On Sat, Mar 06, 2010 at 02:32:29PM -0500, Terrence Brannon wrote:
> re:
> http://www.complang.tuwien.ac.at/forth/gforth/Docs-html/General-Loops-Tutorial.html
>
> First, the enormous amount of stack dancing really made this program
> hard to follow.
You mean, in this example:
: log2 ( +n1 -- n2 )
\ logarithmus dualis of n1>0, rounded down to the next integer
assert( dup 0> )
2/ 0 begin
over 0> while
1+ swap 2/ swap
repeat
nip ;
You ain't seen nothing yet. Or, in other words, this is not an
example that would be considered as having a lot of stack dancing by
most Forth programmers.
The pattern
( x1 x2 ) update-x1 swap update-x2 swap ( x1' x2' )
is typical when two values have to be updated.
> As a person deciding what language to study, I
> immediately began to think "I think this stack dancing is very very
> confusing. Maybe I should look at another language." So, I tried to
> rewrite it, but for some reason, the stack is empty when it
> terminates.
Your rewrite appears to be an example of overfactoring. If you want
to get rid of stack manipulation, you can do it with locals within one
colon definition, e.g.:
: log2 { n1 -- n2 }
\ logarithmus dualis of n1>0, rounded down to the next integer
assert( n1 0> )
n1 2/ 0 begin { n count }
n 0> while
n 2/ count 1+
repeat
count ;
This is pretty Gforth-specific, though (locals withing control
structures are not universally supported). If you wonder why COUNT is
visible after the loop, read
http://www.complang.tuwien.ac.at/forth/gforth/Docs-html/Where-are-locals-visible-by-name_003f.html
- anton
