Why won't this simple statement work?

**nick753** · 10-18-2010

Thanks. So what would be the most simple/beginner way to not allow a character? Without using try catch or anything.

I tried using something like

Code:

bool valid = true;
if(isalpha(a)) valid = false;

while(!valid)
{
   cout << enter a num\n";
   cin >> a;
}

That won't work either. It gives me a crazy runtime error.

The only reason i'm doing this is because someone who just started learning c++ asked me about this. It seemed really simple at first, but after playing around with the most basic ways I can think of, I cannot get it to work! What is the most simple way to handle a ?

**BMJ** · 10-18-2010

Code:

#include <iostream>
#include <limits>
using namespace std;



int main()
{
    int a;
    cout << "Enter number\n";

    while (!(cin >> a))
    {
        cout << "number only\n";
        cin.clear();
        cin.ignore(numeric_limits<streamsize>::max(),'\n');
    }
}

If the stream formatter can't make the necessary type conversion then the stream goes into the fail state and the data in the buffer remains, thus to "start over" you have to clear() the fail state, then empty out the buffer, which you can do by calling ignore(), it ignores the maximum stream size number of characters or until it reaches the delimiter (EOL in this case)

**Swarvy** · 10-18-2010

I haven't tried this code so there are no guarantees, but on the face of it, this seems like a simple way of doing it. Although I wonder how it would hold up if you put in '>', '#', ';' or basically any other type of unusual symbol.

Code:

char a[BUFF_SIZE];
int a_num;

do
{
       cout << enter a num\n";
       cin >> a;
} while( isalpha(a) );

a_num = (int)atof(a);

Edit: BMJ's method is much cleaner. Use his. lol.

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