char input makes the loop not stop in C++

Question

The C++ program is as follows

#include <iostream>

using namespace std;

int main(void) {
    /* temporary storage for the incoming numbers */
    int number;

    /* we will store the currently greatest number here */
    int max = -100000;

    /* get the first value */
    cin >> number;

    /* if the number is not equal to -1 we will continue */
    while(number != -1) {

        /* is the number greater than max? */
        if(number > max)

            /* yes ¬ñ update max */
            max = number;

        /* get next numbet */
        cin >> number;
    }

    /* print the largest number */
    cout << "The largest number is " << max << endl;

    /* finish the program successfully */
    return 0;
}

If I enter some number such as 69 10 -1. It will work. But when I enter some char, even I enter -1, it didn't stop. For example a a -1 -1 -1 Why?

Linked: http://stackoverflow.com/questions/13031414/cin-loop-never-terminating . In particular, see the answer by Dietmar Kühl. I am not voting to close (for now), because if I did so, this question would be closed instantaneously. — David Hammen, Jan 30 '16 at 18:13
`a` is not a valid representation of an integer, so extraction fails and `std::cin` enters a permanent failure state - further extractions also fail. But yeah, this is a duplicate, so look around for solutions. — Archimaredes, Jan 30 '16 at 18:14

score 2 · Accepted Answer · answered Jan 30 '16 at 18:13

You need to check the stream state after each read.

The letter a is not a proper decimal digit, so the input fails. When the input fails, it sets a failure bit, which causes all subsequent inputs to fail or not occur.

Always check the input status after reading a variable.

The recovery is to clear the status if you want to continue.

score 1 · Answer 2 · answered Jan 30 '16 at 18:14

1

Because you want the input statement as your while condition, something like:

while (cin >> number) {

    if(number == -1)
        break;

    if (number > max)
        /* yes ¬ñ update max */
        max = number;
}

answered Jan 30 '16 at 18:14

Paul Evans

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char input makes the loop not stop in C++

2 Answers2