What's a good way to check if a double is an integer in C#?

Question

Possible Duplicate:
How to determine if a decimal/double is an integer?

I have a variable of type double and am wanting to check whether it is an integer.

At the moment I have

public bool CheckIfInteger(double number)
{
    return number.ToString().Contains(".") == false;
}

Is there a better way?

UPDATE: Sorry I didn't realise the potential for confusion, by integer I meant the mathematical definiton of integer, that is the natural numbers together with the negatives of the non-zero natural numbers.

Answer 1

return Math.Truncate(number) == number;

As mentioned in the comments, you might need to take account of the fact that a double representation of your number might not be an exact integer. In that case you'll need to allow for some margin-of-error:

double diff = Math.Abs(Math.Truncate(number) - number);
return (diff < 0.0000001) || (diff > 0.9999999);

Answer 2

If you want to check for an Integer, this will do it:

public bool IsInteger(double number)
{
    return (number % 1 == 0);
}

If you additionally want to check if the number could be converted into an Int32:

public bool IsInt32(double number)
{
    return (number % 1 == 0) && number >= Int32.MinValue && number <= Int32.MaxValue;
}

Answer 3

The pitfall of working with string representations is the locale used and yoou have to take care of rounding issues. 0.999999999 can be considered to be integer 1 in most cases. Here is a small snippet taking into account rounding errors:

Math.Abs(number - Math.Round(number)) < EPSILON

where EPSILON is a double value that is small enough for your purpose 0.00001 for example

See also this for some more information: http://msdn.microsoft.com/en-us/library/system.double.epsilon.aspx

Answer 4

Try:

public bool CheckIfInteger(double number)
{
    return ((double) (int) number == number);
}

Or the prettier:

public bool CheckIfInteger(double number)
{
    return (Math.Floor(number) == number);
}

Answer 5

I'd use TryParse:

  double value = 2.0;
  int number;
  bool result = Int32.TryParse(value.ToString(), out number);

Answer 6

    public static bool CheckIfInteger(double number)
    {
        return number - Math.Truncate(number) == 0;
    }

Answer 7

I think that a better question is: How can I tell if a double is close enough to an integer to be considered an integer for my purposes? Because otherwise, you are bound to run into ambiguities. So I'd recommend something like this:

return Math.Abs(someDouble - Math.Round(someDouble)) < TOLERANCE;

Answer 8

I'm liking abatishchev's idea to use CurrentCulture.

return number.ToString().Contains(CultureInfo.CurrentCulture.NumberFormat.NumberDecimalSeparator) == false;

Would this not solve the epsilon problem (which I didn't even consider initially)?

Answer 9

I use the following string extension method which checks against a RegEx pattern

   public static bool IsInteger(this string inputString)
    {
        Regex regexInteger = new Regex(@"^[-]?\d+$");
        Match m = regexInteger.Match(inputString);
        return m.Success;
    }