CODEWITHSUNDEEP
c++while-loop
jose hui
jose hui

Reputation: 1

Why my while loop in c++ wont run correctly?

I am just a newbie to programming and I was trying to write a while loop that runs as long as the input (num) is not a integer which doesn't ends with zero. What happen is when I enter a number that ends with zero, the program runs the loop correctly, but when i enter something nonsense such as rofl the program only print The input is not valid. and won't repeat the loop. I have tried to look for solutions but I am still stuck after a hour. Anyone can help me here? Thx so much!

void rev_sum() {
    int num;
    int a = 1;
    while (a < 2) {
        cout << "Please input a natural number without zero at the end:\n";
        cin >> num;
        if (!cin) {
            cout << "The input is not valid.\n";
            cin.clear();
            cin.ignore(INT_MAX);
        }
        if (num % 10 == 0) {
            cout << "The number cannot have zero at the end\n";
        } else {
            cout << "gj\n";
            break;
        }
    }
}

Upvotes: 0

Views: 136

Answers (5)

Neil Kirk
Neil Kirk

Reputation: 21813

cin.ignore(numeric_limits<streamsize>::max())

Upvotes: 0

ZipZapZam
ZipZapZam

Reputation: 11

You set a = 1 at the start of the loop and then never change a which means the only way to leave your while loop is if you hit the break statement. If your loop is not looping then it must be stuck somewhere. I'm unfamiliar with the statements if (!cin) and cin.ignore(...) so those are top suspects to check (or change). The statement cin >> num; completes no matter what they type so you can check to see what 'num' is equal to when you enter "rofl". Then after it fails, you still use num so you are processing this unintentional entry. You can add continue; after cin.ignore(...) to jump back to the top of the while loop and ask the question again. You can also print something after the while loop in order to know when you get out.

With all that said, I would never trust the user to enter acceptable information and I would never trust cin to process it for me. Personally, I would read cin as a string using cin.getline(buffer, buffer_size); And then I would complain to the user if they filled the buffer or gave me something that was not an integer (which you can check with a function like scanf()). Then you can spit back exactly what they gave you and you can be specific about your complaint.

Upvotes: 1

Abdulrahman Alhemeiri
Abdulrahman Alhemeiri

Reputation: 675

Try replacing

cin.ignore(INT_MAX);

With

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

And change the 

if (num % 10 == 0)

To

else if (num % 10 == 0)

Your final code should look like this:

void rev_sum() {
    int num;
    int a = 1;
    while (a < 2) {
        cout << "Please input a natural number without zero at the end:\n";
        cin >> num;
        if (!cin) {
            cout << "The input is not valid.\n";
            cin.clear();
            cin.ignore(numeric_limits<streamsize>::max(), '\n');
        }
        else if (num % 10 == 0) {
            cout << "The number cannot have zero at the end\n";
        } else {
            cout << "gj\n";
            break;
        }
    }
}

Upvotes: 3

James Robinson
James Robinson

Reputation: 36

Would it not make more sense to only check that the number ends in 0 if the input was valid....

void rev_sum() {
    int num;
    int a = 1;
    while (a < 2) {
        cout << "Please input a natural number without zero at the end:\n";
        cin >> num;
        if (!cin) {
            cout << "The input is not valid.\n";
            cin.clear();
            cin.ignore(INT_MAX);
        } else {
            if (num % 10 == 0) {
                cout << "The number cannot have zero at the end\n";
            } else {
                cout << "gj\n";
                break;
            }
        }
    }
}

Upvotes: 0

F&#233;lix Adriyel Gagnon-Grenier
F&#233;lix Adriyel Gagnon-Grenier

Reputation: 8893

if (num % 10 == 0) {

could be

else if (num % 10 == 0) {

otherwise the other else case might get executed

Upvotes: 1

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