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Taking Logarithms of relatively small numbers in different languages/architectures/operating systems

Answer 1

+7 A:

You're looking at logarithms with different bases:

Java's System.out.println(Math.log(249.0/251.0)); is a natural log (base e)
C#'s Math.Log (x,y); gives the log of x with base specified by y
Google's Log(249.0/251.0) gives the log base 10

Though I don't get the result you do from C# (Math.Log( 249.0, 251.0) == 0.998552147171426).

Michael Burr 2010-07-02 05:40:19

my apologies <- my C# code was miscopied (I had really complicated variable names and input translation on that line, though it does potentially (and certainly partially) answer my question: Google and Java are different bases) so the question is now Java vs C#

piggles 2010-07-02 05:44:40

I get the same result in C# as in Java with `Math.Log(249.0/251.0)` - at least to 16th decimal place.

Michael Burr 2010-07-02 05:50:47

Hmmm interesting. I will spend another five hours poring over the C# code, in that case. Would someone be able to assure me that, say, an AMD processor, would return the same as an Intel processor?

piggles 2010-07-02 05:52:22

Instead of poring over the C# code, you might be able to get your answer much more readily stepping through and/or setting appropriate breakpoints in the debugger to ensure the `x` and `y` values are correct (as well as the result) when passed to `Math.Log()` and that that result isn't being modified at some point before being displayed.

Michael Burr 2010-07-02 05:57:10

unfortunately I can't seem to run .Net on my Mac, which makes me sad.

piggles 2010-07-02 06:08:20

@Mechko, basically you can count on the same program generating the same output regardless of processor (unless you do exotic stuff with SSE3 etc). The "single step in debugger" advice is probably the best you can get right now.

Thorbjørn Ravn Andersen 2010-07-02 06:08:30

@Thor Thanks for confirming the architecture stuff.

piggles 2010-07-04 04:35:34

Answer 2

+3 A:

You have a mistake somewhere in your C# program between where the log is calculated and where it is printed out. Math.Log gives the correct answer:

class P
{
  static void Main()
  {
      System.Console.WriteLine(System.Math.Log(249.0/251.0));
  }
}

prints out -0.00800004266707626

Eric Lippert 2010-07-02 05:51:29

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Taking Logarithms of relatively small numbers in different languages/architectures/operating systems

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