On 06-09-18 10:50, Chris Angelico wrote:
> On Thu, Sep 6, 2018 at 6:44 PM, Marko Rauhamaa <[email protected]> wrote:
>> Chris Angelico <[email protected]>:
>>
>>> On Thu, Sep 6, 2018 at 2:29 PM, Marko Rauhamaa <[email protected]> wrote:
>>>> Marko Rauhamaa <[email protected]> (Marko Rauhamaa):
>>>>> Steven D'Aprano <[email protected]>:
>>>>>> 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.
> And even more idiomatically, Python doesn't require a new scope just
> for a new variable.
You may have overlooked where Marko wrote:
Actually, this is a more *direct* translation
--
Antoon.
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