Random double

[Ivan Kazmenko]

On Tuesday, 23 April 2013 at 14:43:15 UTC, qznc wrote:
> I want to generate a random "double" value, excluding wierdos 
> like NaN and Infinity. However, std.random.uniform seems to be 
> useless. I tried things like
>
>   std.random.uniform( double.min, double.max);
>   std.random.uniform(-double.max, double.max);
>   std.random.uniform(0.0, double.max);
>
> However, I just get Inf values. :(

I've just tried to reproduce that. The first line gives a 
deprecation warning, the second gives infinities, but the third 
line does give me non-infinities.  I've checked that under DMD 
2.062 on Windows, and the hardware is Intel Xeon E5450.

Here is an example:

-----
import std.random;
import std.stdio;

void main ()
{
	foreach (i; 0..5)
	{
		double x = uniform (-double.max, +double.max);
		writefln ("%25a %30.20g", x, x);
	}

	foreach (i; 0..5)
	{
		double x = uniform (          0, +double.max);
		writefln ("%25a %30.20g", x, x);
	}
}
-----

And an example output is:

-----
                       inf                            inf
                       inf                            inf
                       inf                            inf
                       inf                            inf
                       inf                            inf
   0x1.9d6fad0f9d6f9p+1023     1.4516239851099787345e+308
   0x1.b53e4c11b53e3p+1020      1.919017005286872499e+307
   0x1.5dc7e6d15dc7dp+1020     1.5351529811186840176e+307
   0x1.bd608187bd606p+1023     1.5637717442069522658e+308
   0x1.e38d5871e38d4p+1023     1.6978092693521789428e+308
-----

Perhaps "std.random.uniform(-double.max, double.max);" call does 
indeed cause overflow.  If the example does not work for you, try 
"double.max / 2" or "double.max / 4" boundary instead.

That said, note that the values generated this way will have 
exponent close to the maximal possible (1024), and that may be 
not the only case you wish to cover.  Your suggestion to generate 
64-bit patterns instead of real numbers, probably excluding 
special values (all-0 or all-1), sounds like the way to go.  I'd 
include some important cases (like +-0 and +-1) manually too.

Ivan Kazmenko.