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Topic: Fast exponent and logarithm, given initial estimate
Replies: 29   Last Post: Nov 8, 2004 2:31 AM

 Messages: [ Previous | Next ]
 bv Posts: 227 Registered: 12/7/04
Re: Fast exponent and logarithm, given initial estimate
Posted: Oct 18, 2004 9:40 PM

Glen Low wrote:
&gt;
&gt; I need a fast exponent and logarithm routine for floats. It should
&gt; preferably use only adds, subtracts and multiplies; divides and square
&gt; roots are permissible if necessary. It should not use large lookup
&gt; tables. An additional wrinkle (or help) is that there's a exponent and
&gt; a logarithm estimate available.
&gt;
&gt; Newton-Raphson seems of no use since I can't use the inverse function.

It may be of more use than you think. For y = ln(x), the recursion

y = y - (1 - x*exp(-y))

will work quickly since you can (probably) work with normalized numbers
for which the series approx for exp(-y) won't need many terms to get the
precision you want. With decent estimates this will take only a few
iterations.

Date Subject Author
10/18/04 Glen Low
10/18/04 Jeremy Watts
10/19/04 Peter Spellucci
10/19/04 Glen Low
10/18/04 bv
10/19/04 Glen Low
10/19/04 George Russell
10/19/04 Glen Low
10/20/04 George Russell
10/20/04 Glen Low
10/21/04 Christer Ericson
10/21/04 Glen Low
10/22/04 Christer Ericson
10/19/04 Martin Brown
10/19/04 Glen Low
10/19/04 Richard Mathar
10/19/04 Glen Low
10/20/04 Gert Van den Eynde
10/20/04 Glen Low
10/20/04 Richard Mathar
10/21/04 Gert Van den Eynde
10/21/04 bv
10/22/04 Glen Low
10/22/04 Peter Spellucci
10/22/04 Glen Low
10/23/04 bv
10/24/04 Gert Van den Eynde
10/25/04 Peter Spellucci
10/20/04 Gert Van den Eynde
11/8/04 Glen Low