quick logarithms

Which CPU are you using ?

exp(x) =3D 1 + x + x2/2 + x3/6 + x4/24 + x5/120 + =85 + xn/n! + =85

Would doing a binary search over a small table be fast enough ?

That might do the trick

I am using a dsPIC30 with the clock at 64MHz. The measured values are integers. Pretty much as soon as I hit the send key, I thought again about the table.

This happens too often. I should pay more attention to the golden rule:

"think twice, speak once"

Anyway... Since the range of measured values is 0-1023, I can afford a lookup table. I just hadn't considered it until I wrote it down. I shall be sure not to give up the day job too soon.

Thanks for your help

Pete

Peter Harris> I have a problem that generates measurements of distance. The measured

The following is a fixed point log routine from our transcendental library. This might not be the latest version.

FIX_MATH needs to be defined as an accum data type (integer.fract) in a compiler that supports fract and accums. Without accum support it will probably work with integers by scaling the constants to and math represent a fixed point value.

If you have any questions contact me off line.

FIX_MATH fix_log(FIX_MATH x)

/* (C) Copyright 2006 Byte Craft Limited Waterloo, ON, Canada, N2L 6H7 Walter Banks Email: snipped-for-privacy@bytecraft.com URL :

{ int n=1; FIX_MATH a,log1,log2; log1 = 0.0; if (x = 2.0) { x/=2.0; log1 += __fix_log2; } while(x < 1.0) { x*=2.0; log1 -= __fix_log2; } x =(x-1.0)/(x+1.0); a=2*x; x*=x; do {log2=log1; log1 += a/n; a*=x; n+=2; }while(log1!=log2); return (log1); }

Regards,

How quick is "quick", i.e. how many operations of what type can you afford?

How dirty is "dirty", i.e. what error threshold can you tolerate?

On what platform? What's the format of the input and output?

I find it hard to believe you couldn't find any algorithms to compute logarithms. There's one right there on the wikipedia page for the binary logarithm!

Me too. Some days I seem to do bad searches, some days good ones.

I didn't get as far as the computers section. In any case, I don't really understand what they mean. I can find the integer part easily enough anyway.

Still, I posted in haste withut due consideration. Something I generally try to avoid.

Thanks (all) for your time

Pete Harrison

And now I have found the page I guess you were referring to. It hadn't occured to me to call it a binary logarithm. Ah well...

I collected some stuff.

Rene