Checking data types and errors

This is a discussion on Checking data types and errors within the C++ Programming forums, part of the General Programming Boards category; how do i check a variable to make sure it is the correct data type? i'm sure this question is ...

  1. #1
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    Checking data types and errors

    how do i check a variable to make sure it is the correct data type?

    i'm sure this question is answered somewhere on this board but i couldn't find it...

    double number1;
    cin >> number1;


    if i enter a number the program runs fine.. but if by accident i type in 34D23KJ or something the program tries to put this into number1 then realizes it is not a valid double and crashes...

    is there a function i can run to return true or false?!?!?

    like number1.fail() or something... i'm a bit lost

  2. #2
    Code Goddess Prelude's Avatar
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    Code:
    #include <iostream>
    using namespace std;
    
    int main() 
    {  
      double d;
      cin>>d;
      if ( !cin.fail() )
        cout<<"Valid data type"<<endl;
      else
        cout<<"Invalid data type"<<endl;
      return 0;
    }
    or
    Code:
    #include <iostream>
    using namespace std;
    
    int main() 
    {  
      double d;
      if ( cin>>d )
        cout<<"Valid data type"<<endl;
      else
        cout<<"Invalid data type"<<endl;
      return 0;
    }
    -Prelude
    My best code is written with the delete key.

  3. #3
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    thanx

    thanx man.. exactly what i needed gracias

  4. #4
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    Sorry to butt in, but I had a quick (meaningless?) question: what's the difference between
    Code:
     !cin
    and
    Code:
    cin.fail()
    ?
    Self Learner--patience required

  5. #5
    Code Goddess Prelude's Avatar
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    Technically, if you check the failbit of cin with fail() then you can call clear() to reset it and be able to read from cin again. cin is a bit finicky in this area though, which is why I prefer getline() and parsing

    -Prelude
    My best code is written with the delete key.

  6. #6
    Registered User rmullen3's Avatar
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    Well, I wrote this little program:

    Code:
    int main()
    {
    	int a = 0;
    	cin >> a;
    	if (!cin)
    		cout << "NOT CIN\n";
    	if (cin.fail())
    		cout << "CIN FAILED\n";
    	return(0);
    }
    And both NOT CIN and CIN FAILED were outputted if I inputted a character instead of an integer. So... I'm not sure if there is one.

  7. #7
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    Prelude, I'm sorry I just didn't understand you. Can you dumb it down a bit?
    Self Learner--patience required

  8. #8
    Code Goddess Prelude's Avatar
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    Both of the choices you offered do the same thing. But the difference is in how you can recover from an error. http://www.cplusplus.com/ref/iostream/ios/clear.html

    -Prelude
    My best code is written with the delete key.

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