Groups > comp.lang.java.programmer > #34919

Re: Will Math.random return 0.0?

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From	Eric Sosman <esosman@comcast-dot-net.invalid>
Newsgroups	comp.lang.java.programmer
Subject	Re: Will Math.random return 0.0?
Date	Sat, 16 Jul 2016 14:19:05 -0400
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On 7/16/2016 2:15 PM, Arne Vajhøj wrote:
> On 7/16/2016 12:49 PM, Thomas Richter wrote:
>> Am 16.07.2016 um 17:24 schrieb Stefan Ram:
>>>   I wrote the following program.
>>>
>>> public final class Main
>>> { public static void main( final java.lang.String[] args )
>>>   { while( java.lang.Math.random() != 0.0 );
>>>     java.lang.System.out.println( "0.0" ); }}
>>>
>>>   What do you expect it's (average) runtime to be
>>>   on average hardware?
>>>
>>>   Can we be sure that it will ever terminate?
>>
>> No, you cannot.
>
> Sure?
>
>>             If random() is implemented by the average (=bad) linear
>> congruence generator, then zero will never be the output of random().
>
> Not true.
>
> First LCG's can have seed zero and a good cycle as long as the add part
> is non-zero.
>
> Second even for an MLCG (with add part zero) where seed can not be
> zero, then the random numbers can include zero simply by subtracting
> one.

     Third, no law requires that *all* the state bits become part
of the output.  (Indeed, for java.util.Random *no more than* 32 of
the 48 bits of state contribute to the output of next(), and in the
particular case of nextDouble() only 53 high-order bits from two
consecutive 48-bit states participate.)

-- 
esosman@comcast-dot-net.invalid
"Don't be afraid of work. Make work afraid of you." -- TLM

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Re: Will Math.random return 0.0? Thomas Richter <thor@math.tu-berlin.de> - 2016-07-16 18:49 +0200
  Re: Will Math.random return 0.0? Arne Vajhøj <arne@vajhoej.dk> - 2016-07-16 14:15 -0400
    Re: Will Math.random return 0.0? Eric Sosman <esosman@comcast-dot-net.invalid> - 2016-07-16 14:19 -0400

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