[REBOL] Re: Question and comment about randomizing a block of values
From: joel:neely:fedex at: 19-Jun-2001 12:07
Hey, Scott,
Which part? See below
GS Jones wrote:
> From: "Jeff Kreis"
> > random/seed now/time
> > c: 0
> > x: random reduce head insert/dup copy [] [c: c + 1] 52
> >
> > ; -- produces a block of 1 - 52 randomly ordered
>
> I've been thinking about this on and off for two hours, and I
> still do not understand how this statement works. Can someone
> state what is going on in other words?
>
The deep magic (the part I didn't know about ;-) is that RANDOM
will take a block and return a "randomized" copy of the block:
>> a: [1 2 3 4] == [1 2 3 4]
>> random a == [3 2 4 1]
>> random a == [3 2 1 4]
>> random a == [4 3 2 1]
>> random a == [3 1 2 4]
in contrast to RANDOM/ONLY which returns a random selection from
the block
>> random/only a == 3
>> random/only a == 1
>> random/only a == 4
>> random/only a == 3
The cool trick on the right-hand side is to use an anonymous
block that is extended with instructions to count up, and then
is reduced. Note the addition of PROBEs...
c: 0
x: probe random
probe reduce
probe head insert/dup copy [] [c: c + 1] 8
which yields
[
c: c + 1
c: c + 1
c: c + 1
c: c + 1
c: c + 1
c: c + 1
c: c + 1
c: c + 1]
A block containing 8 copies of C: C + 1 will reduce to
[1 2 3 4 5 6 7 8]
(if C is initialized to zero, of course). This randomizes to
[7 6 4 3 5 2 1 8]
Cool, huh? Thanks again, Jeff!
-jn-
___ ___ ___
\/ 2 + \/ 2 = 4 (for sufficiently large approximations of \/ 2 )
joel'dot'neely'at'fedex'dot'com
