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From: chidsey@smoke.brl.mil (Irving Chidsey)
Newsgroups: comp.lang.fortran
Subject: Re: Random Number Functions--Generate versus Input
Message-ID: <14408@smoke.brl.mil>
Date: 9 Nov 90 13:13:55 GMT
References: <1990Nov8.170334.24579@athena.mit.edu> <1990Nov8.215752.7075@athena.mit.edu>
Organization: U.S. Army Ballistic Research Laboratory, APG, MD.
Lines: 35

In article <1990Nov8.215752.7075@athena.mit.edu> oliver@athena.mit.edu (James D. Oliver III) writes:
<I guess I need to make this clear: the random number genertor itself is
<vectorizable, however, the values of interest to me are *functions* of
<these randome variables.  To be more specific, I'm generating a series of
<randomly oriented 3-D unit vectors.  The fastest way to do this is to
<generate two random numbers for x and y and calculate z as long as x**2 +
<y**2 <= 1.  This process, however, involves a conditional loop back to the
<ranf() function and thus is not vectorizable.  However, the fact that this
<method is not vectorizable is just of incidental interest, since it remains
<the fastest way of doing it.
<
<What I'd like to know is, *given* this way of generating my values, should
<I expect it to be faster than reading the values from a file?  Like I said,
<for this particular case I can run a benchmark, but I was wondering if
<there was a general rule of thumb for the speed of I/O operations.
<
<	Jim Oliver   
<	oliver@athena.mit.edu /	joliver@hstbme.mit.edu
<	oliver%mitwccf.BITNET@MITVMA.MIT.EDU

	The general rule is 'Keeping the random numbers in a file is faster,
but takes more space; computing them on the fly is compact but time consuming;
it doesn't matter if you don't need many.'

	Why not code it so that if ranfilnam  is ' ' you compute, if it
contains a name you read from the file; then you can choose one or the other
depending on conditions when you submit the job.

						Irv

-- 
I do not have signature authority.  I am not authorized to sign anything.
I am not authorized to commit the BRL, the DOA, the DOD, or the US Government
to anything, not even by implication.
			Irving L. Chidsey  <chidsey@brl.mil>