Program goes into infinite loop when I input "9+"

Question

#include <iostream>
using namespace std;

void check_positive () {
    cout << "Number given must be positive. Please try again." << endl;
}

int main() {
    int pesos, interest, compound, year;
    do {
        cout << "How many pesos did you deposit? ";
        cin >> pesos;
        if (pesos <= 0) {
            check_positive();
    }
    } while (pesos <= 0);
    do {
        cout << "What is the interest? ";
        cin >> interest;
        if (interest <= 0) {
            check_positive();
        }
    } while (interest <= 0);
}

Whenever I run this code and enter "9+" as the input during the first loop, the first loop ends but then goes into an infinite loop immediately after the second loop starts. Why does this happen?

Because you never check whether your input ooeration succeeds. — n. m. could be an AI, Oct 12 '15 at 09:53
What do you expect to happen? Be as specific as you can. What code do you think handles this case and how do you think it handles it? — David Schwartz, Oct 12 '15 at 09:53
http://stackoverflow.com/questions/19521320/why-do-i-get-an-infinite-loop-if-i-enter-a-letter-rather-than-a-number already discussed here — FabiF, Oct 12 '15 at 09:57

score 0 · Accepted Answer · answered Oct 12 '15 at 09:57

0

You entered characters 9+ that are not a number, and tried to load them into an integer variable int pesos which can only accept numbers. Cin couldn't convert 9+ to a number and so it entered fail state which you can check by changing your first loop like so:

do {
    cout << "How many pesos did you deposit? ";
    cin >> pesos;

    if (cin.fail()) {
        cout << "You didn't enter a number!";
        return EXIT_FAILURE;
    }

    if (pesos <= 0) {
        check_positive();
}

be wary that you can also have the same problem in the second loop and thus you need to check cin.fail() again there

For additional reading: ios::fail() reference

answered Oct 12 '15 at 09:57

Ritave

1,333
9
25

But why didn't the first loop repeat instead of the second loop since the variable "pesos" was the one that had the unacceptable value? – Joshua Franco Pili Oct 12 '15 at 10:23
I think it depends on the implementation of cin, but probably because cin read `9` and put it into pesos variable, and then got into fail state due to `+`, since now `pesos > 0` it exits the first loop. And when you tried to use cin in failed state to read into `int interest` it returns immediately without reading any input. – Ritave Oct 12 '15 at 10:31
Oh okay, thank you for all the help :) – Joshua Franco Pili Oct 12 '15 at 10:45

Program goes into infinite loop when I input "9+"

1 Answers1