Re: RANDOMU bug (and HTML help)

[davidf(at)dfanning.com (David Fanning)]

Andy Schmitt (ajschmitt@my-dejanews.com) writes:

> This is not the worst of it. For some of us, it is important to be able
> to have the same seed so that the same "random" sequence is produced.
> However, RSI quietly changed the manner in which the SEED variable
> interacts with the RANDOM functions between version 5.0 & 5.1.
> 
> Consider the following behavior from IDL v 5..0.3:
> IDL> seed = 2 & print, randomu(seed, 3)
>      0.342299     0.402381     0.307838
> ...doing this multiple times will always give the same result.
> 
> However, in IDL v.5..1 & later,. using this several times in a row
> produces different results each time.:
> IDL> seed = 2 & print, randomu(seed, 3)
>     0.0594004     0.982075     0.358593
> IDL> seed = 2 & print, randomu(seed, 3)
>      0.831999     0.303037     0.506712
> 
> ...etcetera. It turns out that you now have to specify a NEGATIVE seed
> in order for it to have any influence on the generated sequence:
> 
> seed = -2 & print, randomu(seed, 3)
>      0.342299     0.402381     0.307838
> seed = -2 & print, randomu(seed, 3)
>      0.342299     0.402381     0.307838

For what it is worth, this line produces the very same
result over and over again in my Windows NT IDL 5.2a version:

IDL> seed = 2 & print, randomu(seed, 3)
     0.342299     0.402381     0.307838
IDL> seed = 2 & print, randomu(seed, 3)
     0.342299     0.402381     0.307838
IDL> seed = 2 & print, randomu(seed, 3)
     0.342299     0.402381     0.307838
IDL> seed = 2 & print, randomu(seed, 3)
     0.342299     0.402381     0.307838

Perhaps it is fixed in IDL 5.2? When you say "IDL v5.1 & later"
did you test it in IDL 5.2? I've been having my own problems
with unannounced changes in the way !D.N_Colors works from
IDL 5.0 to 5.1 to 5.2. :-(

Cheers,

David

-- 
David Fanning, Ph.D.
Fanning Software Consulting
Phone: 970-221-0438 E-Mail: davidf@dfanning.com
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