endless loop

[pavik]

The code below works fine with numeric input but gets in an endless loop with alphabetical input. Why?

void getUserInput(vector<int> &ui)
{
    int entry=0;
    bool isEntryValid=true;
    system("cls");

    do
    {
        if(isEntryValid==false)
        {
            cout<<"Invalid entry!"<<endl;
        }

        cout<<"Enter numeric input: <1-100>"<<endl;
        cin>>entry;

        if(entry < 1  || entry > 100 )
        {
            isEntryValid=false;
            system("cls");
        }
        else
        {
            isEntryValid=true;
        }

    }
    while (isEntryValid==false);
    ui.push_back(entry);

}

[jlb]

but gets in an endless loop with alphabetical input. Why?

Because the insertion operator>> knows that a number can't contain a non-digit value and if it encounters a non-digit value it places the stream into a failure state. When the stream is in a failure state no further processing will occur with that stream until the error flags are cleared and the offending data is removed from the stream.

[jonnin]

if you want to handle the errors, you can read everything as a string, then check to see if the string is a valid value and if so, convert & move it to the target, if not, request a new value until it passes.

different compilers may handle this differently. Mine just assigns zero to entry when junk is put in.

[Handy Andy]

Hello pavik,

Welcome to the forum.

In addition to what jlb says something you could do at like 16 is:

while (!cin)
{
    std::cout<<" An error message"<<std::endl;  // <--- Message of your choice.

    cin.clear();
    std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');  // <--- Requires header file <limits>.

    std::cout<<"Enter numeric input: <1-100>"<<endl;
    std::cin >> entry;
}

This will catch any non numeric entry and repeat until a valid number is entered.

"cin >> entry" is a formatted input which means since "entry" is defined as an int it expects a number and nothing else.

Hope that helps,

Andy

[pavik]

Thx for the quick reply. Meanwhile I also found two other topics in connection with my question.

http://www.cplusplus.com/forum/articles/6046/
http://www.cplusplus.com/articles/D9j2Nwbp/

I'll check all the given hints.

[pavik]

I modified my code in two ways. First of all converting string to number (http://www.cplusplus.com/forum/articles/6046/) Then in another solution I used Handy Andy's hint. Both are much better than my original one (what else could be :-) but neighter of them are perfect.

When user enters a junk input - leading number - (i.e 9asd) the codes handles inputs like this as a valid entry (i.e. 9)

Why, and what modifications needs to get rid of this phenomenon?

[jonnin]

why is because it is valid .. stoi etc type functions can pull out a partial valid response of 9 and ignore the bad stuff. Sometimes that is useful, sometimes not. You can read online help / manuals on functions like that to see what they do with various input, such as http://en.cppreference.com/w/cpp/string/basic_string/stol

a really lazy way to do what you want is a double conversion.
1) convert string to int
2) convert that int back to a string
3) is result of 2 == to original input? if yes, valid, else not... you may need to trim whitespace or something for this to work.