I understood that cin.getline() function doesn't clean the buffer and for example in the code below the program skip the line 4:
char name[10];
char id[10];
std::cin >> name;
std::cin.getline(id,10);
std::cout << name << std::endl;
std::cout << id << std::endl;
the output (if I enter "Meysam" as name variable):
Meysam
so because the cin.getline() doesn't clean the buffer then we can't enter the id variable but if we use two cin.getline() like below we can enter name and id variable.
char name[10];
char id[10];
std::cin.getline(name,10,'\n');
std::cin.getline(id,10,'\n');
std::cout << name << std::endl;
std::cout << id << std::endl;
here is the output ( we entered Meysam and 12345 as name and id):
Meysam
12345
but why is this?
I mean because the cin.getline() doesn't clean the buffer we should be able to enter the name variable, but then the program should skip the next cin.getline() which is for id, because the buffer is already filled by previous cin.getline().NO?
I want to know where I didn't understand correctly
Thank you.
In the first case, cin>>name does not consume the newline character and it is still there in the buffer.
The fourth line is not skipped, instead cin.getline() reads the \n in the buffer and stops reading further as the default delimiter of getline in \n. As such the id only contains the newline character.
In the second case, cin.getline() reads the complete line and stops after consuming the newline character. There is nothing left in the buffer now, so the next cin.getline() reads your id.
Refer here
std::cin >> name; will read from input until a delimiter is found, normally space or newline chars, but the delimiter will be left in the input stream, so the next call to getline reads the newline character and returns an empty line, thus effectively clearing the id variable.
The second program works because getline extracts the newline delimiter so the next call doesn't sees it and waits for input.
You can see a better explanation on cppreference (see "notes" section.)
Related
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
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 take the input of the two sentences one after the other,but while printing it is printing a blank space and in the next line it is printing the first sentence and the loop is exiting.
Here is my code:
int main()
{
char b[100000];
char c[100000];
int t;
cin>>t;
while(t--)
{
cin.getline(b,100000);
cin.getline(c,100000);
cout<<b<<"\n"<<c<<"\n";
}
}
The input:
1
run run
good sentence
The output:
Blankspace
run run
cin >>t;
This will prompt the user for some input. Assuming the user does what's expected of them, they will type some digits, and they will hit the enter key.
The digits will be stored in the input buffer, but so will a newline character, which was added by the fact that they hit the enter key.
cin will parse the digits to produce an integer, which it stores in the num variable. It stops at the newline character, which remains in the input buffer.
cin.getline(b,100000);
cin.getline(c,100000);
Later, you call cin.getline(b,100000);, which looks for a newline character in the input buffer. It finds one immediately, so it doesn't need to prompt the user for any more input. So it appears that the first call to getline didn't do anything, but actually it did.
Big arrays are not good idea. Try use std::string instead.
#include <iostream>
#include <string>
int main() {
std::string lineOne;
std::string lineTwo;
std::getline(std::cin, lineOne);
std::getline(std::cin, lineTwo);
std::cout << lineOne << "\n" << lineTwo;
return 0;
}
The reason is that cin>>t stops at the first non-numeric character, which is the newline. The next cin.getline(b,100000) will read this newline character into b and "run run" to c.
To avoid this, you can first read the newline character into somewhere else. Like
cin >> t;
// read the newline character
getchar();
while(t--){...}
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.