Re: SimpleGenerator methods: what is getThinDoubleSample?

[Sven N. Thommesen]

At 08:44 AM 10/17/01, Paul Johnson wrote:

> Now, the question. It appears to me that "getDoubleSample" takes twice
> as long as the others, becuase it uses 2 calls to fill the mantissa.
> getThinDoubleSample does not.  getFloatSample does not either.
>
> So, if one had a program with lots and lots and lots of random number
> draws, one could conceivably make it twice as fast by using floatSample
> or ThinDoubleSample, correct?  What problems could flow from this usage?

A call to get an unsigned random value returns 32 bits of "randomness". An 
IEEE double has 53 bits of mantissa, so a single unsigned int cannot fill 
all those bits. When I created the random library in its present form I 
therefore allowed you to choose (via a compilation switch in the random 
library) whether you are satisfied with the precision you get from using a 
single unsigned to fill the double, or if you want to fill all of the 53 
bits, using two unsigneds. You decide what's needed for the purposes you 
have in mind.

As you observe, the default of using two calls per doublesample does take 
more time. You may recompile to speed things up.

I'd advise that if you are going to add more distribution objects to the 
library, you should make the new ones able to use either a single or a 
double call to getUnsignedSample, to maintain uniform functionality across 
the library.

Sven Thommesen


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