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random number trap [message #4986]

Tue, 29 August 1995 00:00

peter
Messages: 80
Registered: February 1994

Member

A warning about the random number generator in IDL/PV-Wave (not a bug,
per se, but something to watch out for).

As the documentation states, if the seed value given to randomu is
undefined, it is derived from the system time. The time only changes
once per second, however. So if you repeatedly call a procedure that
calls randomu, the return will be the same if the calls occur within a
second of each other, but will be different if they are in different
seconds.

This can lead to random numbers being a *lot* more structured than you
expect. I had naively expected that the seed value would change each
microsecond, so this behavior came as a bit of a surprise.

The solution is to place the seed variable in a common block so that it
is preserved from call to call, and then each returned sequence will
truly be random.

Peter

--------------------------------
Peter Webb, HP Labs Medical Dept
E-Mail: peter_webb@hpl.hp.com
Phone: (415) 813-3756

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[Message index]

		random number trap By: peter on Tue, 29 August 1995 00:00
		Re: random number trap By: Adam Shane on Wed, 27 September 1995 00:00
		Re: random number trap By: James Theiler on Wed, 04 October 1995 00:00

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