On Mon, Apr 23, 2007 at 01:06:07PM -0500, Dan Drake wrote: > Hello everyone,
Hi! > I have some code in which the bottleneck is the factorial function. I > began by using a naive > > fac n = product [1..n] > > but it looks like there are faster ways to do it. I could try to look up > those faster algorithms and implement them, but I'm guessing that using > libgmp's factorial function is the best way to do it. I'm a poor > programmer, so I don't trust myself to implement those algorithms > properly. Hence I need to use FFI. I'm curious: what is your application? I've never seen one in which factorials actually need be computed. In physics, one factorial is generally divided by another (e.g. for combinatorics), so it's rarely wise to take the actual factorials. And to be honest, we usually take the thermodynamic limit and then use Stirling's approximation. I guess they also show up in Taylor expansions, but then we never go far enough for the factorial to be expensive. > Is there an easy way to do this? It might also be faster to use a lookup > table, since most of the time I'll be taking factorials of relatively > small numbers. If you really have small numbers and need speed, I'd switch to using Ints. That'd gain you a lot more than an optimized Integer factorial. -- David Roundy Department of Physics Oregon State University
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