My cin is being ignored inside a while loop

Question

I am trying to code a simple question and number checker into my first C++ program. Problem is, when I type a string like one two, or three, the program becomes and infinite loop and it ignores the cin function to re-assign lives to a number.

cout << "How many lives would you like 1 (hard), 2 (medium), or 3 (easy)?" << endl;
cin >> lives;


while(lives != 1 && lives != 2 && lives != 3 && !isdigit(lives))
{
    cout << "You need to input a number, not words." << endl;
    cout << "How many lives would you like 1 (hard), 2 (medium), or 3 (easy)?" << endl;
    cin >> lives;
}

Here is my current code with your suggestions:

    cout << "How many lives would you like 1 (hard), 2 (medium), or 3 (easy)?" << endl;
std::cin.ignore();
std::cin.clear();
if (std::cin >> lives)
{


    while(lives != 1 && lives != 2 && lives != 3)
    {
        cout << "You need to input a number, not words." << endl;
        cout << "How many lives would you like 1 (hard), 2 (medium), or 3 (easy)?" << endl;
        cin >> lives;
    }

}

I have tried converting it to a string and the problem persists. — Logan Saso, Aug 03 '13 at 19:51
lives type is probably integer or something. Use cin.get() instead of cin, because your return value (enter press) stays in the stdin... — DaMachk, Aug 03 '13 at 19:52
Ok so, using cin.get it does not repeat infinitely. However, it gives me the error message within the loop about 5 or 6 times and refuses to take even the normal number when it stops. — Logan Saso, Aug 03 '13 at 19:58

score 10 · Accepted Answer · answered Aug 03 '13 at 20:05

10

#include <iostream>
#include <limits>

int main()
{
    int lives = 0;
    std::cout << "How many lives would you like 1 (hard), 2 (medium), or 3 (easy)?" << std::endl;


    while(!(std::cin >> lives) || lives < 1 || lives > 3)
    {
        std::cout << "You need to input a number, not words." << std::endl;
        std::cout << "How many lives would you like 1 (hard), 2 (medium), or 3 (easy)?" << std::endl;
        std::cin.clear();
        std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');
    }

    return 0;
}

Alright. std::cin.clear(); takes care of resetting the fail bits. std::cin.ignore removes any wrong input left in the stream. And I've adjusted the stop condition. (isDigit was a redundant check, if lives is between 1 and 3 then clearly it's a digit).

answered Aug 03 '13 at 20:05

Borgleader

15,826
5
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Borg, it is saying that max() is expecting an identifier and will not compile – Logan Saso Aug 03 '13 at 20:12
@LoganSaso I don't know what you're using, but this compiles for me in Mingw/QtCreator, it compiles on [ideone](https://ideone.com/zjGfqq), and VS2012. – Borgleader Aug 03 '13 at 20:14
I am using Visual Studio 2012, it is a picky thing. – Logan Saso Aug 03 '13 at 20:14
2

Probably pulling in the `#define` for `min` and `max`. Try adding `#undef max` before including limits and after including and windows specific headers. – Captain Obvlious Aug 03 '13 at 20:17
using namespace std; I have iostream and limits – Logan Saso Aug 03 '13 at 20:18
Wait I just got it, I did't change the weird _tmain and stuff to just main – Logan Saso Aug 03 '13 at 20:19
I do not know what the #undef does specifically, but it works perfectly so... Good Job – Logan Saso Aug 03 '13 at 20:22
@LoganSaso max is a macro, something like `#define max(x,y) x > y? x: y;` and it was conflicting with the numeric_limits thing, undef removes it. Also you can accept the answer if it works ;) – Borgleader Aug 03 '13 at 20:23
It does work but I do not know how to accept the answer. Just made my stack profile today. – Logan Saso Aug 03 '13 at 20:34

score 3 · Answer 2 · answered Aug 03 '13 at 19:51

3

When std::istream can't read a value, it goes into failure mode, i.e., std::failbit is set and the stream yields false when tested. You always want to test if a read operation was successful:

if (std::cin >> value) {
    ...
}

To restore the stream to good state you'd use std::cin.clear() and you probably need to ignore bad characters, e.g., using std::cin.ignore().

answered Aug 03 '13 at 19:51

Dietmar Kühl

150,225
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Diet, it has the same issue is add the two lines before the while loop. What would "value" bee in your example there. Lives...? – Logan Saso Aug 03 '13 at 19:56
`value` is whatever you want want to read, e.g., an `int`. BTW, your test for `isdigit()` won'twork if `lives` is an `int`: this function operates on `char`s. I'm pretty sure that the issue is as described and you may want to print what was received... – Dietmar Kühl Aug 03 '13 at 20:01
Ok, When using the if statement, it never goes to the while loop at all. Even if I type three or tree, it just says my lives are saved which wont work later with my lives--. – Logan Saso Aug 03 '13 at 20:05
Are you trying to enter `three` to be read as an `int`? That will fail! Try using `3`. If you want to support entering `one`, `two`, and `three` you'll need to implement the parsing from strings yourself. – Dietmar Kühl Aug 03 '13 at 20:22
No that was not what I was doing. – Logan Saso Aug 03 '13 at 20:35

My cin is being ignored inside a while loop

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