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Geek Culture / Math help please

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Hawkblood
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Posted: 14th Apr 2014 22:54
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This has to be the stupidest thing. I can't for the life of me figure out how to do a base 2 log, so I created a process to get the answer. I would like to know if there is a math function already out there......

this process gives the integer value of the input number.

The fastest code is the code never written.
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Green Gandalf
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Playing: Malevolence:Sword of Ahkranox, Skyrim, Civ6.
Posted: 14th Apr 2014 23:37
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Try this:

Quote: "Matrix1 Utility Plug-ins


LOG2

Syntax

Value = LOG2 ( Value )

Description

This function will return the base-2 logarithm of the number provided.
"


Matrix1 is the answer to all my ills.



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Seditious
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Posted: 14th Apr 2014 23:44
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It's already included in the standard maths header (math.h or cmath): http://www.cprogramming.com/fod/log2.html
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Green Gandalf
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Posted: 15th Apr 2014 00:06
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Didn't notice you'd written C++ or a variant. Oops.

I was talking about DBPro.

I'm sure all real computing languages have logs to base 2 as a standard function.



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Hawkblood
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Posted: 15th Apr 2014 00:10
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I think it's been taken out of math.h and cmath, because when I #include them, I still can't get it. I made functions to cover it though:


The fastest code is the code never written.
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Green Gandalf
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Posted: 15th Apr 2014 01:44
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None of those will give the correct values for arbitrary arguments.

For example, log2(3.0) should be about 1.58496.



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Hawkblood
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Posted: 15th Apr 2014 03:23
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You are correct. The purpose of the functions was to give a number for mip generation purposes, not to be mathematically correct. For the functions to work, I would have to:

to be more accurate.....

The fastest code is the code never written.
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Green Gandalf
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Posted: 15th Apr 2014 12:10
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That should do it.



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Dar13
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Posted: 15th Apr 2014 13:58
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Hawkblood
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Posted: 15th Apr 2014 23:29
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Mine eliminates the extra calculation of log(2) for faster results.

The fastest code is the code never written.
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Neuro Fuzzy
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Posted: 25th Apr 2014 06:37
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If you want an integer log2 (floored) this is the same as, for a positive integer, finding the position of the leftmost (largest figure) one bit!

There are a few bitwise formulas for this in the book Hacker's Delight.


"I <3 u 2 bbz" - Dark Frager
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Dar13
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Posted: 25th Apr 2014 07:50
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Quote: " Mine eliminates the extra calculation of log(2) for faster results."

I would hope that the standard library would use a look-up table for simple values of log().

@Neuro Fuzzy
Cool!

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Hawkblood
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Posted: 27th Apr 2014 02:38
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Quote: "I would hope that the standard library would use a look-up table for simple values of log()."

It's still a call to a function (maybe in-line, but still a call). I'm sure that multiplier isn't giving me a huge advantage, but I try to optimize as much as possible.

The fastest code is the code never written.
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Seditious
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Posted: 27th Apr 2014 06:10
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Quote: "It's still a call to a function (maybe in-line, but still a call). "


If it's inline (as it will be with full optimization) it isn't a call.
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