How to terminate reading from cin?

Question

I've tried a bunch of methods listed on here but none of them work. It's always waiting for more input.

I've tried while(std::getline(std::cin, line)) and the method below, nothing seems to work:

#include <iostream>
#include <sstream>

using namespace std;

int main(){
  long length = 1UL<<32;
  int array[length];
  // memset(array, 0, (length-1) * sizeof(int));

  for(int i = 0; i < length; i++)
    array[i] = 0;
  string line;
  int num;
  while(!cin.eof()){
    getline(cin,line);
    stringstream ss(line);
    ss >>num;
    array[num]++;
  }
  for(int i = 0; i < length; i++)
      if(array[i]){
          cout << i << ": ";
          cout << array[i] << endl;
      }
}

How are you ending input? With bash, you can use ctrl+d, or just pipe a file to your process. — Brendan Long, Nov 15 '13 at 01:16
Yes, I'm using ctrl d when doing it by hand, or i redirect a file. neither works — ordinary, Nov 15 '13 at 01:17
I think you can set a particular string such as "quit" as the ending for input. — yanchong, Nov 15 '13 at 01:26
You don't need that loop to clear your array. Just declare it like this: `int array[length] = {0};` — paddy, Nov 15 '13 at 01:29
`1UL << 32` causes undefined behaviour on systems with 32-bit long — M.M, May 27 '18 at 04:44

score 5 · Answer 1 · answered Nov 15 '13 at 01:18

First off, do not use std::cin.eof() to control your loop! It doesn't work. Also, you always need to check for successful input after the input.

That said, to terminate the input you'll need to enter the appropriate end of file character, probably at the start of the line (how it works entirely somewhat depends on the system, some settings, etc.). On Windows you'd use Ctrl-Z, on UNIXes you'd use Ctrl-D.

score 0 · Answer 2 · answered Nov 15 '13 at 01:23

First off, this part of your program attempts to allocate 4 GB of memory on the stack, which doesn't work on my machine (good luck finding any machine with 4 GB of continuous memory space):

long length = 1UL<<32;
int array[length];

If I change that to a much more reasonable:

long length = 32;

Then it works fine for me:

$ g++ -g test.cpp -o test && ./test
2
5
# pressed control+d
2: 1
5: 2
$

So I'm guessing something else is wrong.

Note: Unless you actually plan to use all of those indexes, you may want to considering using an unordered_map, so you only use the space you actually need.

score 0 · Answer 3 · answered Nov 15 '13 at 05:04

The condition you are looking for can be most easily tested by evaluating "std::cin" as a bool, i.e. while (cin). But it won't do this until you have tried to read beyond EOF, so expect an empty getline:

#include <iostream>
#include <string>

int main() {
    std::string input;
    while (std::cin) {
        std::cout << "Type something in:\n";
        std::getline(std::cin, input);
        if(input.empty())
            continue;
        std::cout << "You typed [" << input << "]\n";
    }
    std::cout << "Our work here is done.\n";

    return 0;
}

Live demo: http://ideone.com/QR9fpM

this is the same issue as [here](https://stackoverflow.com/questions/5605125/why-is-iostreameof-inside-a-loop-condition-considered-wrong). It may or may not flag EOF after reading the last line; this code may or may not output the last line twice — M.M, May 27 '18 at 04:40

How to terminate reading from cin?

3 Answers3

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