Problems with input check in c++

Question

I want the program to loop until the input is valid (x is an int, and x>0), but my program accepts the input when I give it 1,1 and loops infinitely when I give a string to it, repeating "Wrong input!".

#include <iostream>

using namespace std;

int main()
{
    bool fail;
    int x;
    do{
        cin >> x;
        fail=(cin.fail() || x<=0);
        if (fail){
            cout << "Wrong input!" <<endl;
            cin.clear();
        };
    }while(fail);
    return 0;
}

Vlad from Moscow · Accepted Answer · 2016-03-01T09:50:30.863

2

Use standard member function ignore in the if statement. For example

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

or just

std::cin.ignore();

It is used to skip invalid symbols in the buffer.

edited Mar 01 '16 at 09:50

answered Mar 01 '16 at 09:45

Vlad from Moscow

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Shall I use it instead of cin.clear()? – R2DToo Mar 01 '16 at 09:50
@R2DToo Insert the call before cin.clear() – Vlad from Moscow Mar 01 '16 at 09:51
Still keeps looping forever for the input "asd" .. And for "1.1" it gives back 1 – R2DToo Mar 01 '16 at 09:53
@R2DToo I am sorry. Place the call of ignore AFTER the call of clear. – Vlad from Moscow Mar 01 '16 at 10:02
@R2DToo ignore without arguments skips only one symbol from the buffer. Use the first form of the call if you want to ignore all the buffer with invalid data. – Vlad from Moscow Mar 01 '16 at 10:04

score 0 · Answer 2 · answered Mar 01 '16 at 09:56

You may read an entire line and convert the line with a string stream:

#include <iostream>
#include <sstream>

int main()
{
    bool fail;
    int x;
    do{
        std::string line;
        fail = ! getline(std::cin, line);
        if( ! fail) {
            std::istringstream s(line);
            fail = ! (s >> x) 
                || x < 0  
                || ! s.eof(); // The entire line represents an integer
        }
        if(fail) {
            std::cout << "Wrong input!" << std::endl;
        };
    } while(fail);
    return 0;
}

Problems with input check in c++

2 Answers2