| Large Numbers [message #64994] |
Fri, 06 February 2009 15:12 |
David Fanning
Messages: 11724
Registered: August 2001
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Senior Member |
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Folks,
I made a big mistake and signed up for an Applied Statistics
class this semester. Now I pretty much spend every free
waking moment doing stats homework. :-(
Anyway, for lunch today I decided to grab a sandwich and
give my youngest some support by calculating how many
girls he had to ask out to have an 80% chance of getting
a date for Saturday night.
I made some conservative assumptions (I learned later
my ideas about the college social scene apply more to the
1970s than they do to today), and off I went writing a
couple of short IDL programs to do the calculations for
the Binomial and Geometry Distributions, etc. All pretty
straightforward.
But then I started getting screwy results. (This, in itself,
is not all that unusual in this particular class. In fact, I've
begun to consider it something of a minor miracle if I'm within
an order of magnitude of the right answer.) But even I know
that negative probabilities don't show up until the second
semester. What in the world!?
It turns out that the recursive function I naively wrote to
process a factorial calculation was overflowing my long
integers, even with a simple calculation like 20! (twenty
factorial). Yowser!
Now, of course, the formula I was using has a large
factorial number divided by another large factorial
number, so the *actual* number I wanted to use in the
calculation is not that big. But it begs the question:
what strategy do computer scientists use to deal with
one very, very big number divided by another very, very
big number?
I've solved my immediate problem for my little toy problem
by using LONG64 variables. But this can't be the right solution.
Does anyone know?
Cheers,
David
--
David Fanning, Ph.D.
Fanning Software Consulting, Inc.
Coyote's Guide to IDL Programming: http://www.dfanning.com/
Sepore ma de ni thui. ("Perhaps thou speakest truth.")
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