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From: forbesmc@matai.vuw.ac.nz
Newsgroups: comp.lang.pascal
Subject: Re: Real LOGn functions in TP 5.5 needed
Message-ID: <1991Mar22.092631.347@matai.vuw.ac.nz>
Date: 21 Mar 91 21:26:31 GMT
References: <1991Mar20.145939.83@gnv.ifas.ufl.edu> <1991Mar20.220800.24027@midway.uchicago.edu> <1991Mar21.122959.346@matai.vuw.ac.nz> <1991Mar21.042630.6174@midway.uchicago.edu>
Organization: Victoria University of Wellington,  NZ
Lines: 31

In article <1991Mar21.042630.6174@midway.uchicago.edu>, daryl@zaphod.uchicago.edu (Daryl McLaurine) writes:
> In article <1991Mar21.122959.346@matai.vuw.ac.nz> forbesmc@matai.vuw.ac.nz writes:
>>Look up the required function in any standard maths book or 'log tables' -
>>I think it goes something like this ;
>>
>>       LOGn(x) = LN(x) / LN(n)
>>
>>where LN is the natural log function and n is the log base required,
>>x is the number whose log you require.
>>
> This is the function I have been using, but I am getting some unexpected 
> results. (Perhaps I mis-remebered the formula?). Also, I am doing real-time
> analysis, and I thought that TP used a table for LN(), and making 2 passes
> to that function would be murder (Not using a '387...I know...I know...).
> I want a foolproof LOGn function that is fast, compact, and isn't dependent
> on the LN() function.
> 	-dm
Several points : if you are going to use the same log base (ie n) all the
time in your program, calculate 1 / LN(n) ONCE and store it in a variable.
Secondly, all the functions I know for LN (or LOG) are infinite series
expansions of various convergence rates so if TP does use a lookup table,
then it will always be faster than any expansion you may use - even if
written in machine code.

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