Does getline() not extract the delimiter?

Question

So I was taking input some integers and then taking input some sentences.

This code works fine:

#include<bits/stdc++.h>
using namespace std;
main(){
    int c,b,n,i;string s;
    cin>>n>>b>>c;
    for(i=0;i<n;i++){
        cin>>ws;
        getline(cin,s,'\n');
        cout<<s; 
     }
    }

Example:

3 3 3
This is weird
This is weirdDefinitely makes
Definitely makesNo sense
No sense

However, when I try to omit the cin>>ws inside the forloop, it doesn't work properly, eg this code segment,

#include<bits/stdc++.h>
using namespace std;
main(){
    int c,b,n,i;string s;
    cin>>n>>b>>c;
    for(i=0;i<n;i++){
            getline(cin,s,'\n');
            cout<<s;
    }
}

Example:

3 3 3
This is weird
This is weirdDefinitely makes
Definitely makes

..and terminates there instead of taking all three inputs. Why is that? cin>>ws extracts all whitespace from the input but isn't getline() doing that too? So why does it not work properly when I omit cin>>ws in the forloop?

What is the actual problem: in what way does the code with uses of `std::ws` not work as expected? — Dietmar Kühl, Nov 09 '18 at 05:03
Are you redirecting input from a file or actually typing at the console? — Mark Ransom, Nov 09 '18 at 05:08
@DietmarKühl the problem is if I omit cin>>ws inside the forloop, it doesn't take input properly, why is that? — Little_idiot, Nov 09 '18 at 05:13
What does "it doesn't take input properly" mean? Ideally, provide input and expected output or, even better, demo the problem using a `std::istringstream`. — Dietmar Kühl, Nov 09 '18 at 05:16
You made a change to the code as well as adding the examples; that change was critical. Next time *please* post your exact code! — Mark Ransom, Nov 09 '18 at 05:27
Your first visible output is the second input. See [Why does `std::getline()` skip input after a formatted extraction?](https://stackoverflow.com/questions/21567291/why-does-stdgetline-skip-input-after-a-formatted-extraction) — molbdnilo, Nov 09 '18 at 05:36
@Little_idiot: I have updated my answer to explain what's going on. — Dietmar Kühl, Nov 09 '18 at 05:39

Dietmar Kühl · Answer 1 · 2018-11-09T06:59:11.803

std::getline() extract characters until it extracted the first delimiter character (by default '\n'). The delimiter is not stored in the result but it is extracted. It does not extract whitespace in general or multiple delimiter characters.

As an aside: always check whether input works after trying to read a value.

In the example printed, the issue is is that after formatted input, i.e., using the >> operator, whitespaces are not extracted. That is, the first calls to std::getline() extracts the empty string terminated by the initial newline. It generally is necessary to extract trailing whitespace when switching between formatted and unformatted I/O. That is, You'd want code like

if (cin>>n>>b>>c >> std::ws) {
    for(i=0;i<n;i++){
        if (getline(cin,s,'\n')) {
            cout << "i=" << i << ":'" << s << "'\n";
        }
    }
}

I can't recommend input operations without adding check for success. The output is changed to make it more easily visible what is going on: try the code with/without this particular std::endl to see what is happening.

Clears it up. Thanks :D – Little_idiot Nov 09 '18 at 05:45 — Little_idiot, Nov 09 '18 at 05:45

Gardener · Answer 2 · 2018-11-09T14:39:24.510

The default delimiter for getline() is '\n', so there is no need to include that in the getline call, though, it should not change the functionality.

See for example Same as getline(input, str, input.widen('\n')), that is, the default delimiter is the endline character.

The change in formatting from the integer input to the getline() input leaves some whitespace (endl) after the integer as explained by @DietmarKühl.

You can change the getline() call to eliminate the delimiter to

getline(cin,s);

which will cause getline() to use '\n' as the default delimiter.

I have modified the 'n' variable to count and removed the other integers to make the code a little simpler to read:

#include <iostream>

int main()
{
    int i;  // index
    int count;  // number of strings to accept
    std::string str;

    std::cout << "Input the number of strings you would like me to process: " << std::endl;
    std::cin >> count;

    if (std::cin >> count >> std::ws) {
        for (i = 0; i < count; i++) {
            if (getline(std::cin, str)) {
                std::cout << "i=" << i << ":'" << str << "'\n";
            }
        }
    }
}

That is actually incorrect: `getline()` has two reason to stop: either the delimiter is found or the end of the stream is found. It doesn't stop on newline if it isn't the used delimiter. — Dietmar Kühl, Nov 09 '18 at 04:59

Allamo Olsson · Answer 3 · 2018-11-09T15:20:06.807

0

Cin doesn't extract all white spaces, it just gets the first word until the first white space. It is like having a getline with a space delimiter(not quite but close to).

Getline takes the whole line and has the default '\n' delimiter like mentioned above.

Ex:

string a = "Stack Overflow is awesome";

can give you Stack and getline will give you everything at that line

edited Nov 09 '18 at 15:20

answered Nov 09 '18 at 04:59

Allamo Olsson

193
1
10

Re: " cin just extracts the word" -- `std::cin` doesn't extract anything. `>>` and `getline` extract characters from an input stream; when you pass `std::cin` to those functions they extract characters from it. – Pete Becker Nov 09 '18 at 14:36

score 0 · Answer 4 · answered Nov 09 '18 at 05:30

When you use cin >> it doesn't remove any whitespace after the input. This means the newline that terminated the first 3 inputs is still in the buffer, waiting to be read by the first getline. Since there's nothing before the newline, the first getline delivered an empty string. Your output should have included a newline so you could have seen the empty line, then it would have made sense.

Originally the code you posted showed a cin >> ws just before the for loop which would have eliminated this problem.

Does getline() not extract the delimiter?

4 Answers4