Infinite loop while reading integers from file

Question

I have two files. main.cpp:

#include <iostream>
#include <fstream>

using namespace std;

int main()
{
    ifstream file;
    file.open("file.txt");

    if (!file.good()) {
        cout << "Error." << endl;
        return 1;
    }

    int n;
    while (!file.eof()) {
        file.clear();

        file >> n;
        if (!file.good() && !file.bad()) {
            continue;
        } else {
            cout << "Hardware error." << endl;
            break;
        }
        cout << n << endl;
    }

    file.close();

    return 0;
}

and file.txt:

a 1 2 321b9 ac.de ef#@g 5 #3

I'd like to read only integers from this file and write them out to the console. When file contains only integers program works well but when it contains any invalid characters then I get infinite loop. How can I fix that?

http://stackoverflow.com/questions/5605125/why-is-iostreameof-inside-a-loop-condition-considered-wrong — Blacktempel, Jul 24 '15 at 11:10
Do you want the output to be "1 2 321 9 5 3" or "1 2 3219 5 3" or "1 2 5" or what? — Beta, Jul 24 '15 at 11:17

score 1 · Accepted Answer · answered Jul 24 '15 at 11:11

1

The loop is because the stream doesn't extract the character that is not an integer. You need to extract it before attempting to read another integer.

A small tweak could be all that is needed;

// on a failed read...
file.clear();
char dummy;
file >> dummy;
continue;

A side note on the use of while (!file.eof()); it is generally not recommended to do this. There are several Q&A on SO on this issue.

answered Jul 24 '15 at 11:11

Niall

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Now I'm getting "Hardware error.". But when I remove else condition then program works well... – tdudzik Jul 24 '15 at 11:17
@tdudzik. I think the check in the `if()` is not quite spot on. Try first with just `file.bad()`. – Niall Jul 24 '15 at 11:18
@tdudzik. Excellent, glad that helped. – Niall Jul 24 '15 at 11:25
It's not that it's "generally not recommended to do this"; it's that it's _wrong_ in most cases (including this one), corrupting the program's semantics and giving you a broken, additional loop iteration. – Lightness Races in Orbit Jul 24 '15 at 15:23

Infinite loop while reading integers from file

1 Answers1