std::cin >> c; gets ignored after reading numbers into array

Question

I'm trying to input some numbers into an array. Pretty simple task.

int array[100], n = 0, length = 0;
std::cout << "Input numbers: " << std::endl;
while (std::cin >> n) {
    array[length++] = n;
}

I press shift + F10 in CLion, trying to run it and

it won't end the while (I pressed enter 5 times in a row), it goes on forever. am I doing something wrong here?

I tried using std::cin.ignore() after each input. It does not seem to have any effect.

Thank you!

EDIT: It does successfully end when I press ctrl-D, but I've encountered another problem. (I edited the title)

I have this program:

void read_input(int arr[], int &length) {
    int n = 0;
    std::cout << "Read input: " << std::endl;
    while (std::cin >> n) {
        arr[length++] = n;
    }
}

int main() {
    int array[100], length = 0;
    int c = -1;

    while (true) {
        std::cout << "Menu: (TO DO)\n";
        std::cout << "Your option: ";
        std::cin >> c;

        if (c == 1) {
            read_input(array, length);
        }
        if (c == 0) break;
    }
}

What happens is, I enter the option:

1
Read input:
1
2
3
4
^D

Menu: (TO DO)
Your option:
Read input:
Menu: (TO DO)
Your option:
Read input:
Menu: (TO DO)
Your option:
Read input:
...

Basically, after it goes into read_input() and I give it some numbers, I press ctrl-D and it won't ask me again for an input/option, it will just read 1 for std::cin >> c; again and again and it will go on forever.

Use the platform-specific EOF keyboard shortcut (`Ctrl-D` on Linux/macOS, or `Ctrl-Z` on Windows). — Some programmer dude, Mar 02 '21 at 08:42
When you give the EOF input the input stream is in a state where it can't be used again. You need to [*clear*](https://en.cppreference.com/w/cpp/io/basic_ios/clear) that state before you can use it again. — Some programmer dude, Mar 02 '21 at 10:02
I don't know if I'm doing it right, but I put `std::cin.clear();` right at the end of `read_input()` after the while loop, so I can make the input usable again, If I understood it right. But now, after I press `ctrl-D` it just quits the program (it gets out of the while(true) loop). — yierstem, Mar 02 '21 at 10:47
Are you also `ignore()`'ing the `Ctrl-D` itself so it does not linger in the input buffer? — Remy Lebeau, Mar 04 '21 at 22:09
I've tried to put std::cin.ignore() everywhere, it still loops to infinity. Perhaps i'm doing it wrong, how should I do it? — yierstem, Mar 04 '21 at 22:20

score 1 · Accepted Answer · answered Mar 06 '21 at 17:55

The approach of reading user inputs until Ctrl+D is not a good idea since it causes many problems to your program and may have potential errors when running on other platforms. Ctrl+D closes the system level pipe so I am not sure if there is a proper way to restore the input stream after being stopped by EOF. As I said, even if you find a solution that works in your environment but there is still a chance of getting a potential bug. Therefore, my suggestion is to use a sentinel (here, eof as string) to replace Ctrl+D.

#include <iostream>
#include <string>


void read_input(int arr[], int &length) {
    std::cout << "Read input (enter `eof` when you are done): " << std::endl;

    std::cin.clear();
    std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');

    std::string number; 
    while (std::getline(std::cin, number) && number != "eof")
    {
        arr[length++] = std::stoi(number);
    }
}

int main() {
    int array[100], length = 0;
    int c = -1;

    while (true) {
        std::cout << "Menu: (TO DO)\n";
        std::cout << "Your option: ";
        std::cin >> c;

        if (c == 1) {
            read_input(array, length);
        }
        if (c == 0) break;
    }
}

Output:

Menu: (TO DO)
Your option: 1
Read input (enter `eof` when you are done): 
1
2
3
eof
Menu: (TO DO)
Your option: 1
Read input (enter `eof` when you are done): 
1
2
3
4
5
eof
Menu: (TO DO)
Your option: 2
Menu: (TO DO)
Your option: 0
Process exited with status 0

