On 2018-09-04 14:26, Steven D'Aprano wrote:
> 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
>
>
> I've tried reading up on SML and can't make heads or tails of the
> "let...in...end" construct.
I don't know SML specifically, but my guess would be that
let X = Y in A end
means "A, evaluated in a local scope where X refers to Y". Reasoning:
often (e.g. in JavaScript), 'let' sets something in a local scope, and I
think Haskell has a construct like this? It's been a while since I
played around with Haskell though... In an as-direct-as-possible Python
translation, (lambda X=Y: A)()
So,
isqrt = (lambda n: 0 if n == 0 else
(lambda r=isqrt(n//4):
2*r if n < (2*r+1)**2 else 2*r+1)())
This is obviously the same as your multi-expression function, with the
addition of the integer division as suggested by Marko.
Cheers,
Thomas
>
> The best I've come up with is this:
>
> def isqrt(n):
> if n == 0:
> return 0
> else:
> r = isqrt(n/4)
> if n < (2*r+1)**2:
> return 2*r
> else:
> return 2*r+1
>
> but I don't understand the let ... in part so I'm not sure if I'm doing
> it right.
>
>
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