Patrick Elliott wrote:
> 
> Actually the code for loops you had looked more
> C-ish. ;) Though, out of context it is hard to tell how cryptic it really is.

Well, here's some context.  And it's straightforwardly-written code, so
anything that's "cryptic" is the fault of Forth, not me.

In Forth, \ to end-of-line is a comment.  You can also enclose comments
in parens, which is how I will demonstrate what the stack looks like.

[For more on stack-based programming, please see
<URL:http://www-2.cs.cmu.edu/~koopman/stack_computers/contents.html>,
especially Section 1.2.]

N \ This puts the address of N (where the variable is stored) onto the
stack.
( 9238 ) \ Made-up memory address
@ \ This uses up the address of N, and fetches what was IN that address.
( 5 ) \ Made-up value of N.
0 \ This is zero.  It gets put on the stack, too.
( 5 0 )
DO \ "DO" takes two numbers off of the stack and loops between them.
   \ In this case, it will loop from 0 to N-1, inclusive.
( ) \ Nothing on the stack.
5 \ This puts 5 on the stack.
( 5 )
SQRT \ This replaces 5 with what happens when we run the SQRT word.
     \ If we're lucky, it'll turn out to be 2.
     \ (Stacks are usually integers, y'see.)
( 2 ) \ The SQRT of 5.
I \ This gives you the value of the looping variable, which is always
  \ (for singly-nested loops) called I.  It puts it on the stack.
( 2 0 ) \ First time through, it's 0.  After that it's 1.  And so on. 
:)
17 \ Puts 17 on the stack.
( 2 0 17 )
* \ Multiply the top two numbers, and replace them with the result.
( 2 0 ) \ 17 * 0 == 0
23 \ Puts 23 on the stack.
( 2 0 23 )
+ \ Replace the top two numbers with their sum.
( 2 23 )
DOSOMETHING \ DOSOMETHING (notionally) takes the top two numbers, and
            \ does something with them.  We will also imagine that it
            \ doesn't bother to return anything.
( )
LOOP \ Finish the loop started with "DO".

You know what?  That's really not very helpful.

Here's a better idea.  Go to
<URL:http://www.softsynth.com/pforth/pf_tut.htm> and poke around
"Arithmetic", "Variables" and "Loops".  Phil Burk does a much better job
of explaining it than I just did.  Sorry about that.

Also, since this has nothing more to do with scene files, please direct
all followups to povray.off-topic or to me personally, via email.

> Annoying thing being
> that the changed everything (including all new computer systems) the year after I
> graduated. :p

Always the way.  That happened to me in elementary, junior high, and
college.  In high school, they upgraded the last year I was there.  Heh.

Deaken

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