C++ isdigit command

[herby490]

Switching the two works Thanks

[herby490]

Ok i just realized that if i type a number then a letter the cin still fails how can i stop this

[Ginto8]

Ok i just realized that if i type a number then a letter the cin still fails how can i stop this

Use atoi() and getline(). I'm pretty sure this would work (just an example, and I didn't see the original code):

Code: Select all

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

int main()
{
    string temp = "";
    int x = 0;
    cout << "Enter input starting with an integer value: ";
    getline(cin, temp);
    x = atoi(temp);
    cout << "\nYou entered " << x << ".\n";
    getchar();
    return 0;
}

That should be what you're looking for.

[herby490]

I would rather not have to convert it to a number is then any other way to do it?

[Ginto8]

Not really. that's the only way to make it work if a user inputs a number than some set of characters.

sorry if you really don't want to do that.

[herby490]

Its hard to believe that is the only way to do it. You would think that this would be a problem that people want to address.

[Scoody]

Here you go:

Code: Select all

int intval = 0;
while (intval != -1) {
  cout << "Number please: ";
  cin >> intval;
  if (cin.fail()) {
    cout << "Something bad happened ..." << endl;
    cin.clear(); // clear the failstate
    cin.sync(); // clear cin's buffer of nastiness
  }
}

This loop will run until intval is equal to -1. You just add your own check in the loop-head that suits your needs.
If the input is something like this: 123 abc lol test haha 321
It will go through every chunk that's separated by a space, and since "abc" isn't valid the buffer will be cleared and "lol test haha 321" gets lost.
But if you only want to read one number at a time you can add cin.sync(); after cin >> intval; and the user will be forced to enter a new number until it meets your criteria.

[MarauderIIC]

You know, I didn't even know that sync() existed. Thanks.

[Kros]

You know, I didn't even know that sync() existed. Thanks.

Wow. Me either. What the heck?

Handy.

[Maevik]

Another good way to make sure you're getting a number is to use if( cin.fail() ) with a flag loop and a "Try again retard" message that prompts them again.

Code: Select all

int main()
{
     double a = double();
     bool again = true;

     while ( again )
     {
          cout << "For the quadratic equation, please enter to value for a: ";
          cin >> a;
          again = false;

          if (cin.fail())
          {
                cout << "Invalid Data, the value for a must be numerical. idiot" << endl;
                cin.ignore(1000, '\n');
                cin.clear();
                again = true;
          }
     }

     //carry on with your program

}

it's a lot of code, but it's effective and it gives you a chance to berrate your user. The cin.fail() function checks to see if the data in the input stream caused a fail state, if it does it tells them they're doin' it wrong, ignore's the next 1000 characters in the stream, or until it gets to a newline (which would normally be the end of the stream anyway ) clears the input stream, and prompts the loop to go back and ask the question again.

[MarauderIIC]

Maevik, thanks but already posted :) http://elysianshadows.com/phpBB3/viewto ... 402#p35402

[Maevik]

Maevik, thanks but already posted http://elysianshadows.com/phpBB3/viewto ... 402#p35402

Ahh, ok. I didn't realize !cin worked the same way. If you use this method, do you still need cin.clear() and cin.ignore() ?

[herby490]

That works if I type something that is a letter or has a letter but now the only number that it will accept is negative 1. I have tried if(isdigit(a)) break;
but it wont work. How can i break the loop if the user entered a valid number?

[MarauderIIC]

Did you get the right condition? "while (intval != -1) {"
Essentially it checks to see if intval got assigned to. You could also put a break in an else-condition on Scoody's code.

Maevik, !cin is shorthand for cin.fail() (or !cin.good(), perhaps)

[herby490]

Ok I still can't get it to work. I have tried to make it so it would break the loop if the person entered a valid input. Heres the code

Code: Select all

a=0;
while(a!=-1)
	{
		cout << "Enter an A value: ";
	cin >> a;


		if(!cin)
		{
			cout <<"Congratulations you failed the cin statment.\nPlease enter a real number" << endl;
			cin.clear();
			cin.sync();
		}
        else if(a!=-1)//Tried those once
        break;
        else //tried this another time
        break;
	}

Note i did not use the else and the else if statement in the same program I just put it there for convenience.