CODEWITHSUNDEEP
c++if-statementbreakdo-whilecin
Danial Razavi
Danial Razavi

Reputation: 47

do-while infinite loop with if c++

In this loop the user should enter an int and program checks if it be odd it will go to next parts, but I can't understand why if the user enter a character that's not int, the program is falling in an infinite loop!

int num;
do {
    cout << "PLEASE enter the num: ";
    cin >> num;
    if (num % 2 == 0)
        cout << "Number should be odd!" << endl;
    else
        break;
} while (true);
//...

Cause anyway char/int != 0 and if it be ==0 too it should stop at next cin but it wont stop! I tried ws(cin) too but it didn't help me. Please tell me how I can fix that problem, and why that's happening.

Upvotes: 3

Views: 666

Answers (3)

Danial Razavi
Danial Razavi

Reputation: 47

i found a way to fix my problem,I changed input type to string and made a function as blow:

int isNum(string input)
{
    int digit, value = 0;
    for (int i = 0; i < input.length(); i++)
    {
        digit = input[i] - '0';
        if (digit >= 0 && digit <= 9)
            value = value * 10 + digit;
        else
        {
            value = 0;
            break;
        }
    }
    return value;
}

it will return the input as int if all characters be between 0 and 9, and else it will return 0; to me, so i use that returned int as my num.

Upvotes: 0

Parag Agarwal
Parag Agarwal

Reputation: 69

In your program, the standard input stream is expecting an integer. When you give it a character, cin cannot put it in the integer variable, ie it fails(error flag is set) and leaves the entered character in the buffer (for next cin assignments). cin repeatedly considers the character as input and keeps failing and the integer variable is assigned the value 0.

To prevent that you can modify your program as 

int num;
    do{
        cout << "PLEASE enter the num: ";
        while(!(cin>>num)){
        cin.clear();
        cin.ignore(INT_MAX,'\n');
        cout<<"Invalid. Enter a number";
    }
        if (num % 2 == 0)
            cout << "Number should be odd!" << endl;
        else
            break;
    }
while(true);

include limits.h for using INT_MAX or you can use a large value like 10000.

The cin.clear clears the error flag and makes it ready for taking input. cin.ignore(INT_MAX,'\n') ignores everything(upto INT_MAX characters) until the next new line. This makes sure that the program will work fine if the user inputs a long line instead of a number.

Upvotes: 0

Mats Petersson
Mats Petersson

Reputation: 129314

Becasue your program does not check the result of cin >> num;. Since cin >> num; (where num is an integer) will read all available digits, and if the input is not a digit at all [and not a whitespace character, which are skipped over by cin >>], then it keeps repeatedly trying to read the input.

It's not clear from your question exactly what you want to do when the user has entered "askjhrgAERY8IWE" to your program - there are two solutions that come to mind:

Alt 1. Ask again after removing the errant input. This is probably right for this application as it is interactive, but a terrible idea for an automated program that reads its input from a file.

if(!(cin >> num))
{
     cout << "That deoesn't seem to be a number..." << endl;
     cin.clear();
     cin.ignore(10000, '\n');
}

Alt 2. Exit with an error message. This is clearly the right thing when the typical input is a datafile.

 if(!(cin >> num))
 {
      cout << "That deoesn't seem to be a number..." << endl;
      exit(1); 
 }

You should also add code to deal with end of file in this code - for file-input that should be acceptable [unless the file has specific data to mark the end]. For interactive input (where the user is typing directly to the program), end of file may or may not be considered an error - really depends on what happens next.

Upvotes: 4

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