How to check input double value contain how many digits?

Question

I need a double value to contain 2 digits after ".", such as 2.15, 20.15. If the input value is 3.125, then it should print an error message.

My code is:

    private static bool isTwoDigits(double num)
    {
        return (num - Math.Floor(num)).ToString().Length <= 4;
    }

If you input 2.15, then it will be 2.15 -2 = 0.15 <= 4 - which works. But when I change num to 20.15 it doesn't, because (num - Math.Floor(num)) here will return 0.14999999999.

Any other good ideas?

This has been asked before http://stackoverflow.com/questions/3359916/format-to-two-decimal-places — user430788, Apr 24 '14 at 23:51
This has been asked before. http://stackoverflow.com/questions/3359916/format-to-two-decimal-places — user430788, Apr 24 '14 at 23:52
@user430788: this is not exactly the same question. The question here is not how to round the number, but how to assert the number of decimal places in a given number. — jods, Apr 24 '14 at 23:53
OH, fair enough. Then yeah, regexp it and count the length of whats after the . — user430788, Apr 24 '14 at 23:53
@user430788, thanks anyway though that not what i am looking for ! — user3567012, Apr 25 '14 at 00:00

score 1 · Accepted Answer · answered Apr 24 '14 at 23:51

This is the nature of binary floating points number. Just like 1/3 can't be exactly written out as a finite decimal number, 0.1 can't be exactly represented by a finite binary expansion.

So depending on what you are trying to achieve exactly, you could:

If you are validating some string input (e.g. a textbox), you can process the information at the string level, e.g. with a RegEx.
You can store your numbers in the decimal datatype, which can store decimal values exactly.
You can do your computation on a double but you have to give yourself a tolerance. If you expect only 2 digits of precision, you can do something like Math.Abs(x - Math.Round(x, 2)) < 0.00000001). The definition of this tolerance margin depends on your use case.

Sam I am says Reinstate Monica · Answer 2 · 2014-04-24T23:53:26.957

0

If you're really worried about the number of decimal places, on a base-10 number, use decimal instead of double.

the decimal is for calculating financial calculations, and the reason it's called decimal in the first place is so that it can better handle base-10 calculations such as dollars and cents.

And you can also check if the number is 2 digits a bit more simply.

return num % 0.01m == 0.0m;

edited Apr 24 '14 at 23:53

answered Apr 24 '14 at 23:48

Sam I am says Reinstate Monica

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I use your solution "return num % 0.01 == 0;" but it doesn't work ! – user3567012 Apr 24 '14 at 23:56
it works only _on the decimal datatype._ That's what the `m` suffix on constants means. – jods Apr 25 '14 at 00:00

score 0 · Answer 3 · answered Apr 24 '14 at 23:59

SO as has already been said, you can use regexp to ensure the entire format is correct.

But if you know there will only be 1 decimal because its already a number you can also just use String.IndexOf

eg

double foo = ....  ;
string fooString = foo.ToString();
if (fooString.Length - fooString.IndexOf(".") != 3)  => error.

(Its 3 because Length is max index + 1 )

How to check input double value contain how many digits?

3 Answers3