[web-devel] haskell memory allocation

[Henning Thielemann]

On Wed, 25 Sep 2019, anaaugusto2012 at bol.com.br wrote:

> I want to allocate more memory because my teacher challenged me to show 
> him the first 12 and last 12 digits of the expression 2 ^ 17179869184

Maybe better suited for haskell-cafe?

I would be pretty waste of memory to compute the power completely.

You can get the first 12 digits using approximate multiplications and you 
can get the last 12 digits using modulo arithmetic.

It holds 17179869184 = 2^34.

Prelude> mapM_ print $ take 35 $ iterate (\x -> let y = x*x in if y>=10 then y/10 else y) 2
2.0
4.0
1.6
2.5600000000000005
...
9.274368853264574

That is, the first 12 digits should be 927436885326. Hopefully, 
computation was precise enough. If not, you might retry more precise 
computations using Integer.

Prelude> mapM_ print $ take 35 $ iterate (\x -> mod (x*x) (10^12)) (2::Integer)
2
4
16
256
...
266397474816

The last 12 digits are 266397474816.