.
However, there's an issue in the use of cin.ignore. While it is true that it will help you a lot when used after cin statements, it can also be quite annoying when used incorrectly.
Consider a case where you have not used a cin statement and the first statement in your code is a getline. Now, if you use cin.ignore before the getline statement, the cin.ignore will wait for a character to be entered, will remove it from the input stream and then the remaining code will be executed.
This is because in such a scenario, there was no '\n' character stored in the input stream and hence, input stream was empty. Therefore, cin.ignore waited for a character to be entered before proceeding further.
For example, consider this code,
int main()
{
string var;
cin.ignore();
getline(cin, var);
}
Suppose you enter "David" for your "var" input. Now, since at the time of cin.ignore, there was nothing in input stream, so cin.ignore waited for a character to come. As soon as you pressed D, cin.ignore removed the D from the input stream and then proceeded further. Now, avid will be taken into the input stream and as soon as you pressed Enter key ('\n'), getline will store "avid" in your "var" variable, ultimately giving an incorrect reuslt.
Consider the same example but with an cin statement before,
int main()
{
int a;
string var;
cin >> a;
cin.ignore();
getline(cin, var);
}
In this case, when your cin >> a statement will execute, it will store the '\n' character in the input stream. Now, cin.ignore will notice the '\n' character in the input stream, so it will remove that character and will proceed further. Now, if you enter "David" and then press enter, your complete word "David" will be stored in the "var" because cin.ignore ignored the '\n' character stored by the cin statement and will not interfere with your "David" string.
But, suppose you do the same without cin.ignore() statement,
int main()
{
int a;
string var;
cin >> a;
// cin.ignore();
getline(cin, var);
}
As always, the cin >> a statement will store a '\n' character in the input stream. Then it will proceed further. There is no cin.ignore in this case however so it will move directly to getline.
getline statement will notice a '\n' in the input stream, so it will think that the user has already entered his input and will store whatever's in the input stream in the variable "var". Input stream had nothing in it except the '\n' character so your var variable will be empty
This is the reason why you should use cin.ignore() but it should always be used with care.
Moving on to your example, your name variable was being ignored because there must have been a "\n" character stored in the input stream and hence, getline was assuming that you have already entered your input. Using cin.ignore was necessarily in this case.
However, after that command, there was no cin statement, and hence input stream was empty. But in your for loop, you have again used cin.ignore before taking input by using getline statement. Since the input stream is empty, so cin.ignore will always ignore the first character of whatever input you enter and the remaining input will then be stored in the variable you are using with getline.
Just to be careful and to avoid this error, it is always good to use cin.ignore right after your cin statements rather than using before getline statements. In this way, you will be ignoring the '\n' stored by cin easily and cin.ignore will not interfere with your getline statements either (which might happen if you use it right before getline statements)
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'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.
I've noticed that whenever I write a program that uses std::cin that if I want the user to press Enter to end the program, I have to write std::cin.ignore() twice to obtain the desired behavior. For example:
#include <iostream>
int main(void)
{
int val = 0;
std::cout << "Enter an integer: ";
std::cin >> val;
std::cout << "Please press Enter to continue..." << std::endl;
std::cin.ignore();
std::cin.ignore(); // Why is this one needed?
}
I've also noticed that when I'm not using cin for actual input but rather just for the ignore() call at the end, I only need one.
Discl: I'm simplifying what really happens.
The first serves to purge what the extraction operator (>>) hasn't consumed.
The second waits for another \n.
It is exactly the same when we do a std::getline after an extraction: a the_stream::ignore(std::numeric_limits<streamsize>::max(), '\n'); is required before the call to std::getline()
That's strange. What platform are you running on? By definition, ignore extracts and discards n characters from the input stream or if it hits EOF it stops. If you do not specify any parameters it extracts 1 character. On Windows, line ending involves both a \r and a \n -- a total of two characters (a carriage return followed by a newline).