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[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]# Any SML coders able to translate this to Python?

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Chris Angelico <rosuav at gmail.com>: > On Thu, Sep 6, 2018 at 2:29 PM, Marko Rauhamaa <marko at pacujo.net> wrote: >> Marko Rauhamaa <marko at pacujo.net> (Marko Rauhamaa): >>> Steven D'Aprano <steve+comp.lang.python at pearwood.info>: >>>> I have this snippet of SML code which I'm trying to translate to Python: >>>> >>>> fun isqrt n = if n=0 then 0 >>>> else let val r = isqrt (n/4) >>>> in >>>> if n < (2*r+1)^2 then 2*r >>>> else 2*r+1 >>>> end >>> [...] >>> You must make sure "r" doesn't leak outside its syntactic context so: >>> >>> def isqrt(n): >>> if n == 0: >>> return 0 >>> else: >>> def f2398478957(): >>> r = isqrt(n//4) >>> if n < (2*r+1)**2: >>> return 2*r >>> else: >>> return 2*r+1 >>> return f2398478957() >> >> Actually, this is a more direct translation: >> >> def isqrt(n): >> if n == 0: >> return 0 >> else: >> def f2398478957(r): >> if n < (2*r+1)**2: >> return 2*r >> else: >> return 2*r+1 >> return f2398478957(isqrt(n//4)) >> > > I don't understand why you created that nested function instead of > something simple like renaming the variable. Is there a difference > here? Yes, in understanding the semantics of "let." "let" is used to introduce local bindings in some functional programming languages. I must admit I'm not fully versed in ML but it looks like the analogue in Lisp variants. This is how the above function would be written in Scheme: (define (isqrt n) (if (= n 0) 0 (let ((r (isqrt (quotient n 4)))) (if (< n (expt (1+ (* 2 r)) 2)) (* 2 r) (1+ (* 2 r)))))) Now, Lisp's "let" can be implemented/defined using "lambda": (let ((X A) (Y B) ...) . BODY) => ((lambda (X Y ...) . BODY) A B ...) which gives us: (define (isqrt n) (if (= n 0) 0 ((lambda (r) (if (< n (expt (1+ (* 2 r)) 2)) (* 2 r) (1+ (* 2 r)))) (isqrt (quotient n 4))))) Python does have a limited form of "lambda" and even a conditional expression so--as others have mentioned--this particular function could be translated pretty directly into Python using its lambda. More generally and idiomatically, though, Python's functions are named. So that explains the version I give above. Marko

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