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3.2.43. RANDOM(max)

Returns a pseudo-random whole number. If called with only the
first parameter, the first format will be used, and the number
returned will be in the range 0 to the value of the first
parameter, inclusive. Then the parameter max must be a non-
negative whole number, not greater than 100000.

If called with more than one parameter, or with one parameter,
which is not the first, the second format will be used. Then min
and max must be whole numbers, and max can not be less than min,
and the difference max-min can not be more than 100000. If one or
both of them is unspecified, the default for min is 0, and the
default for max is 999. Note that both min and max are allowed to
be negative, as long as their difference is within the
requirements mentioned.

If seed is specified, you may control which numbers the pseudo-
random algorithm will generate. If you do not specify it, it will
be set to some “random” value at the first call to RANDOM()
(typically a function of the time). When specifying seed, it will
effect the result of the current call to RANDOM().

The standard does not require that a specific method is to be used
for generating the pseudo-random numbers, so the reproducibility
can only be guaranteed as long as you use the same implementation
on the same machine, using the same operating system. If any of
these change, a given seed may produce a different sequence of
pseudo-random numbers.

Note that depending on the implementation, some numbers might have
a slightly increased chance of turning up than other. If the REXX
implementation uses a 32 bit pseudo-random generator provided by
the operating system and returns the remainder after integer
dividing it by the difference of min and max, low numbers are
favored if the 2^32 is not a multiple of that difference.
Supposing that the call is RANDOM(100000) and the pseudo-random
generator generates any 32 bit number with equal chance, the
change of getting a number in the range 0-67296 is about
 0.000010000076, while the changes of getting a number in the range
67297-100000 is about 0.000009999843.

A much worse problem with pseudo-random numbers are that they
sometimes do not tend to be random at all. Under one operating
system (name withheld to protect the guilty), the system’s pseudo-
random routine returned numbers where the last binary digit
alternated between 0 and 1. On that machine, RANDOM(1) would
return the series 0, 1, 0, 1, 0, 1, 0, 1 etc., which is hardly
random at all. You should therefore never trust the pseudo-random
routine to give you random numbers.

Note that due to the special syntax, there is a big difference
between using RANDOM(10) and RANDOM(10,). The former will give a
pseudo-random number in the range 0-10, while the latter will give
a pseudo-random number in the range 10-999.

Also note that it is not clear whether the standard allows min to
be equal to max, so to program compatible, make sure that max is
always larger than min.

RANDOM()       –>   ‘123’     /*Between 0 and 999 */
RANDOM(10)          –>   ‘5’  /*Between 0 and 10 */
RANDOM(, 10)   –>   ‘3’  /*Between 0 and 10 */
RANDOM(20, 30) –>   ‘27’ /*Between 20 and 30 */
RANDOM(,, 12345)    –>   ‘765’     /*Between 0 and 999, and sets
seed */