Mastering E-Prime: PRNG vs. standard randomization functions

[David McFarlane]

E-Basic offers two sets of randomization functions: The standard set
from VBA (Randomize(), Rnd(), Random()), and the PRNG object (with
methods GetSeed, SetSeed, Reset, GetNext, GetPrev, Random()). How do
these differ? Do they use the same underlying pseudo-random number
generator? Why use one rather than the other?

As demonstrated by Paul Groot at
http://groups.google.com/group/e-prime/browse_thread/thread/ccda3fe57a595c54
, both sets of functions *do* use the same underlying pseudo-random
number generator (but you should still test that out for yourself!),
and provide equivalent results. So what's the point? Well, the
standard VBA set does have a confusing interface. I posit that
E-Basic provides the PRNG object simply to gather all these functions
into one tidy E-Object, and along the way provides one extra function
lacking from the standard set. E-Basic still includes the standard
set for those VBA programmers who prefer it, as well as to allow
legacy code to work.

Some further notes (in the following, x indicates an arbitrary
number, +x a number greater than 0, -x a number less than 0):

- PRNG.GetSeed
has no equivalent in the standard VBA set.

- PRNG.SetSeed x
does the same as
Randomize x
(note that "Randomize" without an argument uses the system clock
for the seed; PRNG has no simple equivalent to this --
"PRNG.SetSeed" without an argument sets the seed to 1).

- PRNG.Reset
is a synonym for
PRNG.SetSeed 0
or
Randomize 0

- PRNG.GetNext
does the same as
Rnd(+x)

- PRNG.GetPrev
does the same as
Rnd(0)
i.e., they simply repeat the value of the latest random number
generated (e.g., from the latest use of PRNG.GetNext or Rnd(+x)).
(Note also that after PRNG.Random() or Random() it returns the
full double value, not the truncated long integer value.)

- PRNG.Random(min,max)
does the same as
Random(min,max)
0 <= min < max, or they produce the run time error "Invalid Range
Specified".

- Rnd(-x)
returns the same as
Randomize Abs(x) ' (same x as above)
Rnd(+y)
which in turn does the same as
PRNG.SetSeed Abs(x)
PRNG.GetNext

So which should you use? That comes down to your personal
programming taste or style: Would you rather use well-known standard
VBA statements & functions, or keep things organized as methods from
one E-Object? Whichever you choose, you might help others by
explaining your choice with comments in your program.

-- David McFarlane, Professional Faultfinder