How to handle wrong data type input

Question

In C++, how do you handle wrong inputs? Like, if the program asks for an integer, when you type a character it should be able to do something and then loop to repeat the input but the loop goes infinite when you input a character when an integer is need and vice versa.

chris · Accepted Answer · 2015-03-23T12:16:37.637

60

The reason the program goes into an infinite loop is because std::cin's bad input flag is set due to the input failing. The thing to do is to clear that flag and discard the bad input from the input buffer.

//executes loop if the input fails (e.g., no characters were read)
while (std::cout << "Enter a number" && !(std::cin >> num)) {
    std::cin.clear(); //clear bad input flag
    std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n'); //discard input
    std::cout << "Invalid input; please re-enter.\n";
}

See the C++ FAQ for this, and other examples, including adding a minimum and/or maximum into the condition.

Another way would be to get the input as a string and convert it to an integer with std::stoi or some other method that allows checking the conversion.

edited Mar 23 '15 at 12:16

answered Apr 27 '12 at 11:34

chris

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i have done some research before asking here. I saw that they put cin.ignore(1000, '\n'); what does this do? Also !(cin >> num) returns a boolean? i didn't know that – Zik Apr 27 '12 at 11:42
2

@Marvin, `cin.ignore (1000, '\n')` ignores/discards characters in the input buffer until either 1000 have been discarded, or a newline has been encountered, whichever comes first. It's a good way to get rid of a line. You'll see in the parashift example, they use the maximum size of a stream instead of 1000 to account for a line of max length. I use `cin.sync()` because when doing this, I want to be on equal footing with the user (not read the next line yet), so I discard everything. Finally, `cin` has an `operator void *`, so it cann convert to a bool. – chris Apr 27 '12 at 11:48
but how does the while loop check !(cin >> num) ? – Zik Apr 27 '12 at 12:12
3

@Marvin, `cin >> num` fails if a user types, say 'a' when it was expecting an int. It provides a conversion operator to allow it to be implicitly converted to a `void *`. If `cin` is in a bad state, it will return `NULL`. If not, it will return the object. This can then be taken and converted to bool: true if not NULL, false if NULL. The loop can then use that to evaluate the bool expression it needs. – chris Apr 27 '12 at 12:15
@chris mind that ignore() has to be passed before clear() for this to work, the reference you have given has the correct form in it ;) – maja Mar 23 '15 at 07:17
@Zik It could just be XCode, but although this code works cin>>num does not return a boolean as in bool x = (cin>>num) will not compile. Anyone have any idea why? – Supamee Apr 03 '17 at 18:53
@Supamee, `cin >> num` returns `cin`, which is contextually-convertible to `bool`. That means `if (cin)` compiles and does what you expect, but `bool b = cin;` doesn't compile without an explicit cast. – chris Apr 03 '17 at 19:45
What about using std::getline() instead of std::cin and then checking for proper datatype? eg: std::getline(std::cin, input); if (input......YOUR CONDITION.... – Nov 04 '20 at 19:40
1

@VaisakMohan, That would be fine too; it's one way to do the last paragraph. However, do be aware of whether you want to count multiple inputs on one line. It would be a shame if you read a line of valid inputs, convert the first part of it, and toss the others. `getline` works best for actual line-based input instead of token-based. You also have to be aware of whitespace, which could be handled differently in your conversion. Beyond that, reading line by line means you have to do the conversion _and_ error checking (minus discarding bad input from the stream) instead of only the latter. – chris Nov 04 '20 at 22:45
1

In addition, with `getline`, you have to be careful not to leave a stray newline in the stream before using it. If using `getline`, it's generally recommended to go all-in instead of trying to mix and match it with token-based input. – chris Nov 04 '20 at 22:46

score 10 · Answer 2 · edited Jan 25 '15 at 10:09

The top voted answer covers the solution really well.

In addition to that answer, this may help visualize what's going on a little better:

int main()

    int input = 1;//set to 1 for illustrative purposes
    bool cinState = false;
    string test = "\0";
    while(input != -1){//enter -1 to exit
        cout << "Please input (a) character(s): ";//input a character here as a test
        cin >> input; //attempting to input a character to an int variable will cause cin to fail
        cout << "input: " << input << endl;//input has changed from 1 to 0
        cinState = cin;//cin is in bad state, returns false
        cout << "cinState: " << cinState << endl;
        cin.clear();//bad state flag cleared
        cinState = cin;//cin now returns true and will input to a variable
        cout << "cinState: " << cinState << endl;
        cout << "Please enter character(s): ";
        cin >> test;//remaining text in buffer is dumped here. cin will not pause if there is any text left in the buffer.
        cout << "test: " << test << endl;
    }
    return 0;    
}

Dumping the text in the buffer to a variable isn't particularly useful, however it helps visualize why cin.ignore() is necessary.

I noted the change to the input variable as well because if you're using an input variable in your condition for a while loop, or a switch statement it may go into deadlock, or it may fulfill a condition you weren't expecting, which can be more confusing to debug.

score 0 · Answer 3 · answered Apr 21 '23 at 08:50

Here is my approach to this matter.

Below is a C++ program that allows an end user to answer Yes or No to a simple "Yes" or "No" question.

This program will re-ask the same question, as long as the answer is neither Yes nor No.

Since C++ is case sensitive, I have also set up the program to accept both upper and lower case letters of the first letter of the allowed answers, since they both basically signify the same thing.

 #include <iostream>

 #include <string>

 using namespace std;

 int main() {
     
   string input;

   do {
       cout << "Are you interested in learning how to solve problems through coding?" <<endl;
       
       cout << "Choose Yes or No" <<endl;
       
       cin >> input;

       if ((input == "Yes") || (input == "yes"))
       
       {

        cout << "That is one of the ways of becoming a software engineer.";
        
        break;

       } 
       
       else if ((input == "No") || (input == "no"))
       
       {

        cout << "You have little possibility of becoming a computer engineer."<<endl;
        
        break;
        
       }

        } while ((input != "Yes" && input != "No") || (input != "yes" && input != "no"));

   return 0;
 }

score -6 · Answer 4 · answered Apr 27 '12 at 11:36

-6

Test the input to see whether or not it is what your program expects. If it is not, alert the user that the input they provided is unacceptable.

answered Apr 27 '12 at 11:36

score -6 · Answer 5 · answered Apr 27 '12 at 11:40

-6

You can check it through the ASCII value if the ascii value s between 65 t0 90 or 97 to 122 the it would be character.

answered Apr 27 '12 at 11:40

jiten

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How do you know the system is using ASCII? – Toby Speight May 31 '18 at 16:07

How to handle wrong data type input

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