What happened when press ctrl+D in while(cin>>i) loop in C++?

Question

I am new to C++ and feel a little confused with this question. I use Mac OS X and command "g++ -std=c++11" to compile the code.

May be this question is too broad, I don't know, but if someone can explain with following example please?

int temp;
while (cin >> temp) {
    cout << temp << endl;
}

When I input

1 2 3<ENTER>

It prints

1
2
3

as I expected, and press

<CTRL+D>

to quit.

But if I input

1 2 3<CTRL+D>

It prints

1D
2

following with my input character "3", and then I press

<ENTER>

the last character "3" will be printed. Then I should press

<CTRL+D>

again to quit.

And besides, there is a similar example as following:

vector<int> list; int temp;
while (cin >> temp) {
    list.push_back(temp);
}
for (auto e : list) {
    cout << e << endl;
}

When I input

1 2 3<ENTER><CTRL+D>

It prints

1D
2
3

So my question is:

What happened in these three cases then I press

<CTRL+D>

and

<ENTER>

?

Why there is a "D" character in the second and third case? And how to prevent it from happening?

Are you sure you fail to press correctly `CTRL+D` and maybe you pushed `D` too? Try again and see if the behaviour is repeated! — BiagioF, Aug 11 '16 at 15:51
@BiagioFesta This cannot be true since the list is of integers. How can he push "D"? — Liran Funaro, May 14 '17 at 08:49
"D" is not a character your program prints (it cannot, it only prints numbers). It's something your terminal driver prints. — n. m. could be an AI, May 14 '17 at 09:05
@LiranFunaro I disagree but you always can use the reopen button. — n. m. could be an AI, May 14 '17 at 09:15
@n.m He asked why it prints `D`, not what it does or why isn't it working. I can't repoen it, no such button for me. — Liran Funaro, May 14 '17 at 09:19

Liran Funaro · Answer 1 · 2017-05-14T10:14:26.783

It is probably due to overlapping between the console output and yours.

When you press ctrl+D, the console prints ^D but print a return character ('\r') immediately afterward. This means that everything that will be printed after that, will be printed over the ^D.

Your program first prints 1, so it only overlaps the ^, leaving the D intact. Try inserting a two digit number and you will see that the D disappears.

Some consoles wipe the line after the return character, but some don't as probably your console does.

Solutions

(1) You can simply print '\n' (std::endl) before you start printing the list. This will leave the ugly ^D in the console but it will not overlap your print.

vector<int> list; int temp;
while (cin >> temp) {
    list.push_back(temp);
}
cout << endl; // Print before the list.
for (auto e : list) {
    cout << e << endl;
}

(2) You can print an empty line first to delete this print. Just print: " \r" (i.e., two spaces then a return character).

int temp;
while (cin >> temp) {
    cout << "\r  \r" << temp << endl; // Print in each line
}

Neither of the solutions seem to work except for the special case of pressing Ctrl-D at the first line of the input. — n. m. could be an AI, May 14 '17 at 10:00
@n.m. I've edited the solutions with explicit code to prevent misunderstanding. — Liran Funaro, May 14 '17 at 10:15

What happened when press ctrl+D in while(cin>>i) loop in C++?

1 Answers1

Solutions