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Forum => Whither newLISP? => Topic started by: Kirill on July 22, 2009, 05:34:53 AM

Title: TDWTF: Russian Peasant Multiplication
Post by: Kirill on July 22, 2009, 05:34:53 AM
Please see this post: Russian Peasant Multiplication (//http)



It ends with:


Quote
Your challenge: write a function that multiplies two numbers using the Russian peasant algorithm. There is no language restriction, though anything on the esoteric language list will probably be ignored. Spoiler alert: the solution(s) will undoubtedly appear in the comments.


There is no LISP solution there yet...
Title:
Post by: cormullion on July 22, 2009, 10:57:31 AM
You can find Lutz' (I assume!) typically elegant solution there (//http) now.
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Post by: xytroxon on July 22, 2009, 01:37:00 PM
Quote from: "cormullion"You can find Lutz' (I assume!) typically elegant solution there (//http) now.


Re: Programming Praxis: Russian Peasant Multiplication

2009-07-22 14:51 • by newlisp.org (unregistered)

   

278283 in reply to 277861

Reply Quote

improvement for some problems starting with even number:


Code Select

(define (rmul x y , (s 0))

  (until (= x 1)

    (unless (zero? (% x 2))

      (inc s y))

    (setq x (>> x) y (<< y)))

  (+ y s)

)


I think I found it... 9 pages of code entries posted so far? Al Gore won't be very happy over all the CO2 generated ;)



-- xytroxon
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Post by: TedWalther on July 22, 2009, 01:47:15 PM
Quote from: "cormullion"You can find Lutz' (I assume!) typically elegant solution there (//http) now.


Thanks for the link.  I was so inspired, I dashed off this one-liner:


Code Select

(define (rmul x y) (if (= x 1) y (+ (if (zero? (% x 2)) 0 y) (rmul (>> x) (<< y)))))


Ted
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Post by: newdep on July 22, 2009, 01:56:33 PM
QuoteI think I found it... 9 pages of code entries posted so far? Al Gore won't be very happy over all the CO2 generated ;)


I think I all people on this planet would stop talking for 1 day we solved the CO2 problem already..thats including Al.. ;-)





I Like these brain trainers.. But Cormullion posted already the trick...<<iekss>>..now I need to wait for another one to come by.. I was too quick in clicking..;-)
Title:
Post by: TedWalther on July 22, 2009, 01:58:21 PM
Here is the same solution, but with more normal indenting.  Still ends up being 6 line, like Lutz' solution, but uses tail recursion and maybe does fewer assignments?


Code Select

(define (rmul x y)
  (if (= x 1)
      y
    (+
     (if (zero? (% x 2)) 0 y)
     (rmul (>> x) (<< y)))))
Title:
Post by: TedWalther on July 23, 2009, 11:00:39 AM
Even better, the even? predicate could be written like this:


Code Select

(= (& x 1) 0)


I really wish that 0 could be included as a value for "false" in numeric contexts.  That is the one thing from C and other languages that I miss.



I often find myself wishing that "" and 0 as well as nil were counted as false values.



Ted
Title:
Post by: TedWalther on July 23, 2009, 11:05:16 AM
Desired False values:

0 (integer or float)

"" (empty string)



Current False values:

false (symbol)

nil (symbol)

() (empty list)



I do know that there are reasons for not allowing "" or 0 as false values.  Can someone fill me in?  As a former C programmer, I find them very comfortable idioms.



Ted
Title:
Post by: TedWalther on July 23, 2009, 11:20:07 AM
Even shorter version of the russian peasant:


Code Select

(define (rmul x y)
  (if (= x 1)
      y
    (+
     (* (& x 1) y)
     (rmul (>> x) (<<y)))))
Title:
Post by: TedWalther on July 23, 2009, 11:20:46 AM
accidentical duplicate, please delete this post.
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Post by: Lutz on July 23, 2009, 11:22:50 AM
QuoteI do know that there are reasons for not allowing "" or 0 as false values. Can someone fill me in? As a former C programmer, I find them very comfortable idioms.




try 'null?' which combines nil?, empty? and zero?


Code Select
(map null? '(() nil "" 0 0.0)) => (true true true true true)



ps: note that 'false' is not a built-in symbol, it only works, if nobody has set it.



ps2: maintenance release 10.1.1 is on its way
Title:
Post by: TedWalther on July 23, 2009, 11:35:03 AM
Thanks Lutz.  Maybe I'll start using (if-not-null ...) in my code now.  Not as satisfying as a straight (if) statement, not as impressive in a code contest like the russian peasant one, but it will work.  Are there reasons somewhere for not having (not (null? ..)) as the default for (if, or, and, case) and relatives?  Just LISP tradition?  I did read a JavaScript book by a LISP guy where he was really negative about that very feature of JavaScript, but I couldn't quite make out why he thought it was a bad feature.



Ted


Quote from: "Lutz"
QuoteI do know that there are reasons for not allowing "" or 0 as false values. Can someone fill me in? As a former C programmer, I find them very comfortable idioms.




try 'null?' which combines nil?, empty? and zero?


Code Select
(map null? '(() nil "" 0 0.0)) => (true true true true true)



ps: note that 'false' is not a built-in symbol, it only works, if nobody has set it.



ps2: maintenance release 10.1.1 is on its way
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