You can replace eof by whatever you like except numbers for this task.

mrkubax10 · Answer 2 · 2021-03-02T08:59:41.497

0

You should use for loop:

for(int i=0; i<5; i++)
{
  std::cin>>n;
  array[i]=n;
}

Or if you want to finish input on newline:

std::string input="";
while(input!="\n")
{
  getline(std::cin,input);
  n=std::stoi(input);
  array[length++]=n;
}

edited Mar 02 '21 at 08:59

answered Mar 02 '21 at 08:46

mrkubax10

1
2

`sizeof(array)` will give you `400` on any standard PC, which will go way out of bounds of the array. And not stop if the user want's to stop early. – Some programmer dude Mar 02 '21 at 08:49
Your edit won't work either, since the `>>` operator for strings read *space-delimited* strings, and newline is a space and won't be read. – Some programmer dude Mar 02 '21 at 08:57

Alireza Abbasi · Answer 3 · 2021-03-05T17:43:48.300

The whitespace will not be discarded from the beginning of the input sequence. You can discard the whitespace with using the std::ws function which is a manipulator designed to extract and discard leading whitespace from the beginning of an input stream.

You can use from getline() for do it.

#include <iostream>
#include <string>

void read_input(int arr[], int& length) {
    int n = 0;
    std::cout << "Read input: " << std::endl;
    std::string num_str{};
    while (std::getline(std::cin >> std::ws, num_str)) {
        try
        {
            n = std::stoi(num_str);
            arr[length++] = n;
        }
        catch(...)
        {
            break;
        }
    }
}

int main() {
    int array[100], length = 0;
    int c = -1;
    
    while (true) {
        std::cout << "Menu: (TO DO)\n";
        std::cout << "Your option: ";

        std::string str;
        std::getline(std::cin >> std::ws, str);
        try
        {
            c = std::stoi(str);
        }
        catch(...)
        {
            c = -1;
        }
        
        if (c == 1) {
            read_input(array, length);
        }
        if (c == 0) break;
    }
}

score 0 · Answer 4 · answered Mar 06 '21 at 09:30

0

You most likely just need to clear the stream at the end of the read_input loop and make sure that you ignore whatever is left in the input buffer:

void read_input(int arr[], int &length) {
  int n = 0;
  std::cout << "Read input: " << std::endl;
  while (std::cin >> n) {
    arr[length++] = n;
  }
  std::cin.clear();
  std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');
}

Although Ctrl-D to mark the end of the sequence seems an unusual choice, why not just use a dedicated character to mark the end of input and safely discard it.

answered Mar 06 '21 at 09:30

jester

3,491
19
30

Because I don't know what is the proper way of doing it. I added cin.clear() and cin.ignore() as proposed and now, it goes back to the menu, no infinite loops, but If i enter another option (number) it won't do anything, it won't read the value, even if I press `ctrl-D`. – yierstem Mar 06 '21 at 10:59
@yierstem I did not quite get that, could you maybe clarify that with a sample output? – jester Mar 06 '21 at 18:20
@yierstem The second part of my answer was about the use of Ctrl-D to stop the input sequence - it's possibly not the best since it closes the system level pipe in most *nix systems. Hence the suggestion to use a dedicated character/string as a marker to mark the end of the input. That approach seems to have been elaborated in the answer by Thang. – jester Mar 07 '21 at 08:50

alle_meije · Answer 5 · 2021-03-07T19:44:22.950

0

Your problem seems to be the value of n; is that 0 when you type \n?

I like the solution given here: to capture cin with getline() which returns a string. Then you can check with isempty() if there was any input. You will need to convert to a number with stof() or stol() but that should be doable.

edited Mar 07 '21 at 19:44

answered Mar 07 '21 at 14:28

alle_meije

2,424
1
19
40

std::cin >> c; gets ignored after reading numbers into array

5 Answers5