Peter Seibel wrote:
>Well, I find that Common Lisp is an excellent language for dealing
>with pseudo-code because it provides many different control
>constructs. For an example look at the section "Local Flow of
>Control" from chapter 20 of Practical Common Lisp[1] where I show how
>you can create a very literal translation of some very low-level
>pseudo code from Knuth's _Art of Programming_ and then transform it
>into "regular" Lisp code. The nice thing is that the first version,
>which is a quite literal translation of Knuth's code, is something
>you can actually run and test. Then you can refactor bit by bit and
>know that you've always got a working version.
>
>-Peter
>
>[1] <http://www.gigamonkeys.com/book/the-special-operators.html>
>
>
>
Well, while tagbody is as beautiful as any goto, it isn't essential.
Any language with decent local function definition abilities will do:
Peter's example from PCL:
(defun algorithm-s (n max) ; max is N in Knuth's algorithm
(let (seen ; t in Knuth's algorithm
selected ; m in Knuth's algorithm
u ; U in Knuth's algorithm
(records ())) ; the list where we save the records selected
(tagbody
s1
(setf seen 0)
(setf selected 0)
s2
(setf u (random 1.0))
s3
(when (>= (* (- max seen) u) (- n selected)) (go s5))
s4
(push seen records)
(incf selected)
(incf seen)
(if (< selected n)
(go s2)
(return-from algorithm-s (nreverse records)))
s5
(incf seen)
(go s2))))
Peter's example using thunks instead of labels:
(defun algorithm-s (n max)
(let (seen selected u (records ()))
(flet ((s1 ()
(setf seen 0)
(setf selected 0))
(s2)
(s2 ()
(setf u (random 1.0))
(s3))
(s3 ()
(if (>= (* (- max seen) u) (- n selected))
(s5)
(s4)))
(s4 ()
(push seen records)
(incf selected)
(incf seen)
(if (< selected n)
(s2)
(nreverse records)))
(s5 ()
(incf seen)
(s2)))
(s1))))
A decent compiler will turn the calls into jumps anyway
(insert protracted argument over tail call optimization here)
Matt
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