From help-octave-request at bevo dot che dot wisc dot edu  Thu Jan 11 10:37:12 2001
Subject: Re: Wrong 'log' results when using .oct files
From: Przemek Klosowski <przemek at jazz dot ncnr dot nist dot gov>
To: javier dot perez at pipebeach dot com, help-octave@bevo.che.wisc.edu
Date: Thu, 11 Jan 2001 11:37:09 -0500


   I'm trying to compute the logarithm in base 2 of a number, in C++
	....
   Commenting out the line 
	   #include <octave/config.h>
   everything works fine, and I obtain the result "3". 

	   jape at shiraz:~/cur/# ./test_log 
	   (int) log2(8): 3 3

   The problem is when including that file, because the result is "2".

	   jape at shiraz:~/cur/# ./test_log
	   (int) log2(8): 3 2

   Any idea why is this way?

While I don't know why including octave headers changes your result, the mere
fact that a floating number truncated to an integer behaves unpredictably
is not at all strange. When you print 'j', the printing routine rounds the 
least significant bits so that you get a printout of '2' instead of
2.00000000000000000001 or 1.999999999999999999997. When you truncate such
numbers to integers, however, you get different results (2 and 1, respectively).
Which exact floating point number you get is a tossup, and may depend on order
of expressions, level of optimization used in the compilation, phase of the moon,
or any number of circumstances beyond your control.

Use rounding, or (int) (j+0.5) for a more reliable operation; truncation is really
a very blunt operation, and should be avoided.



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