I am trying to read the n-th line from the standard input in the following program. However, the console will print out "current line is" before I input any number...not sure what's wrong. Thanks for help.
int main()
{
string currentLine;
int n;
cin >> n;
cout << n << endl;
while (n > 0)
{
getline(cin, currentLine);
cout << "current line is" << currentLine << endl;
n--;
}
return 0;
}
The formatted input using operator>>() stops as soon as its format can't be satisfied by the next character. For integers it stops when there is no further digit, e.g., when the next character is a whitespace like the newline from entering the line.
std::getline() reads until it finds the first newline. There was one left right before when reading the integer. You probably want to extract this newline and potentially other whitespace. You could, e.g., use
if (std::getline(std::cin >> std::ws, currentLine)) {
// do something with the current line
}
else {
// deal with a failure to read another line
}
The manipulator std::ws skips leading whitespace. As indicated above, you should also verify that the input was actually successful before processing input.
In order to get n, you have to input a number and press the Enter button. As #Kuhl said, the operator>> stops as soon as its format can't be satisfied by the next character.
This means the first time getline(cin, currentline) runs will get '\n' !
Then the program will output "current line is\n" while the '\n' will not be shown on the console.
If you want to get n and 'currentline', you may choose the #Kuhl's answer or write the program like this:
getline(cin, currentline);
while(n>0) {
// anything you want
}
The getline(cin, currentline) will help you to skip the '\n' followed by the number 'n'.
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 have a problem using getline method to get a message that user types, I'm using something like:
string messageVar;
cout << "Type your message: ";
getline(cin, messageVar);
However, it's not stopping to get the output value, what's wrong with this?
If you're using getline() after cin >> something, you need to flush the newline character out of the buffer in between. You can do it by using cin.ignore().
It would be something like this:
string messageVar;
cout << "Type your message: ";
cin.ignore();
getline(cin, messageVar);
This happens because the >> operator leaves a newline \n character in the input buffer. This may become a problem when you do unformatted input, like getline(), which reads input until a newline character is found. This happening, it will stop reading immediately, because of that \n that was left hanging there in your previous operation.
If you only have a single newline in the input, just doing
std::cin.ignore();
will work fine. It reads and discards the next character from the input.
But if you have anything else still in the input, besides the newline (for example, you read one word but the user entered two words), then you have to do
std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');
See e.g. this reference of the ignore function.
To be even more safe, do the second alternative above in a loop until gcount returns zero.
I had similar problems. The one downside is that with cin.ignore(), you have to press enter 1 more time, which messes with the program.
int main(){
.... example with file
//input is a file
if(input.is_open()){
cin.ignore(1,'\n'); //it ignores everything after new line
cin.getline(buffer,255); // save it in buffer
input<<buffer; //save it in input(it's a file)
input.close();
}
}
I know I'm late but I hope this is useful.
Logic is for taking one line at a time if the user wants to enter many lines
int main()
{
int t; // no of lines user wants to enter
cin>>t;
string str;
cin.ignore(); // for clearing newline in cin
while(t--)
{
getline(cin,str); // accepting one line, getline is teminated when newline is found
cout<<str<<endl;
}
return 0;
}
input :
3
Government collage Berhampore
Serampore textile collage
Berhampore Serampore
output :
Government collage Berhampore
Serampore textile collage
Berhampore Serampore
i think you are not pausing the program before it ended so the output you are putting after getting the inpus is not seeing on the screen right?
do:
getchar();
before the end of the program
The code is correct. The problem must lie somewhere else. Try the minimalistic example from the std::getline documentation.
main ()
{
std::string name;
std::cout << "Please, enter your full name: ";
std::getline (std::cin,name);
std::cout << "Hello, " << name << "!\n";
return 0;
}
Only the first call to getline() appears to read anything in from std::cin. Is the fact that buffer contains something a problem - why doesn't getline() just overwrite the contents of buffer?
How can I get the second call to getline() to read something in?
My code:
const int recordLen = 20;
// truncate the input to 20 chars
void getText(char line[])
{
std::cout << "Enter something for getText: ";
std::cin.getline(line, recordLen+1);
for(int i = std::strlen(line); i < recordLen; i++)
{
line[i] = ' ';
}
line[recordLen] = '\0';
}
int main()
{
char buffer[340];
getText(buffer);
std::cout << buffer;
std::cout << "Now enter some more text:";
// put more text into buffer
std::cin.getline(buffer, 30);
std::cout << "you entered : " << buffer << std::endl;
return 0;
}
So - example output of program:
Enter something for getText: alskdjfalkjsdfljasldkfjlaksjdf
alskdjfalkjsdfljasldNow enter some more text:you entered :
After the display of "Now enter some more text:", the program immediately displays "you entered:". It does not give me the opportunity to enter more text, neither does it display any characters that were truncated from the previous call to getline().
std::cin.getline(line, recordLen+1);
Here, if the input is longer than recordLen chars, the remaining characters will not be read and will remain in the stream. The next time you read from cin, you'll read those remaining characters. Note that, in this case, cin will raise its failbit, which is probably what you're experiencing.
If your first input is exactly recordLen chars long, only the newline will remain in the stream and the next call to getline will appear to read an empty string.
Other than that, getline does overwrite the buffer.
If you want to ignore anything beyond the first recordLen chars on the same line, you can call istream::clear to clear the failbit and istream::ignore to ignore the rest of the line, after istream::getline:
std::cin.getline(line, recordLen+1);
std::cin.clear();
std::cin.ignore( std::numeric_limits<streamsize>::max(), '\n' );
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).