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Re: Bizarre (?) behavior of randomu [message #57831 is a reply to message #57829]

Fri, 04 January 2008 01:54

wita
Messages: 43
Registered: January 2005

Member

The trick is indeed to specify a random seed during the first call to
RANDOMU and preserve the seed value through subsequent calls to
RANDOMU. If you do not specify it in the first call (like in your
example) the seed value will be taken from some system values (like
the systime or similar) and RANDOMU will produce different sets of
random numbers with each call. This is generally undesirable,
if you want your results to be reproducible.

example:
IDL> seed=1
IDL> print, randomu(seed, 10)
0.415999 0.0919649 0.756410 0.529700 0.930436
0.383502 0.653919 0.0668422 0.722660 0.671149
IDL> print, randomu(seed, 10)
0.383416 0.631635 0.884707 0.519416 0.651519
0.237774 0.262453 0.762198 0.753356 0.909208
IDL> seed=1
IDL> print, randomu(seed, 10)
0.415999 0.0919649 0.756410 0.529700 0.930436
0.383502 0.653919 0.0668422 0.722660 0.671149
IDL> print, randomu(seed, 10)
0.383416 0.631635 0.884707 0.519416 0.651519
0.237774 0.262453 0.762198 0.753356 0.909208

Note that while I specify seed=1, it will be replaced by a LONARR(36)
after the first call to RANDOMU.

A more elegant solution without using COMMON blocks is to wrap the
random number generator in an IDL object.

with best regards,

Allard

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[Message index]

		Re: Bizarre (?) behavior of randomu By: weitkamp on Fri, 04 January 2008 06:44
		Re: Bizarre (?) behavior of randomu By: wita on Fri, 04 January 2008 01:54
		Re: Bizarre (?) behavior of randomu By: M. Katz on Wed, 02 January 2008 14:24

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