Get number of digits in an unsigned long integer c#

Question

I'm trying to determine the number of digits in a c# ulong number, i'm trying to do so using some math logic rather than using ToString().Length. I have not benchmarked the 2 approaches but have seen other posts about using System.Math.Floor(System.Math.Log10(number)) + 1 to determine the number of digits. Seems to work fine until i transition from 999999999999997 to 999999999999998 at which point, it i start getting an incorrect count.

Has anyone encountered this issue before ?

I have seen similar posts with a Java emphasis @ Why log(1000)/log(10) isn't the same as log10(1000)? and also a post @ How to get the separate digits of an int number? which indicates how i could possibly achieve the same using the % operator but with a lot more code

Here is the code i used to simulate this

Action<ulong> displayInfo = number => 
 Console.WriteLine("{0,-20} {1,-20} {2,-20} {3,-20} {4,-20}", 
  number, 
  number.ToString().Length, 
  System.Math.Log10(number), 
  System.Math.Floor(System.Math.Log10(number)),
  System.Math.Floor(System.Math.Log10(number)) + 1);

Array.ForEach(new ulong[] {
 9U,
 99U,
 999U,
 9999U,
 99999U,
 999999U,
 9999999U,
 99999999U,
 999999999U,
 9999999999U,
 99999999999U,
 999999999999U,
 9999999999999U,
 99999999999999U,
 999999999999999U,
 9999999999999999U,
 99999999999999999U,
 999999999999999999U,
 9999999999999999999U}, displayInfo);

Array.ForEach(new ulong[] {
 1U,
 19U,
 199U,
 1999U,
 19999U,
 199999U,
 1999999U,
 19999999U,
 199999999U,
 1999999999U,
 19999999999U,
 199999999999U,
 1999999999999U,
 19999999999999U,
 199999999999999U,
 1999999999999999U,
 19999999999999999U,
 199999999999999999U,
 1999999999999999999U
}, displayInfo);

Thanks in advance

Pat

Answer 1

log10 is going to involve floating point conversion - hence the rounding error. The error is pretty small for a double, but is a big deal for an exact integer!

Excluding the .ToString() method and a floating point method, then yes I think you are going to have to use an iterative method but I would use an integer divide rather than a modulo.

Integer divide by 10. Is the result>0? If so iterate around. If not, stop. The number of digits is the number of iterations required.

Eg. 5 -> 0; 1 iteration = 1 digit.

1234 -> 123 -> 12 -> 1 -> 0; 4 iterations = 4 digits.