How about shifting left, and counting how many shifts it takes to get to the first "1" digit? This will work, if you have an unsigned two's-complement integer, won't it?

Ed Poor

-----Original Message----- From: Chris Bourke [mailto:cbourke@cse.unl.edu] Sent: Thursday, November 14, 2002 10:19 AM To: wikitech-l@wikipedia.org Subject: [Wikitech-l] logarithmic approximations

I'm working on a project right now using GNU's multiprecision library. However, and unfortunately, there are no library functions to do any logarithmic functions.

I'm merely trying to get a ceiling (or floor) or the lg (base 2) of a large integer. Of course, I could always send it into a loop requiring O(lg^3(n)) to get it, but I know there must be a way to do a simpler approximation, especially since I don't need very high precision.

I asked this question of some math associates and they suggested the usual taylor series, etc. However, I am unable to find a satisfactory series for a large n (taylor only works for x<1, right?).

Any help would be appreciated.

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Chris Bourke cbourke@cse.unl.edu 402-432-0795